Air Quality

Particulate matter (PM) is a complex mixture of very small solid particles and liquid droplets made up of many different chemicals, including elemental carbon, organic compounds, metals, sulphates, and nitrates. These particles are small enough to remain suspended in the atmosphere for an appreciable period of time. PM is emitted directly from sources (primary PM, e.g., black carbon or soot) or formed in the atmosphere from precursor compounds that undergo chemical reactions (secondary PM). Both primary PM and secondary PM precursors have a large variety of natural and anthropogenic sources, such as wildfires, motor vehicle exhaust, and coal combustion. The health effects of PM are directly associated with the size of the particles, as smaller particles penetrate deeper into the lungs. PM_2.5 is of significant concern for human health, has been extensively researched and reviewed, and is causally associated with the greatest range of health effects of any of the major (or “criteria”) air pollutants.

Risk assessments published by Health Canada (2013) and the US EPA (2019) have evaluated the weight of evidence of health effects from short-term and long-term exposure to PM_2.5, and their conclusions are summarized here. Evidence indicates that short-term exposure (hours to days) to ambient PM_2.5 increases the risk of premature death, including all-cause, cardiovascular, and respiratory mortality. In addition, short-term exposure increases the risk of adverse cardiovascular and respiratory effects, including aggravation of pre-existing cardiovascular and respiratory diseases (e.g., asthma or chronic obstructive pulmonary disease [COPD]), changes in cardiac and lung function, increased respiratory symptoms, and deterioration in indicators of cardiovascular health. Short-term exposure also results in increased medical interventions. For example, population-based studies show that increases in short-term PM_2.5 exposure are associated with increases in emergency room visits and hospital admissions relating to respiratory and cardiovascular conditions.

Overall, the evidence indicates that long-term exposure (months to years) to ambient PM_2.5 increases the risk of non-accidental and cardiovascular mortality, and it may also be associated with respiratory mortality. As well, long-term PM_2.5 exposure increases the risk of adverse respiratory effects, including increased respiratory symptoms and effects on lung development in children, as well as cardiovascular effects, such as health parameters relating to atherosclerosis progression. Chronic exposure to PM_2.5 increases the risk of lung cancer, and the International Agency for Research on Cancer has classified outdoor PM as carcinogenic in humans (IARC, 2016).

In addition to the extensive evidence examining respiratory and cardiovascular outcomes of PM_2.5, there is emerging evidence that long-term exposure to PM_2.5 may be associated with adverse effects on the nervous system, including dementia (Fu et al., 2019). Exposure to PM_2.5 has also been increasingly associated with other health effects, including metabolic diseases such as diabetes (Chen et al., 2013a) and pregnancy outcomes such as low birth weight (Stieb et al., 2016; Lavigne et al., 2019).

Ozone is a gas that is not directly emitted by air pollution sources. Rather, it forms in the atmosphere from reactions between precursor compounds and sunlight. The weight of evidence of health effects of short term and long-term exposure to ozone have been evaluated by Health Canada (2013) and the US EPA (2020a), the conclusions of which are summarized here. Short-term exposure to ozone increases the risk of non-accidental and cardiopulmonary mortality. The evidence from short-term ozone-exposure studies in humans also demonstrates reduced lung function, increased respiratory symptoms (e.g., cough, shortness of breath), and airway hyper-responsiveness. Epidemiological studies provide evidence of exacerbation of asthma and COPD, respiratory infections, and increases in hospitalizations and emergency room visits for respiratory conditions such as asthma. There is also evidence that short-term ozone exposure is associated with metabolic effects and may also be associated with adverse effects on the cardiovascular system. Long-term exposure to ozone has been associated with reduced lung function growth in children (Gauderman et al., 2000), onset of asthma in subgroups that spend more time outdoors (e.g., children, outdoor workers), and increased respiratory symptoms in children with asthma. There is some evidence that long-term ozone exposure may contribute to premature mortality, in particular from respiratory causes (Turner et al., 2016). 

Evidence indicates that various subpopulations are at increased risk of health effects from poor air quality (Health Canada, 2013; US EPA, 2019; US EPA, 2020a). Increased risk of PM-related health effects has been identified for older adults and young children, as well as for individuals with pre-existing cardiovascular or pulmonary diseases, or genetic factors that make them more sensitive to the effects of PM. Similarly, some individuals have increased sensitivity to ozone exposure due to underlying health conditions, age, how they spend their time, or genetics. Specifically, ozone exposure increases health risks among children, older adults, subgroups that spend increased time outdoors, individuals with asthma or COPD, and individuals with polymorphisms in genes associated with oxidative stress responses and inflammation. Overall, it has been estimated that about one-third of the Canadian population has at least one risk factor that increases their susceptibility to air pollution (Stieb et al., 2019).

Populations living in areas with higher levels of air pollution are also at increased risk for adverse health effects. Studies of populations in large urban centres in Canada suggest that material and social deprivation are associated with increased exposure to air pollution (Pinault et al., 2016a; Pinault et al., 2017). In addition, children living in low-income areas, racialized populations, and immigrants are exposed to higher levels of air pollution than children in high-income areas, White populations, and non-immigrants, respectively (Pinault et al., 2016b; Pinault et al., 2017).

Indigenous Peoples² in Canada may also be more sensitive to adverse health effects from poor outdoor air quality. Overall, the burden of chronic respiratory disease, including asthma and COPD, has been reported to be disproportionally higher in First Nations people (Carriere et al., 2017) and in Métis people (Gershon et al., 2014). In addition, higher rates of lower respiratory tract infections have been reported in First Nations and Inuit children (Kovesi, 2012; McCuskee et al., 2014). This can increase health risks from air pollution. In addition, the risk of exposure to poor air quality may be elevated for Indigenous Peoples due to multiple factors, including overcrowded housing, inadequate ventilation, exposure to smoke from wood heating, and the geographic proximity of many Indigenous communities to forests and, therefore, increased risks from wildfire smoke (Reading & Halseth, 2013; NCCAH, 2017). The existing health and social inequities experienced by Indigenous populations can increase health risks related to poor outdoor and indoor air quality. Many of these inequities are underpinned by systemic racism and colonization (see Chapter 2: Climate Change and Indigenous Peoples’ Health in Canada and Chapter 9: Climate Change and Health Equity).

PM can affect climate through direct interactions with radiation, predominantly through scattering or absorption of incoming solar radiation (sunlight). Sulphate, nitrate, ammonium, and secondary organic aerosol particulates predominantly scatter incoming solar radiation, some of which is reflected back into space; therefore, these PM species have a net cooling effect on climate (Boucher et al., 2013). Conversely, PM species that absorb incoming solar radiation, such as black carbon and certain types of organic particulates referred to as brown carbon, have a net warming effect (Bond et al., 2013). The presence of clouds influences these effects (Chand et al., 2009), and, under clear skies, so does the nature of the Earth’s underlying surface. PM over highly reflective surfaces such as snow has a stronger net warming effect than over less reflective surfaces because of the relatively larger fraction of incoming solar radiation that would have been reflected back to space without the presence of PM (Haywood & Shine, 1995). In addition, the mix of compounds within individual aerosol particles can also affect the direct radiative effects of PM. For example, absorption of solar radiation by black carbon is enhanced when PM is coated by more weakly absorbing compounds, such as sulphates (Jacobson, 2001; Liu et al., 2017).

PM can also affect climate through its effects on clouds, due to its role as the “seeds” upon which water may condense in either liquid or solid form — acting as cloud condensation nuclei and ice nuclei, respectively. Aside from the mass of particles in the atmosphere, there is also strong evidence that the number of particles influences cloud formation, with increased atmospheric particle number concentrations (total number of particles per unit volume of air) resulting in liquid water clouds with a larger number concentration of smaller cloud droplets, making the clouds more reflective and producing a cooling effect on climate (Boucher et al., 2013). However, PM–cloud interactions are subject to a large number of feedback processes that can locally modify the magnitude and direction of the radiative effects of these interactions, notably through changing precipitation production, which may alter the lifetime or spatial extent of the cloud, and changes involving ice particles in clouds (Fan et al., 2016). For these reasons, the effects of PM on clouds and climate are still uncertain (Seinfeld et al., 2016).

The Fifth Assessment Report³ of the Intergovernmental Panel on Climate Change (IPCC) assessed the global net radiative forcing from anthropogenic PM, including through both direct radiative effects and aerosol–cloud interactions, at −0.90 (5% to 95% confidence interval [CI] −1.9 to −0.1) W/m² (i.e., a net cooling effect). This likely offsets a significant fraction of the estimated +2.83 (5% to 95% CI +2.54 to +3.12) W/m² radiative forcing due to the change in concentration of the well-mixed GHGs⁴ (CO₂, methane, nitrous oxide, and halocarbons) between 1750 and 2011. However, the net forcing from PM combines significant positive forcing (warming) from black carbon emitted from fossil fuel and biofuel use, and negative forcing (cooling) from sulphates, nitrates, and primary and secondary organic aerosols. The net forcing from PM mentioned above does not include additional positive forcing from black carbon deposited on snow, estimated at +0.04 W/m², which has a two to four times larger impact on surface temperatures than CO₂ per unit radiative forcing (Myhre et al., 2013; Skiles et al., 2018).

Due to the short atmospheric lifetime (approximately 1 week) and highly variable spatial distribution of PM, impacts on radiation can be much larger on regional scales than the global average estimates given above. The chemical composition of PM influences the pathway and fate of incoming solar radiation as it travels through the atmosphere. Both PM that strongly scatters and PM that strongly absorbs incoming radiation reduce the amount of solar radiation reaching the Earth’s surface. However, only the latter will warm the atmospheric layer where it occurs. These effects could have significant impacts on surface temperatures and rainfall by reducing the amount of energy absorbed at the surface and, in the case of absorbing PM, heating the atmosphere aloft (Ramanathan et al., 2005; Liu et al., 2012). Modelling studies have shown that, due to the response of the global wind and weather patterns, the effects of SLCFs on climate are not limited to the regions where the emissions occur and may be even larger in regions well removed from the emission sources (Shindell et al., 2015; Kasoar et al., 2016).

Methane, with an atmospheric lifetime of approximately 9.1 years (Prather et al., 2012), is relatively well mixed throughout the atmosphere and is emitted from a large variety of sources (e.g., natural wetlands, geological seeps, rice cultivation, ruminants, and fossil fuel extraction and use) with an approximately equal contribution from natural and anthropogenic sources at the global scale (Saunois et al., 2016). Methane acts directly as a GHG, with an estimated radiative forcing of +0.48 (5% to 95% CI +0.43 to +0.53) W/m² due to the approximately 150% increase in concentrations over the period 1750 to 2011 (Myhre et al., 2013). Methane is also chemically reactive in the atmosphere, participating in the photochemical production of ozone. Ozone, in addition to being an air pollutant, acts as a GHG (Forster & Shine, 1997), and methane, therefore, makes additional contributions to climate forcing through changes in ozone.

Ozone is formed photochemically in the lower atmosphere from precursor compounds, including methane, other VOCs, and NO_x. Ozone is a highly reactive gas, and few reliable measurements are available from before the 1950s (Cooper et al., 2014). As a result, estimates of the change in ground-level ozone and the effects of tropospheric ozone on climate are based on numerical models and are more uncertain than for the long-lived GHG gases (e.g., CO₂). These estimates show that the increase in ozone from pre-industrial levels due to precursor emissions (methane, CO, non-methane VOCs, and NO_x) has resulted in an estimated radiative forcing of +0.50 (5% to 95% CI +0.30 to 0.70) W/m², of which +0.24 (5% to 95% CI +0.11 to +0.37) W/m² is attributed to ozone from methane (Myhre et al., 2013).

Changes in climate can have significant effects on air quality. Studies show a strong correlation between temperature and ground-level ozone (Camalier et al., 2007). This relationship results from various factors: direct temperature effects on chemical reactions that produce ozone; increases in biogenic emissions of ozone precursors (e.g., VOC emissions from vegetation) with temperature; and the correlation of higher temperatures with meteorological conditions that are conducive to the production of ozone (Steiner et al., 2006; Schnell & Prather, 2017). While tropospheric ozone in industrial regions in some seasons is expected to increase with increasing temperatures, climate change alone is expected to result in a decrease in the global background concentration⁵ of ozone, as increases in water vapour reduce levels of ozone through increased photochemical destruction in remote locations (Wu et al., 2008a; Stevenson et al., 2013).

The direct impacts of increased temperature on PM vary by season. Warmer temperatures in winter will result in a larger fraction of certain PM components remaining in the gas phase, resulting in lower PM concentrations. However, warmer temperatures in summer will favour the increased production of sulphates and higher PM concentrations (Dawson et al., 2007). Observations across the United States suggest additional contributions to PM concentrations result from other temperature-related factors (Tai et al., 2010). In particular, increased emissions of VOCs from biogenic sources, such as trees, with increasing temperature have been shown to have a strong impact on the formation of organic components of PM (Day & Pandis, 2011)

Climate change includes many complex changes beyond increasing temperature. By more rapidly warming the Arctic, climate change may lead to a greater persistence of hot and dry conditions associated with slow-moving high-pressure systems (Coumou et al., 2018). Summertime episodes of poor air quality over Southeastern Canada and the Northeastern United States are frequently associated with these large, slow-moving high-pressure systems and are brought to an end by the passage of a mid-latitude cyclone and associated cold front. Less frequent summertime mid-latitude cyclones have been observed in the recent past, negatively affecting air quality (Leibensperger et al., 2008), and less frequent mid-latitude cyclone passage has been projected under future climate scenarios in several studies (Mickley et al., 2004; Wu et al., 2008b). However, the magnitude of these specific effects on future changes in air pollution is uncertain. Tai et al. (2012) found that the changes in the frequency of mid-latitude cyclone passage had only a small impact on PM, while Horton et al. (2014), from an analysis of 15 different climate models, found relatively small increases in stagnation events, which may lead to higher pollutant concentrations for 2080 to 2099 along the west coast of North America, Northeastern United States, and Southeastern Canada.

A large number of studies have used regional-scale air quality models, combined with future climate projections, to estimate future changes in tropospheric ozone and PM across North America. It should be noted that projections of future air quality depend on model projections of changes in the physical climate, and these changes are more uncertain at regional (sub-continental and smaller) scales for a variety of reasons (Deser et al., 2012; Mearns et al., 2013; Giorgi & Gutowski, 2015). Future air quality will also be affected by changes in air pollutant and precursor emissions. Many studies estimate the effects of climate change alone by making future projections using constant, present-day emissions of air pollutants. The impact of climate change alone on air quality, discussed in this section, is frequently referred to as “the climate penalty” (Wu et al., 2008b).

While differences in modelling assumptions and scenarios make detailed comparisons difficult, some general conclusions can be drawn. Available studies generally find that, relative to current conditions, climate change will cause small increases across large parts of North America of 2 to 3 parts per billion by volume in the daytime average ozone concentration during the summer (June to August) by the middle of the century (Hogrefe et al., 2004; Nolte et al., 2008; Lam et al., 2011; Kelly et al., 2012). Isolated increases of about 5 parts per billion by volume were also found across different modelling studies. The largest increases were projected over industrialized regions, such as the Northeastern United States and adjacent regions of Southeastern Canada. These studies have also found some evidence for a larger impact of climate change on days with the highest ozone concentrations (Hogrefe et al., 2004; Nolte et al., 2008) and indications that climate change may lengthen the ozone season (Nolte et al., 2008; Trail et al., 2014).

Estimates of the impact of climate change alone on PM are more uncertain, as there are fewer studies, and the physical and chemical processes affecting PM are more complex and more sensitive to changes in physical climate variables that climate models represent poorly, such as the frequency of precipitation. Tagaris et al. (2007) found decreases of 10% to 20% (1 to 2 μg/m³) in PM_2.5 over central and southeastern parts of the United States by about 2050, due to higher precipitation, although only three summers were simulated. Conversely, Kelly et al. (2012) simulated 10 summers leading to 2050 and found the effects of climate change alone resulted in increases in summer PM_2.5 of 0.5 to 1.0 μg/m³ over much of the Eastern United States, extending into Southwestern Ontario. Increases of 0.3 to 0.5 μg/m³ were projected over the broader region of Southern Ontario and Southern Quebec. The increases in PM_2.5 resulted from the net effect of increases in sulphates and secondary organic aerosols and decreases in nitrates. These changes can be contrasted with those found by Trail et al. (2014), who projected decreases in summer PM_2.5 of 1 to 2 μg/m³ for periods around 2050 across most of the Central United States to the Great Lakes, due to increased dispersion of pollutants in the atmosphere because of projected changes in wind speed and precipitation. These results demonstrate the level of uncertainty in our current understanding of certain aspects of changes in physical climate and associated air quality projections, as represented by existing models. For changes in annual average PM_2.5, the available studies suggest very small changes in regional average values (Tagaris et al., 2007; Lam et al., 2011; Trail et al., 2014).

Two important caveats to the preceding discussion are worth noting. First, ozone and PM precursor emissions have been reduced significantly over the recent past in the United States and Canada because of emissions controls put in place to reduce the environmental and human health impacts of air pollution (Amann et al., 2013; Stieb et al., 2015; Fann et al., 2017; Jiang et al., 2018; Zhang et al., 2018). Air pollutant emissions are projected to decrease further over the coming decades as a result of air quality regulations, and available projections show significant net improvements in air quality can be realized in the future, accounting for the combined effects of anticipated reductions in air pollutant emissions and the negative impact of climate change (Tagaris et al., 2007; Nolte et al., 2008; Lam et al., 2011; Kelly et al., 2012; Pfister et al., 2014; Trail et al., 2014; Yahya et al., 2017). However, additional reductions in air pollution emissions would be required to counteract the effect of the climate penalty. Second, future ozone concentrations in North America will be affected by changes in the global concentration of methane (Yahya et al., 2017). Increases in the concentrations of methane result in a higher global background concentration of ozone (Stevenson et al., 2013), which affects ozone concentrations in North America, as regional-scale episodes of high ozone build on top of the global background.

Another impact of climate change on air quality is the expected increase in the length and severity of the wildfire season (Flannigan et al., 2013). Estimates of the effects of climate change on wildfire emissions of black and organic carbon aerosols suggest increases of 80% to 150% by 2050 (compared to 1997 to 2001) over the Western United States (Yue et al., 2013), with an increase in summer average PM_2.5 concentrations of 20% to 60% (Val Martin et al., 2015). These projections were based on increases in the average annual area burned each year of between 25% and nearly 170%, which are comparable to estimates of the increase in area burned by mid-century for Canada (Boulanger et al., 2014). Note that the Val Martin et al. (2015) study did not include any increase in wildfires in the Eastern United States because any climate change effect was found to be negligible, while projections of wildfires in Canada show increases from coast to coast (Boulanger et al., 2014; Wang et al., 2017). For more information on the impact of climate change on wildfires in Canada and the associated health impacts, see section 5.6 Climate Change and Air Pollution from Wildfires.

Because of the relatively short time that air pollutants remain in the atmosphere, reductions in their emissions quickly affect their atmospheric concentrations and the associated radiative forcing of these compounds. Actions to reduce certain air pollutants that are SLCFs, such as black carbon, are therefore recognized as a way to reduce climate warming over the near term (CCAC, 2020). This would allow more time for international CO₂ mitigation efforts to have an impact before critical temperature thresholds are crossed, while also simultaneously reducing the health and environmental burden of air pollution (IPCC, 2018). Shindell et al. (2012) projected a reduction in global mean warming of 0.5°C by 2050, relative to pre-industrial levels, resulting from a suite of controls on methane and black carbon emissions, and estimated additional, significant benefits for human health and agricultural yields. A smaller cooling benefit of 0.22°C by 2050 from a range of emission controls designed to maximize the climate benefit — largely targeting methane and black carbon — was found by Stohl et al. (2015). The authors also estimated that a warming of nearly 0.7°C would result from the complete removal of land-based anthropogenic SO₂ sources, illustrating the significant cooling effect of sulphate PM from SO₂ precursor emissions.

These results demonstrate the need to evaluate climate change and air quality policies in an integrated manner. Targeting SLCFs that have a net warming effect on climate, namely methane and black carbon, would have simultaneous benefits for climate and air quality. Conversely, certain actions designed to improve air quality may have unintended negative consequences for climate change (Stohl et al., 2015; Partanen et al., 2018). However, the strategic application of air pollution emission controls can have significant co-benefits for climate change, and well-designed GHG mitigation strategies, such as those targeting reductions in the use of fossil fuels, can have clear co-benefits for air quality and, hence, population health (Vandyck et al., 2018). Co-benefits for health associated with GHG reductions are reviewed in section 5.5 Air Pollution Health Impacts Due to Climate Change and the Health Co-Benefits of Greenhouse Gas Mitigation and also highlighted in Chapter 10: Adaptation and Health System Resilience of this report.

One of the most significant risks to air quality throughout Canada associated with climate change is the projected increase in emissions from wildfires. An improved understanding of the effects of climate change on wildfires requires further development of the capacity to simulate interactions between climate and fire risk, taking into account anthropogenic influences such as fire management through suppression, for example, in terrestrial carbon cycle models. A second significant uncertainty in projections of climate change effects on air quality is the impact of future changes in biogenic emissions. As discussed above, increasing biogenic emissions due to warmer temperatures is an important mechanism by which climate change negatively affects air quality. However, the likely effects of higher CO₂ and water stress, as well as changes in the distribution of vegetation types on biogenic emissions, are not well understood (Fiore et al., 2015). Last, many of the studies estimating the impact of climate change on air quality have been based on only a few years of simulation because of the computational cost of the models. The short time periods make it difficult to remove the influence of random year-to-year variability from the effects of climate change (Barnes et al., 2016) or to robustly analyze climate change impacts on more extreme, but less frequent, climate events.

Studies have investigated how a changing climate could influence future population health impacts of air pollution. Researchers have assessed these health impacts both regionally and globally, under various climate projection scenarios, including the Representative Concentration Pathways (RCPs)⁶ and the Special Report on Emissions Scenarios (SRES)⁷ published by the IPCC, and for multiple future time periods. Most studies have examined the influence of climate change on ozone, and some have also addressed PM_2.5. Both premature mortality and morbidity outcomes have been considered. In the studies reviewed here, the authors estimated the incremental air pollution health impacts in the future due to climate change alone (i.e., the impact of a warming climate on air pollution concentrations), keeping other factors constant.

More recent studies have used the RCPs described in the Fifth Assessment Report of the IPCC (2014) and other publications (van Vuuren et al., 2011; van Vuuren & Carter, 2014) to model future climate change projections. The RCPs, identified as RCP2.6, RCP4.5, RCP6.0, and RCP8.5 (referring to different levels of radiative forcing in watts per square metre), represent global climate policy scenarios ranging from more stringent (RCP2.6) to less stringent (RCP8.5) in terms of GHG reductions and concentration trajectories. The RCPs also include projected policies to control air pollution. The more stringent RCP scenarios are also associated with lower air pollutant emissions globally and in Canada, given that many changes in sector activities to reduce GHG emissions (e.g., transition to cleaner energy sources) would also reduce air pollution emissions. The four RCP scenarios project average global temperature increases for 2081 to 2100 (relative to 1986 to 2005) of 1.0°C, 1.8°C, 2.2°C, and 3.7°C, respectively. The RCPs are more in-depth explorations of the impacts of potential mitigation strategies on GHG emissions and are an extension and further development of the GHG emissions-based SRES projections used in previous IPCC assessment reports (IPCC, 2014).

There are no recent published analyses from Canada using the RCP scenarios, but findings have been published for the United States and for global analyses. In a national analysis, Fann et al. (2015) estimated that summer average eight-hour daily maximum ozone concentrations would increase by about 1 to 5 ppb for parts of the United States in the near future (2030) due to climate change, under RCP6.0 and RCP8.5 scenarios. This analysis compared air quality impacts under the 2030 climate, as projected by the RCPs, with air quality impacts under climatic conditions in 2000. The ozone-related health outcomes in 2030 are presented in Table 5.1. The ranges in the projections reflect estimates of the health effects of ozone from multiple epidemiological studies. Overall, health impacts and the associated societal costs under RCP8.5 are projected to be an order of magnitude higher than under RCP6.0.

Much lower US health impacts for the year 2050 (less than 100 ozone-related premature deaths) were reported in a separate analysis of the climate penalty under RCP4.5 and RCP8.5 (Stowell et al., 2017).

Climate change impacts on both ozone and PM_2.5 health effects under the RCP8.5 scenario have also been assessed globally, with estimates of 43,600 and 215,000 excess deaths annually by 2100 due to ozone and PM_2.5 increases, respectively, including almost 10,000 deaths in North America. Global impacts in the near term (2030) were estimated to be almost 60,000 excess deaths annually for the two pollutants combined (Silva et al., 2017). Multiple models show an increase in PM_2.5 health risks due to the climate penalty in nine of the 10 regions of the world, including North America (Park et al., 2020).

Several investigators have applied the older SRES A1B scenario, which is roughly comparable to RCP6.0 and estimates a global temperature increase of 1.59°C in 2050, to project the air pollution health impacts of climate change. These studies estimate hundreds of ozone-related and several thousand PM_2.5-related deaths annually in 2050 in the United States due to the warming climate, along with thousands of hospital visits, bronchitis incidents, and asthma attacks (Selin et al., 2009; Tagaris et al., 2009). Higher estimates of ozone and PM_2.5-related deaths attributable to climate change were reported for the year 2090, with 13,000 deaths in North America and more than 100,000 deaths globally (Fang et al., 2013). A study of 50 United States cities under the SRES A2 scenario in the 2050s, which predicts a rise in average summer temperature for the Eastern United States of 1.6°C to 3.2°C, projected an increase in mortality (0.11% to 0.27%) and hospital admissions for asthma (2.1%) and COPD (0.2% to 1.6%) due to ozone increases under the future climate (Bell et al., 2007).

A Canadian analysis estimated the air pollution health impacts for a hypothetical increase of 4°C in ambient temperatures for 2002, with anthropogenic air pollution emissions held at 2002 levels (Séguin, 2008). Nationally, increases in summer ozone concentrations due to the higher temperature were associated with 156 excess premature deaths and a total economic value of $750 million due to mortality and morbidity outcomes. This increased to 658 premature deaths ($3.2 billion) when increases in biogenic emissions, expected under a warmer climate, were also included. However, the same study projected, with considerable uncertainty, a net reduction in PM_2.5 concentrations under the warmer climate, so that the net effect of the two pollutants under a 4°C increase was estimated to be an excess of approximately 300 premature deaths and a social cost of $1.4 billion.

Overall, these studies show that climate change is expected to worsen air quality and cause considerable deaths and illnesses related to air pollution, with hundreds to thousands of excess annual deaths in North America over this century and 100,000 annual excess deaths estimated globally by 2100. Studies that examined both ozone and PM_2.5 generally estimated that effects of climate change on PM_2.5-related health outcomes are about an order of magnitude higher than those associated with ozone.

Mitigation measures to reduce GHGs and SLCFs help stabilize the climate by reducing pollutants that play a role in increasing global temperature and accelerating climate change. However, climate change mitigation strategies can also have a range of indirect health co-benefits (Markandya et al., 2018). Key among these are health co-benefits from reductions in air pollution because of the following: climate mitigation will limit the climate penalty; some climate pollutants (e.g., black carbon, ozone) contribute to air pollution either directly or as precursors; and changes in industrial and transportation activities to reduce long-lived GHG and/or SLCF emissions can also reduce co-emitted conventional air pollutants (Smith et al., 2014). For example, the widespread use of electric vehicles powered by low-carbon energy can be a strategy for GHG mitigation that could also result in improved air quality. Consequently, climate change mitigation measures represent an opportunity to integrate objectives to stabilize the climate with those to reduce air pollution health impacts, which may offset the costs of implementing the measures (Thompson et al., 2016).

Studies examining air pollution health co-benefits associated with GHG mitigation strategies typically compare a policy scenario that includes strong GHG mitigation and accompanying reductions in air pollutant emissions with a scenario with limited GHG mitigation and associated higher air pollutant emissions. Economic valuation of air pollution health co-benefits allows co-benefits to be incorporated into a broader climate change framework, and the cost-effectiveness and potential optimization of various GHG mitigation measures, which can impose an economic burden on society, can be examined. Studies report monetized health co-benefits as total dollar value or as the marginal benefit per tonne of CO₂ equivalent that is reduced. Both PM_2.5– and ozone-related health co-benefits have been evaluated in the studies reviewed below, unless otherwise stated.

In a global analysis of GHG mitigation measures associated with RCP4.5, it was estimated that Canadian air pollution health co-benefits would range from about 4500 to 6500 avoided premature deaths annually between 2030 and 2100, relative to the reference scenario associated with the RCP projections (West et al., 2013) (Table 5.2). US co-benefits were estimated to be about an order of magnitude higher (West et al., 2013; Zhang et al., 2017). Globally, changes in emissions were estimated to avoid 0.5 million, 1.3 million, and 2.2 million deaths from combined exposure to ozone and PM_2.5 in 2030, 2050, and 2100, respectively. The global average monetized marginal health co-benefits were estimated to be $50 to $380 per tonne of CO₂ (tCO₂) (in 2005 USD), which exceed the marginal GHG abatement costs in 2030 and 2050 in this analysis, and partially offset the costs in 2100 (West et al., 2013). For the United States, the marginal value of avoided ozone- and PM_2.5-related deaths was estimated to be $45 to $137/tCO₂ (in 2005 USD) (Zhang et al., 2017).

Other studies have evaluated the avoided air pollution deaths associated with climate mitigation pathways based on specific targets for increases in global mean surface temperature in the future compared to a reference scenario. The US air pollution heath co-benefits of reducing the climate penalty were estimated for two climate mitigation scenarios, reflecting a 2.5°C and 2.0°C increase in global mean surface temperature (Saari et al., 2019). The two mitigation scenarios reduced the combined ozone and PM_2.5 health impacts by thousands and tens of thousands of deaths in 2050 and 2100, respectively, with 40% greater PM_2.5 benefits by 2100, under the more stringent policy compared to the less stringent policy. In 2100, air pollution (ozone and PM_2.5) risks associated with climate change were attenuated by 70% to 88% in the policy scenarios. Similarly, US health co-benefits of limiting global warming in 2100 to a 1.5°C rise was estimated to result in 11,000 and 52,000 avoided premature deaths across the United States in 2050 and 2100, respectively, compared to a reference scenario with a 6°C increase in global mean surface temperature. These health co-benefits were valued at 150 billion USD and 1.3 trillion USD (in 2005 USD), respectively, which were equivalent to a rate of $25 (95% CI $9–$42) per tonne of CO₂ equivalent (tCO₂e) and $122 (95% CI $45–$207)/tCO₂e, respectively (Garcia-Menendez et al., 2015).

In a global analysis, Vandyck et al. (2018) examined the air quality implications in 2100 of two GHG mitigation scenarios, the first based on GHG emission reductions implied by the Nationally Determined Contributions,⁸ leading to a 2.5°C to 3.2°C rise in global mean temperature by 2100, and the second a 2°C rise by 2100. Globally, avoided deaths due to air pollution in 2050 were estimated at 0.3 to 0.5 million for the Nationally Determined Contributions scenario and 0.7 to 1.5 million for the 2°C scenario, including about 20,000 to 25,000 avoided deaths in the United States. The timing of taking action can affect the magnitude of the accrued health co-benefits. For example, it was estimated that about 150 million deaths due to air pollution globally could be avoided over this century if carbon emission reductions to limit warming to 2°C are implemented sooner rather than later in the 21st century (Shindell et al., 2018).

Another global study specifically examined the potential air pollution health co-benefits of meeting the Paris Agreement temperature change targets of 1.5°C and 2°C, and compared them to the associated mitigation costs of the climate policy scenarios (Markandya et al., 2018). Globally, the economic value of the health co-benefits were estimated to outweigh the mitigation costs for all scenarios examined, with the ratio of health co-benefits to climate change mitigation costs ranging from 1.4 to 2.45. The greatest potential health co-benefits were observed for China and India, but significant co-benefits were also estimated for the European Union and the United States. Overall, climate change mitigation measures to meet the targets established in the Paris Agreement can be cost-effective under the modelled scenarios and, depending on geographical regions, when air quality associated health co-benefits are considered.

Other studies have estimated thousands of avoided deaths due to ozone for North America and hundreds of thousands globally under various climate change mitigation scenarios, including those targeting methane (West et al., 2007; West et al., 2012). The latter were valued at $13 to $17/tCO₂e (2005 USD) in 2030, which outweighed the costs of measures to reduce methane emissions (West et al., 2012).

Some studies have examined the co-benefits of climate change mitigation strategies targeting specific SLCFs or emission sectors. The annual health co-benefits of full electrification of the vehicle fleet in the Greater Toronto and Hamilton Area (Canada) were estimated at 260 and 330 avoided deaths, based on electricity derived from natural gas or from renewables, respectively (Gai et al., 2020). The North American health co-benefits of ambitious clean energy and clean transportation scenarios in the United States were estimated at 175,000 and 120,000 avoided deaths due to air pollution, for the 2015 to 2030 period, respectively, with additional annual benefits thereafter (Shindell et al., 2016). The majority of benefits were from PM_2.5 reductions, with a smaller fraction from ozone reductions, and 3% to 4% of all benefits (thousands of avoided deaths for 2015 to 2030 period) were estimated for Canada. Near-term US benefits of the policies were valued at 250 billion USD (95% CI 140 billion to 1.05 trillion USD) annually.

A global analysis of strategies to reduce methane and black carbon reported that implementation of the measures would avoid 0.7 to 4.7 million deaths per year globally, with the largest impact attributable to black carbon reductions, and a marginal benefit of 700 to 5000 USD per tonne of methane reduction (Shindell et al., 2012). Carbon policies for the United States addressing clean electricity production, on-road transportation, and an economy-wide cap-and-trade system were estimated to have air pollution health co-benefits associated with PM_2.5 and ozone reductions that would offset approximately 26% to 1050% of GHG mitigation costs (Thompson et al., 2014). The marginal health co-benefits of PM_2.5 reductions under the cap-and-trade scenario were estimated at $6/tCO₂, which exceeded the implementation costs, and at $8/tCO₂ (in 2005 USD) under the clean energy scenario, which partially offset the implementation costs (Saari et al., 2015). Marginal health co-benefits of $148/tCO₂ (in 2006 USD) compared to GHG mitigation costs of $126/tCO₂ under a clean energy scenario, and marginal health co-benefits of $80/tCO₂ compared to costs of $15/tCO₂ under the cap-and-trade scenario were reported for a subnational carbon policy in the Northeastern United States. It was noted that costs varied widely due to regional differences in energy consumption (Thompson et al., 2016).

It is important to consider all aspects of emission changes when evaluating the health co-benefits of climate change mitigation strategies. The US air pollution health co-benefits associated with electric vehicle (EV) adoption were found to vary, depending on the source mix of electricity generation (Peters et al., 2020). It was estimated that 25% of EV use powered by the current US electricity grid mix would result in 437 (95% CI 295 to 578) avoided premature deaths due to PM_2.5 and 98 (95% CI 33 to 162) due to ozone reductions annually, with a total value of about 16.8 billion USD. The health co-benefits were estimated to be about twice as high for the same EV adoption rate if the fraction of emission-free energy sources in the grid mix was doubled, although the associated reduction in CO₂ emissions would only increase by about 10%.

Overall, studies of the potential air pollution health co-benefits associated with climate mitigation scenarios targeting maximum increases in global mean temperature of about 1.5°C to 3°C consistently indicate that substantial health benefits would be realized. Annually, the burden of disease would be reduced by up to tens of thousands of premature deaths related to PM_2.5 and ozone in the United States in 2050 and 2100, and by up to 1 million to 2 million premature deaths globally. When directly compared in a study, more stringent mitigation policies confer greater co-benefits, and there is evidence that larger co-benefits will be realized if policies are implemented sooner rather than later. Air pollution health co-benefits represent a significant public health gain for individual countries and globally, and the socio-economic value can contribute to or even offset the costs of mitigation. Moreover, the joint consideration of climate change and air pollution mitigation options provides a framework to optimize the overall public health benefits and to identify potential unintended consequences, such as inadvertent risks to health. This was made evident following the promotion of diesel-fuelled vehicles in some countries because of their lower GHG emissions, which resulted in elevated emissions of air pollutants (Cames & Helmers, 2013; Haines, 2017).

Health Canada undertook an analysis of the potential air pollution health impacts in Canada due to the climate penalty and the potential health co-benefits of a specific GHG mitigation pathway for the purposes of this assessment report. In a previous study, scientists at Environment and Climate Change Canada modelled mid-century air quality in North America to examine the influence of both a warming climate and GHG emission controls (Kelly et al., 2012). Air quality estimates were made using a unified regional air-quality modelling system (AURAMS), specifically, a comprehensive Canadian system. Specifically, air pollution concentrations were modelled for the June to August period under the current climate (based on meteorological data from 1997 to 2006) and under a future climate in 2041 to 2050 (based on meteorological conditions in SRES A2). Air pollutant emissions, which were derived from the 2002 Canadian emissions inventory, were held constant between the two scenarios in order to examine the impact of changing climatic conditions alone. Biogenic emissions and the effect of temperature on those emissions were included, but wildfire emission changes associated with climate change were not addressed. In addition, the study estimated air pollutant concentrations under the same 2041 to 2050 future climate, with a 2050 emissions inventory informed by RCP6.0 (a moderate GHG mitigation scenario), which was used to scale North American air pollutant emissions from 2002 to 2050 levels (Kelly et al., 2012). RCP6.0 was chosen in part due to the high level of detail available in its accompanying emissions activity changes, allowing a more precise calculation of the air pollutant reductions associated with the GHG-emitting activity reductions. This scaling reflects both air pollutant emission reductions associated with GHG mitigation as well as assumed air pollution controls. Air pollutant concentrations for each scenario were estimated on a national grid at 45 km resolution, and effects on both ozone and PM_2.5 were considered. As noted in section 5.4 Interactions Between Climate Change and Outdoor Air Pollution, effects of a changing climate on PM_2.5 are more complex and less well studied than effects on ozone. Therefore, projections of changes in PM_2.5 are more uncertain.

For the current analysis, the air quality output from the three scenarios modelled by Kelly et al. (2012) was used as input for Health Canada’s Air Quality Benefits Assessment Tool (AQBAT 3.0). AQBAT is a national model used to estimate the human health (mortality and morbidity outcomes) and welfare benefits or damages associated with incremental changes in ambient concentrations of air pollutants, including PM_2.5 and ozone, in Canada (Health Canada, 2019). AQBAT includes concentration-response functions (CRFs), which characterize the increase in per capita risk of a given adverse health outcome per unit increase in ambient air pollutant concentration, based on published scientific studies. AQBAT estimates health impacts by combining the following parameters: the baseline risk of the adverse outcome in the population, population counts, CRFs, and the change in air pollutant concentrations between the scenarios. In addition, AQBAT provides economic valuation estimates for the health outcomes; these consider the potential social welfare consequences of the health outcomes, including medical costs, reduced workplace productivity, pain and suffering, and the effects of increased mortality risk.

The air pollutant concentration estimates from these three scenarios were used to answer the following questions.

What would the incremental air pollution health impacts in Canada be in 2050 due to:

• a changing climate, if air pollution precursor emissions remain unchanged (i.e., the climate penalty)?
• reductions in air pollution precursor emissions associated with the RCP6.0 projection, under future climate conditions?
• a changing climate and reductions in air pollution emissions associated with the RCP6.0 projection, compared to recent climatic conditions and emissions?

The Canadian population health impacts calculated for 2050 (June to August) and the associated economic valuation estimates are presented in Tables 5.3 and 5.4.

This analysis estimates that the impact of a warming climate in 2050 would result in 850 excess deaths across the country in the summer months, with clear regional differences. Most of the deaths would occur in Ontario (620) and Quebec (270), and some regions would experience small reductions in air pollution and the associated health impacts (e.g., British Columbia). This is consistent with the air quality changes projected by Kelly et al. (2012), which estimate for North America that the largest increases in ozone and PM_2.5 would occur in industrialized regions such as the Northeastern United States and adjacent regions of Southeastern Canada. Reductions in air pollutant emissions associated with projections under the RCP6.0 scenario would have national population health benefits of about 5200 avoided premature deaths annually in 2050, with benefits in all provinces but particularly in Ontario (2900 avoided premature deaths) and Quebec (1500 avoided premature deaths). These reflect the air pollution health co-benefits of the GHG mitigation strategies in RCP6.0, along with the assumptions in RCP6.0 of more stringent air pollution controls globally over time. The final scenario examined considered the effect of both a warming climate and air pollutant emission reductions. The results show that the benefits of air pollution mitigation are attenuated by the negative effects of a warmer climate. As indicated above, modelled projections of changes in PM_2.5 concentrations are considered to be more uncertain than those for ozone, which is also true for the health impacts derived from those projections.

Exposure to ozone and PM_2.5 also increases the risk of multiple non-fatal outcomes, such as asthma symptoms and cardiorespiratory hospital admissions. These represent important aspects of population health. In this analysis, the effect of the climate penalty on worsening air quality is estimated to result in 1.9 million acute respiratory symptom days and tens of thousands of asthma symptom days and restricted activity days, contributing to a substantial public health burden. This burden is in contrast to the larger annual health co-benefits that would result from air pollution reductions associated with the RCP6.0 scenario, including thousands of avoided cardiorespiratory hospital visits, as well as respiratory illnesses. About 70% of the mortality impacts are attributable to estimated changes in ambient ozone concentrations, with the remainder attributable to PM_2.5.

The economic value of the damage to health due to air pollution in Canada associated with a warming climate, based on analysis of the summer period in 2050, is estimated to be $2.7 billion, with a range of $0.97 billion to $5.2 billion, representing the 2.5th to the 97.5th percentile. This primarily reflects the value associated with increased risk of premature death. Importantly, this analysis also shows the potentially large socio-economic co-benefits that could be realized from air quality improvements delineated in the RCP6.0 projection, which were estimated to be $17 billion, with a range of $6.0 billion to $32 billion, representing the 2.5th to 97.5th percentile, in 2050.

Overall, the scientific evidence shows that climate change will affect atmospheric processes and, consequently, air quality. Multiple studies estimate that hundreds to thousands of ozone-related deaths may result annually in the United States during this century as a result of a warming climate. Fewer studies have examined PM_2.5 health impacts, but those that have suggest that PM_2.5 health impacts will be about one order of magnitude larger than those from ozone (Tagaris et al., 2009; Fang et al., 2013; Silva et al., 2017). Only one study was identified that included an analysis of health impacts in Canada. This focused only on ozone health impacts and estimated that the climate penalty under SRES A1B would result in 45 excess annual deaths in Canada in 2050 (Selin et al., 2009). In a new analysis for Canada conducted for this national assessment report, the net summer season health impacts in 2050 due to the climate penalty are estimated to be 850 excess deaths due to air pollution (total value of impacts $2.7 billion), with approximately 70% of those due to ozone and the remainder to PM_2.5, and with the vast majority of health impacts occurring in Ontario and Quebec. Note that this analysis did not include the air quality and health impacts resulting from wildfire smoke, which is discussed later in this chapter.

Strategies to mitigate GHG emissions can result in significant air pollution health co-benefits, the societal value of which can, in turn, offset the costs of GHG mitigation measures. Again, studies in the scientific literature generally estimate the PM_2.5 co-benefits to be about an order of magnitude larger than those from ozone reductions. The new Canadian analysis, presented above, estimates net benefits of following the RCP6.0 pathway to be about 5200 avoided deaths for the summer season for a single year in 2050, mainly in Ontario and Quebec, with a total value of benefits of $17 billion. Air pollution co-benefits are anticipated for each year following GHG emission reductions, and the cumulative benefits over decades would be expected to be much larger.

Air pollution health co-benefits of climate change mitigation represent important potential near-term gains and are realized locally, where mitigation measures are implemented. The incorporation of health co-benefits into a climate change framework can provide justification for more stringent or accelerated climate change mitigation measures or incentives to implement actions on climate change (Shindell et al., 2018). Appropriate analyses could allow for the strategic selection of climate mitigation pathways in Canada that also optimally target reductions in population health impacts due to air pollution, given the health and economic burden these represent to society. It would also allow for identification of potential unintended adverse health consequences.

The modelling of air quality in future scenarios under the effects of climate change represents a challenge, due to inherent uncertainties with respect to global atmospheric dynamics and chemistry, downscaling from global to regional scales, chemical–climate interactions, emissions projections, and meteorological conditions (Tagaris et al., 2009; Fang et al., 2013; Silva et al., 2016; Zhang et al., 2017). Most studies relevant to Canada have focused only on ozone, while fewer analyses have captured PM_2.5 impacts. Modelled population health impacts are likely underestimated, as not all health outcomes that have been associated with air pollution exposure (e.g., neurological outcomes) can currently be quantified. Variability in the CRFs and monetization of health outcomes used in different studies may influence results (West et al., 2013; Thompson et al., 2014; Silva et al., 2017). Furthermore, the studies undertaken to date have largely used different approaches, assumptions, and modelling methodologies to investigate different questions, limiting the ability to synthesize the information.

On average, 7000 wildfires burn about 2.5 million hectares — about half the size of Nova Scotia — every year in Canada. The area burned by wildfire in Canada has doubled since the early 1970s, and this has been attributed to human-caused climate change (Gillett et al., 2004). For example, human influences on the climate were found to be an important contributor to the severity of the 2017 wildfire season in Canada (Kirchmeier-Young et al., 2019). The increase in area burned results in higher air pollution emissions from wildfires, as well as greater associated health risks to humans (Reisen et al., 2015; Matz et al., 2020). The four factors that influence wildfire emissions include area burned, fuel consumed, combustion completeness (efficiency), and emission factor, which is the amount of pollutant released, measured in grams per kilogram.

The Canadian climate is warming, and this is having profound and immediate impacts on fire activity in Canada. The reasons for increasing wildfire activity due to rising temperatures are threefold. First, warmer temperatures extend the fire season, and a longer fire season has already been observed in parts of Canada. For example, interior British Columbia, Alberta, and Northern Ontario have longer fire seasons today than in 1959 to 2000 (Albert-Green et al., 2013; Hanes et al., 2019). Second, warmer conditions result in increased lightning, and, all things being equal, more lightning will result in more fire (Romps et al., 2014). Third, warmer temperatures lead to drier fuels, unless there is a significant increase in precipitation (Flannigan et al., 2016). Almost all future climate change scenarios for Canada do not have sufficient increases in precipitation to compensate for the drying effect from warmer temperatures. Drier fuels make it is easier for fire to start, spread, and burn more intensely, which, in turn, makes the fires more difficult to control or to extinguish and results in greater air pollution emissions.

Wildfire activity varies temporally and spatially. For the period 1959 to 2015, the number of large fires and area burned by wildfire increased in Western Canada (Hanes et al., 2019). This trend is expected to continue, with the largest increase in wildfires and smoke projected for Western Canada through to 2050. Increasing fire activity is anticipated across all of Canada in the last half of this century (Flannigan et al., 2005; Flannigan et al., 2009).

Scientists are investigating whether recent increases in wildfires in Canada are a direct result of climate change. The observed increases in large wildfires are consistent with what is anticipated with climate change (Flannigan et al., 2009; Hanes et al., 2019). Recent research suggests that extreme fire risk in Western Canada during the last decade increased by 1.5 to 6 times due to human-caused climate change (Kirchmeier-Young et al., 2017). Tan et al. (2019) suggested that extreme spring fire weather in Western Canada in 2016 was very likely an outcome of human-caused climate warming, which increased the occurrence of a weather pattern associated with warmer and drier conditions. Kirchmeier-Young et al. (2019) suggested that human-caused climate change increases the area burned by a factor of seven to 11 times during extreme fire seasons, such as the 2017 fire season in British Columbia.

Amiro et al. (2009) suggested a doubling of wildfire emissions in Canada by the end of this century using the Canadian Global Circulation Model. The increases were largely due to increases in area burned rather than increases in the fuel burned per unit of area (known as depth of burn). Recent research, using three global circulation models (HadGEM2, CanESM2, and CSIRO-MK3.6.0) and three RCP scenarios (2.6, 4.5, and 8.5), however, suggested that the proportion of days in the fire seasons with the potential for significant fuel consumption (depth of burn) by wildfire will increase across Canada’s forests, more than doubling for British Columbia and the rest of the boreal forest by 2100 (Wotton et al., 2017). The doubling of fuel consumption due to depth of burn only may occur as early as the 2030s in British Columbia. Wotton et al. (2017) suggested that the proportion of days with high-intensity wildfires that are difficult to impossible to extinguish will increase by two to three times for British Columbia and the boreal forest by 2100.

Smoke from wildfires can travel long distances and significantly affect communities that can be 1000 km or more from the fire. As a result, large population centres are not immune to the adverse health impacts from wildfire smoke, although they may be in an area with low wildfire risk (Matz et al., 2020). In addition, Indigenous communities, which may be in close proximity to forests, may be more affected by wildfires and the need for and effects of evacuation (see Chapter 2: Climate Change and Indigenous Peoples’ Health in Canada).

Given projected increases in wildfire activity, the potential for interactions between society and fire in Canada (such as more evacuations of communities) will increase. Overall, Canadians need to be prepared for a future with more wildfires and increased levels of wildfire air pollution due to the changing climate.

Wildfire emissions contain many different air pollutants, including PM, CO, NO_X, methane, polycyclic aromatic hydrocarbons, and VOCs, and contribute to the formation of ozone and secondary PM (Naeher et al., 2007). The health effects of some of these air pollutants are discussed in section 5.3 Health Effects of Outdoor Air Pollution. For each wildfire, the exact composition of the smoke is highly variable and determined by many factors, including type of vegetation burning (e.g., wet or green vegetation versus dead or dry vegetation), type of combustion (e.g., flaming versus smoldering conditions), and weather conditions (Adetona et al., 2016; Black et al., 2017).

The health effects of wildfire smoke are an active area of research and have been well reviewed, as they are an important global issue (Benmarhnia et al., 2013; Youssouf et al., 2014; Liu et al., 2015; Adetona et al., 2016; Reid et al., 2016; Black et al., 2017; Cascio, 2018). These reviews considered studies conducted around the world, including in North America, South America, Europe, Australia, and Asia. The methods used to assess exposure to wildfire smoke varied across the individual studies and included use of land-based PM monitors, satellite imagery, air quality modelling, comparison of fire versus non-fire periods, and self-reports. Based on these reviews, epidemiological studies have identified that exposure to wildfire smoke is associated with an increase in all-cause mortality; however, more studies are required to identify which specific causes of mortality are most affected. In addition, the literature indicates a strong association between exposure to wildfire smoke and respiratory morbidity, specifically exacerbations of asthma and COPD, and increased respiratory infections. Numerous studies have reported significant increases in health care utilization, including hospital admissions, emergency room visits, physician visits, and/or medication use, for these respiratory conditions associated with an increase in wildfire smoke. For cardiovascular morbidity, including health outcomes such as myocardial infarction, stroke, heart failure, and heart rhythm disturbances, the association with wildfire smoke remains inconclusive, due to a small number of studies indicating an effect and many studies reporting null findings. Many studies reported no association with cardiovascular diseases as a group, and the results are inconsistent among the studies evaluating specific outcomes. A small number of studies have evaluated other health effects, including birth outcomes, mental health, and diabetes, although further research is required to assess the impact of wildfire smoke on these and other health effects.

Studies conducted in Canada on the health effects of wildfire smoke exposure support the conclusions of the reviews discussed above. Specifically, these studies have identified increased asthma-related physician visits (Henderson et al., 2011; McLean et al., 2015; Dodd et al., 2018a), asthma medication dispensed (Henderson et al., 2011; Elliot et al., 2013; McLean et al., 2015), and respiratory-related physician visits and hospital admissions (Henderson et al., 2011; Dodd et al., 2018a) associated with exposure to wildfire smoke. However, no associations were observed for cardiovascular-related physician visits or hospital admissions (Henderson et al., 2011; Dodd et al., 2018a).

How wildfire smoke causes health effects is not fully understood, although the evidence suggests the mechanisms may be similar to those identified for ambient PM. Studies in humans and animals exposed to wildfire or wood smoke have suggested that the health effects result from increased oxidative stress and inflammatory responses, as well as possible interaction of particulates with the autonomic nervous system and a possible reduction in immune responses (Adetona et al., 2016; Reid et al., 2016; Black et al., 2017; Cascio, 2018).

Health impact analyses have estimated the health burden of wildfires attributable to the increase in air pollutant concentrations from wildfire smoke. On a global scale for 1997 to 2006, the average annual mortality attributable to PM_2.5 from landscape fire smoke was estimated at 339,000 deaths (interquartile range 260,000 to 600,000) (Johnston et al., 2012). For this study, landscape fires included forest, grass, and peat fires, and were associated with an estimated annual wildfire-PM_2.5 exposure concentration of 0 to 45 µg/m³ and a population weighted average concentration of 2.1 µg/m³. The greatest impacts were noted for sub-Saharan Africa (157,000 premature deaths) and Southeast Asia (110,000 premature deaths). The importance of climatic variability on wildfire activities was also evident from the sensitivity analysis of strong La Niña and El Niño years. During the El Niño period, associated with dry conditions and greater fire activity, the estimated annual global mortality attributable to PM_2.5 from landscape fire smoke was estimated at 532,000, compared to an estimate of 262,000 for the La Niña period.

In a national assessment for the continental United States, wildfire episodes in 2008 to 2012 were associated with an annual population-weighted mean wildfire-PM_2.5 exposure of 0.6 to 1.1 µg/m³, depending on the wildfire activity of a given year (Fann et al., 2018). These annual wildfire-PM_2.5 exposures were associated with an estimated 1500 to 2500 premature deaths from short-term exposure; 8700 to 32,000 premature deaths from long-term exposure; 3900 to 8500 respiratory hospital admissions; and 1700 to 2800 cardiovascular hospital admissions. The economic valuation over the five-year period was 63 billion USD (in 2010 USD) for the short-term premature deaths and combined hospital admissions. The long-term premature deaths had a valuation of 450 billion USD (in 2010 USD) across the five-year period. There was considerable regional variation in wildfire smoke exposure and attributable health impacts, with greater wildfire activity occurring in the western and southeastern states.

There is only limited information quantifying the health and associated monetary impacts from wildfire-related air pollution in Canada in the published literature. A 2001 fire in Chisholm, Alberta, was used as a case study to estimate the acute health impacts associated with short-term increases in PM_2.5 (Rittmaster et al., 2006; Rittmaster et al., 2008). This seven-day fire burned approximately 116,000 ha and had significant impacts on the air quality of Edmonton (160 km south of Chisholm), Red Deer (125 km south of Edmonton), and the surrounding area. Health damage associated with PM_2.5 concentrations above the Canada Wide Standard of 30 µg/m³, which was applicable at the time, were estimated to be $2 million to $3 million (in 1996 CAD).

As discussed in the section 5.6.1 Wildfires in Canada in a Changing Climate, wildfire activity in Canada is expected to increase under a warming climate. Currently, there is too much uncertainty to predict with sufficient accuracy where in Canada wildfires will occur by mid-century and hence to estimate the population health impacts of wildfires under potential future climates. However, it is informative to assess the current air pollution health impacts from wildfires to better understand the magnitude of this population health issue. Health Canada and Environment and Climate Change Canada undertook an analysis of the Canadian air quality and human health impacts of air pollution from wildfires in recent years, the results of which are presented in this section (Matz et al., 2020).

Although intermittent in nature, wildfire smoke is recognized as a major contributor to air quality issues in North America. To provide forecast guidance for air quality alerts that could reduce air pollution exposure and protect human health during a wildfire smoke event, Environment and Climate Change Canada developed FireWork, a comprehensive operational air quality forecast system based on near-real-time biomass-burning emissions data. This system delivers operational forecasts of modelled air pollutant concentrations from biomass burning, in particular PM_2.5, on a daily basis over North America (Pavlovic et al., 2016).

A multi-year retrospective analysis of FireWork wildfire-PM_2.5 forecasts was conducted to estimate Canadian population exposure to PM_2.5 from wildfires (Munoz-Alpizar et al., 2017). Emissions from wildfires across North America were included in the modelling, which was limited to a five-month period from May to September, for calendar years 2013 to 2018. Due to substantial changes to the modelling grid used for 2016, there is a high level of uncertainty in the model output. As such, the results for that year were excluded from further analysis.

Comparisons of average monthly forecasted surface PM_2.5 concentrations due to wildfires for the 2013 to 2018 period showed large year-to-year variations in both the timing and the spatial locations of impacts (see Figure 5.2). Additionally, wildfires can sometimes affect the same location many times during a single season. The frequent presence of wildfire-PM_2.5, especially in Western North America, has implications for regional achievement of PM_2.5 air quality standards. The percent of Canadian landmass with wildfire-PM_2.5 and the percent of the Canadian population affected by wildfire-PM_2.5 above specified concentrations are provided in Figure 5.3. For 2013 to 2017, more than 60% of the landmass of Canada had average (May to September) wildfire-PM_2.5 concentrations of 0.2 µg/m³ or more, affecting more than 90% of the population and demonstrating the widespread nature of wildfire smoke. Additionally, for 2013 to 2018, approximately 25% to 40% of the land mass of Canada had average wildfire-PM_2.5 concentrations of 1 µg/m³ or more, affecting approximately 20% to 30% of the population. At the higher-threshold concentration levels, the percentages of landmass and population affected were further reduced. Since the population of Canada is not evenly distributed over the large landmass, the proximity of fire activity to population centres is a key determinant of the population affected by higher wildfire-PM_2.5 levels. For example, in 2017, the wildfire activity and smoke plume affected large population centres in British Columbia, and the proportion of the population affected was greater than the proportion of the landmass. Data for the provinces and territories with wildfire-PM_2.5 concentrations of 1 µg/m³ or more, affecting more than 5% of the landmass and more than 5% of the population, for 2013 to 2018, are presented in Figure 5.4. In multiple years between 2013 and 2018, about 60% to 100% of the landmass and over 80% of the population in the four Western provinces and the Northwest Territories experienced average (May to September) wildfire-PM_2.5 concentrations of at least 1 µg/m³.

The Canadian population health impacts attributable to wildfire PM_2.5 for 2013 to 2015 and 2017 to 2018 were estimated using Health Canada’s AQBAT 3.0. The mortality and morbidity results, including the economic valuation, are presented in Table 5.5 and Table 5.6. Nationally, 54 to 240 premature deaths due to short-term exposure and 570 to 2500 premature deaths due to long-term exposure per year were attributable to wildfire-PM_2.5, as well as many non-fatal cardiorespiratory health outcomes. The most frequent morbidities were days with acute respiratory symptoms and days with restricted activity. The substantial year-to-year variation in wildfire activity is reflected in the health impact analysis, with the greatest impacts estimated for 2017 and much smaller estimates for 2013. This variability is also driven by whether the air pollution plume from the fires disperses over highly populated areas. Over the five calendar years assessed, the economic value of the population health impacts was estimated at $410 million to $1.8 billion per year for acute health impacts and $4.3 to $19 billion per year for chronic health impacts.

A breakdown of estimated national premature deaths, by province and territory, is provided in Table 5.7. In 2013, the greatest impacts were estimated for Ontario and Quebec, reflecting the wildfire activity in Northwestern Quebec that year. For the other years (2014 to 2018), the greatest impacts were estimated in the provinces of British Columbia and Alberta, reflecting the substantial wildfire activity in Western Canada and in the United States during these years. During this period, health impacts were also noted for Saskatchewan, Manitoba, Ontario, and Quebec, indicating that the long-range transport of wildfire-PM_2.5 can affect population health at great distances from the wildfire locations.

In recent studies, projected wildfire activity under climate change scenarios has been used to estimate population exposures to wildfire smoke and health impacts in the future. Mills et al. (2018) projected wildfire smoke exposure across the continental United States under RCP4.5 and RCP8.5 for the years 2050 and 2090. In 2050, a projected 3 million more people would be exposed to wildfire smoke under RCP8.5 compared to RCP4.5 and the difference would increase to 10 million in 2090. Significant regional variation is noted across the country, with the greatest air quality impacts in the northeast and southwest. The study methods took a conservative approach and did not capture long-range transport of wildfire smoke. Ford et al. (2018) also considered the RCP4.5 and RCP8.5 scenarios to model wildfire and biomass burning, and PM_2.5 emissions for the continental United States. Annual average wildfire PM_2.5 emissions were estimated to increase due to projected increases in emissions during the peak fire season and lengthening of the fire season, for 2050 and 2100. The largest projected increases in emissions were noted for the Southeastern United States and along the Canadian border. Additionally, due to projected decreases in anthropogenic sources of PM_2.5 under RCP4.5 and RCP8.5, the relative contributions of fire-PM_2.5 were projected to increase from approximately 25% in 2000 to approximately 50% in 2050 and 2100. From a modelled baseline of 17,000 premature deaths attributable to fire-PM_2.5 in 2000, projections estimated increases in deaths to 42,000 (RCP4.5) or 32,000 (RCP8.5) by 2050, and 32,000 (RCP4.5) or 44,000 (RCP8.5) by 2100.

Liu et al. (2016) used projections of wildfire PM_2.5, based on the SRES A1B climate scenario to estimate respiratory hospital admissions for seniors across the Western United States. Short-term increases in PM_2.5 from wildfires were associated with an increase of 178 respiratory hospital admissions for people 65 years or older for 2046 to 2051 compared to 2004 to 2009. The estimates were greatest for the population centres in Southern and Central California, Western Washington, Central Colorado, and Central Utah.

For some of the air pollutants associated with wildfire smoke, such as PM, numerous epidemiological studies have identified populations that may be at increased risk (see section 5.3 Health Effects of Outdoor Air Pollution). In comparison, fewer studies have investigated possible populations or conditions that may increase the risk of adverse health effects from exposure to wildfire smoke. Limited evidence suggests that young children, seniors, people with pre-existing conditions such as asthma or COPD, and people with lower socio-economic status may be at increased risk (Liu et al., 2015; Reid et al., 2016). A recent review of North American studies reported evidence of a greater effect of wildfire smoke on women compared to men for health care utilization due to respiratory effects in healthy adults and in those with COPD (Kondo et al., 2019). The review also reported a slightly lower relative risk for health care utilization for respiratory effects in youth compared to adults, but the data were insufficient to assess any effect modification due to income, education, access to care, or other personal characteristics. Further research is needed to better identify the subpopulations at greatest risk to the health effects of wildfire smoke.

Indigenous populations may be more sensitive to health effects from wildfire air pollution (see Chapter 2: Climate Change and Indigenous People’s Health in Canada). In Canada, First Nations and Métis people have a higher burden of chronic respiratory diseases such as asthma and COPD (Gershon et al., 2014; Carrière et al., 2017), making them more susceptible to the adverse effects of air pollution overall. A Canadian study suggested that adverse respiratory outcomes, including emergency room visits and clinic visits for cough, asthma, and pneumonia, increased during the prolonged 2014 wildfire season in the Northwest Territories compared to the previous two years, although this study was not specific to Indigenous Peoples (Dodd et al., 2018a). In addition, Indigenous populations living in remote locations may be at increased risk of exposure to wildfire smoke due to proximity.

Given their occupation, wildland firefighters are exposed more frequently and to greater levels of wildfire smoke than the general public. Studies of firefighters have indicated acute health effects of wildfire smoke exposure, including reduced lung function, lung inflammation, pulmonary and systemic oxidative stress, and respiratory symptoms (Youssouf et al., 2014; Adetona et al., 2016; Black et al., 2017; Groot et al., 2019). However, the long-term effects of cumulative occupational exposure to wildfire smoke have not been identified.

Scientific studies have identified numerous adverse health effects associated with wildfire smoke, including premature mortality and respiratory health effects. In addition, new research is evaluating possible associations with cardiovascular health effects, as well as with birth outcomes, mental health outcomes, and diabetes. Furthermore, significant population health impacts attributable to wildfire PM_2.5 have been estimated for Canada, the United States, and globally. Specifically, for the Canadian population over the period from 2013 to 2018, 620 to 2700 deaths per year were attributed to PM_2.5 from wildfires, along with many non-fatal adverse health outcomes. Climate change is anticipated to increase the number and severity of wildfires in Canada and globally, due to greater fire activity and lengthening of the fire season. The increased emissions from wildfires will result in a greater public health burden from air pollution and will require expanded adaptation efforts by public health agencies and other government organizations.

Although increasing fire activity is expected in Canada over this century, it is difficult to estimate population health impacts under future climate change scenarios, as population exposure will depend on the location and size of individual fires, as well as meteorological conditions, all of which are hard to predict with the required spatial resolution. Air quality modelling includes inherent uncertainties, given the complexity of atmospheric processes. In addition, modelled population health impacts are likely underestimated, as not all health outcomes that have been associated with air pollution exposure can be quantified and included in the analysis. In addition, epidemiological studies of the health effects of wildfire air pollution specifically are still limited, and effects on health may differ from those caused by exposure to ambient air pollution. There is global recognition of the increasing importance of wildfire smoke as a source of air pollution exposure, and investigation of the health effects attributable to wildfire smoke is an active area of research. This may result in the development of source-specific CRFs for use in health impact assessments.

Canada developed the Air Quality Health Index (AQHI) to convey the health risks of air pollution to the public on a day-to-day basis and inform decisions to protect health. The AQHI forecast is designed to help Canadians know when to monitor their symptoms, limit their exposure to air pollution, and make other behavioural changes, such as adjusting their exercise activities. The index represents the combined impact of the air pollution mixture and conveys health risks in relative terms. Rather than anchoring the index to threshold values derived from air quality standards, the AQHI calculates risks from epidemiological analysis of the population health impacts associated with short-term (daily) exposure to air pollutants, with a value of 10 based upon the highest-risk day observed during the period 1998 to 2001. The formula incorporates concentrations of NO₂, ozone, and PM_2.5 according to their contribution to increasing the risk of mortality from non-accidental causes (Stieb et al., 2008). The AQHI is reported on a scale of 1 to 10+, and the higher the number, the higher the health risk (see Figure 5.5). The AQHI is therefore an initiative to reduce the risk of air pollution that promotes adaptation through behavioural change; it has garnered international attention for its efficacy and clarity (Chen et al., 2013b; Oakes et al., 2014; Du et al., 2020).

The usefulness of the AQHI as a tool for adaptation depends on the ability to anticipate air quality conditions. Environment and Climate Change Canada forecasts the AQHI across the country (Government of Canada, 2019a), accompanying weather forecasts in all regions except Quebec, which is the only province or territory that has not adopted the AQHI. As part of its Info-Smog Program, Quebec has an air quality index that calculates a subindex for each pollutant relative to a provincial air quality standard (Government of Canada, 2019b).

Health messages and health protection advice communicated through the AQHI distinguish between the general population and the populations that may experience increased risk (Table 5.8). People with heart and lung conditions are described as most affected by air pollution, while other populations at higher risk include those with diabetes, young children, seniors, and people who are active outdoors. Advice to reduce exposure is primarily directed at avoiding elevated exposure to air pollution by changing schedules and relocating activities (e.g., indoors or outdoors and away from traffic), while, at the same time, encouraging appropriate exercise activities. Individuals are encouraged to check the AQHI forecast and become familiar with how their health may be affected at different AQHI values (e.g., recognizing when they experience symptoms). Those who may be at increased risk are encouraged to monitor their symptoms and limit outdoor activities at higher AQHI values. In addition to the information accompanying the AQHI forecasts, some provinces, municipalities, and media outlets post AQHI guides and information on webpages and provide adaptation advice to the public. The Air Health Check awareness campaign, which ran from 2015 to 2019, disseminated additional information on factors that increase vulnerability, symptoms, and protective actions (Scout Environmental, 2019).

Bilateral arrangements between federal and local authorities govern local air quality alerts, also called advisories, warnings, and special air quality statements. Air quality alerts may be based upon a forecast AQHI of seven or above (high risk), or when a specific pollutant exceeds a concentration chosen by the province. The health protection messages for advisories are generally consistent with the AQHI messaging for high or very high risk. However, they may be more detailed, including advice to avoid traffic, stay inside where there is central air conditioning, have an adequate supply of medication on hand, and reduce generation of air pollutants indoors, along with messages advising personal actions to reduce pollution, such as limiting vehicle use and outdoor combustion (OMECP, 2010). The Quebec Info-Smog Program issues advisories when a pollutant reaches, or is forecast to reach, the “poor” category and makes use of similar risk reduction messages (Santé Montréal, 2017).

The AQHI does not measure the effects of odour, pollen, dust, heat, or humidity on human health. Additive health effects have been observed between heat and air pollution, leading to increases in mortality and hospital admissions for cardiovascular and respiratory diseases (European Academies’ Science Advisory Council, 2019). However, seasonal and regional variations in heat responses, as well as other factors, have thus far precluded consideration of a combined index for heat and air quality or the incorporation of a heat indicator into the AQHI. Only minimal messaging on the combined risk of air pollution and heat is included in AQHI factsheets. When air quality and heat warnings are issued simultaneously, messages acknowledge the combined risks of the two hazards; however, integrated adaptation messaging has not been established. Some local jurisdictions have been using ad hoc combinations of content from Health Canada’s work on extreme heat and air pollution (Anderson, 2016), and there is a need to develop joint messaging.

There is no direct experimental evidence to demonstrate the effectiveness of air quality forecast programs such as the AQHI in reducing population health risks. However, many studies show that health risks increase with increasing exposure to air pollution and, therefore, actions that effectively reduce exposure will reduce risk (Abelsohn & Stieb, 2011). Several studies have investigated the effectiveness of air quality advisory programs in altering behaviour (Wen et al., 2009; Spurr et al., 2014; Radisic et al., 2016). They found that individuals who were more vulnerable to air pollution (e.g., asthma sufferers) and those with more knowledge of air pollution were more likely to respond to the information provided. However, few studies have been able to address the question of whether incidence of illness decreases. A study of the impact of phone messaging of alerts to sensitive patients, with follow-up interviews on actions and symptoms, did not detect an effect on health (Mehiriz & Gosselin, 2019). Another study investigated the impact of air quality advisories in Toronto and found that the announcement of alerts was associated with a reduction of emergency department visits for asthma by 25% but could not detect an effect for any other health outcome (Chen et al., 2018).

Enhancing urban green spaces can play a role as an adaptive measure to deal with climate change, while also providing potential health and social co-benefits. There has been considerable research indicating a positive association between exposure to green space and improved health outcomes, such as hypertension and cardiovascular end points (Twohig-Bennett & Jones, 2018). More recently, researchers are attempting to elucidate how green space and air pollution, two key elements of the urban environment, interact to affect health. There is emerging evidence that increased green space may attenuate the effect of air pollution (Crouse et al., 2019). For example, urban green space can provide an environment away from higher-pollution microenvironments (such as near roadways), where people can go, including to exercise. Green infrastructure, such as vegetation barriers along busy roadways, can reduce the transfer of air pollutants to nearby environments (Baldauf, 2016). Green space can also provide a cooling effect to help reduce the urban health island effect (see Chapter 3: Natural Hazards) and function as a carbon sink.

Wildfire smoke has become a frequent summertime feature of air quality conditions over large areas in Canada, particularly in Western Canada, with resulting attention to public health impacts and adaptation measures. Models suggest that wildfires will continue to increase in both severity and frequency due to climate change (see section 5.6 Climate Change and Air Pollution from Wildfires). Adaptation measures in response to wildfire smoke are primarily related to warning individuals and providing them with the means to reduce their exposure, particularly those who may be at increased risk of related health impacts. A qualitative study of the health impacts of the 2014 wildfires in the Northwest Territories observed a decrease in mental and emotional health among the majority of interviewees and recommended comprehensive planning and education to reduce risks in Indigenous and other communities (Dodd et al., 2018b).

Recent years have seen advances in air quality modelling to incorporate wildfire smoke into air quality forecasts. Environment and Climate Change Canada developed its FireWork model as part of its operational air quality forecast system and produces twice-daily forecasts of wildfire PM_2.5 for the next 48 hours. The model output is incorporated into air quality forecasts. Maps and animations of predicted smoke paths are produced, and air quality alerts are issued when necessary (Government of Canada, 2019c). The relative cohesion of a smoke plume over large distances means that wildfire episodes present particular challenges to forecasting and to the adoption of appropriate adaptive measures.

Currently, the AQHI formula is being evaluated for effectiveness in the context of wildfires. Concerns have arisen that, during smoke events, the AQHI readings do not correspond to the sensory experiences of the public in the area. An analysis was conducted with data from British Columbia, and an AQHI+ version of the index, using a formula based on one-hour PM_2.5 only, was determined to have a better fit to asthma outcomes and respiratory physician visits, although not to mortality and cardiovascular outcomes. The new version is now used in British Columbia throughout the year, in conjunction with the AQHI (Yao et al., 2019). The Northwest Territories has developed a self-assessment guide for wildfire smoke and health, based on visibility (NT HSS, 2016a).

Extreme ambient PM_2.5 concentrations can occur, and adaptive measures that go beyond the risk reduction advice associated with the AQHI may be required. The British Columbia Centre for Disease Control (BC CDC) carried out an analysis of public health responses to wildfire events and identified 13 priorities for action. While all are relevant to adaptation to smoke events, three are of particular interest: developing guidance for clean air shelters that can be used during smoke events; engaging public health practitioners in wildfire emergency response; and raising public awareness of protective actions (Maguet, 2018). Evidence reviews of air filtration in institutions were conducted (Keefe, 2014), the use of clean air shelters was explored (Barn, 2014), and advice on outdoor activities and the use of protective masks was developed (BC CDC, 2014). A series of public information factsheets were developed in 2019 (BC CDC, 2019). The Manitoba Office of Disaster Management has developed guidelines for protecting community health and well-being from wildfires (Manitoba Health, 2019), as have the Northwest Territories (NT HSS, 2016b). The federal government, as well as several provinces and territories, have reports, web pages, and factsheets that advise the public on matters relating to health protection and risk reduction (OHLTC, n.d.; Saskatchewan Environment Public Health and Safety, n.d.; Nova Scotia Department of Health and Wellness, 2018; NT HSS, 2018; Alberta Health Services, 2019; Government of Canada, 2019d; Ministère de l’Environnement et Lutte contre les changements climatique, 2019; Yukon Health and Social Services, 2019; Health Canada, 2020).

Photo credit: Nina Dobbin

Building airtightness (also called envelope airtightness), defined as the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope, is an important consideration for indoor air quality. Nearly two-thirds of energy use in residential and commercial buildings in Canada is for heating and cooling (NRCan, 2018). Since the early 1980s, the energy efficiency of Canadian homes has dramatically improved as a result of updated building codes as well as energy efficiency certification programs, including R-2000 and Energy Star (Hamlin & Gusdorf, 1997; Parekh et al., 2007). However, an increase in airtightness often comes at the expense of natural ventilation. Furthermore, the increased use of air conditioning (NRCan, 2016), which may continue to grow due to warm temperatures associated with climate change, can result in reduced natural ventilation, which can increase the accumulation of indoor-generated air pollutants such as VOCs and PM_2.5. Increased airtightness can also trap moisture inside homes, resulting in mould and the proliferation of dust mites (Bone et al., 2010). Heat-recovery ventilators offer an energy efficient means of providing adequate ventilation in homes (CMHC, 1998; Health Canada, 2018), and their use has been associated with improved health, for example, reductions in respiratory disorder symptoms, among occupants, according to Canadian and international studies (Leech et al., 2004; Kovesi et al., 2009; Maidment et al., 2014). As efforts continue in Canada to increase residential energy efficiency in order to support GHG mitigation, it is important to ensure that homes remain sufficiently ventilated to avoid poor indoor air quality and prevent adverse health outcomes associated with inadequate ventilation (Hernberg et al., 2014; Sharpe et al., 2015). Inadequate ventilation could also have implications for the transmission of infectious respiratory diseases, such as tuberculosis, which is a particular concern for First Nations (on- and off-reserve) and Inuit communities (Beggs et al., 2003).

Changes to the outdoor environment due to climate change can alter air quality in the indoor environment. For example, higher outdoor ozone concentrations can result in higher concentrations of indoor pollutants, such as formaldehyde, acrolein, other aldehydes, acids, and ultrafine particles, as outdoor ozone has been shown to react with other compounds as it moves indoors (Nazaroff & Weschler, 2004; Weschler, 2006). Furthermore, ambient conditions, such as temperature, relative humidity, and wind speed, can also influence indoor air quality. For example, higher wintertime outdoor temperatures contribute to lower ventilation rates by reducing infiltration that results from rising warm air within a building, and strong wind gusts increase ventilation rates by creating a pressure differential between the inside and the outside of a building (Health Canada, 2018). Increased indoor temperatures resulting from higher outdoor temperatures have also been associated with higher air pollutant emissions rates from building materials and higher indoor VOC concentrations in homes (Wallace et al., 1996; Heroux et al., 2010; Xiong et al., 2013). Conversely, adaptive measures, such as increased home airtightness and increased use of air conditioning, could reduce the impact of poor ambient conditions on indoor air quality, although retaining pollutants from indoor sources may be an issue if ventilation is inadequate. Overall, these interactions are complex, variable, and influenced by building parameters and local factors.

Climate change in Canada has increased the frequency of some extreme weather events (e.g., extreme heat events, heavy precipitation events) and is expected to continue to do so in the future (Bush & Lemmen, 2019) (see Chapter 3: Natural Hazards). These events can have a variety of indoor air quality impacts that adversely affect human health. Power outages associated with extreme weather events, such as floods or severe wind or ice storms, may result in individuals using portable gas-powered generators, oil and gas space heaters, fireplaces, and/or candles indoors (Warren & Lemmen, 2014). These devices can lead to high levels of indoor air pollutants, such as CO, PM_2.5, black carbon, ultrafine particles, NO₂, and polycyclic aromatic hydrocarbons, which have been associated with a range of adverse health effects, including increased risk of death. For example, in 1998, 28 deaths reported during an ice storm that resulted in large power outages across much of Eastern Canada were largely attributed to CO poisoning (Hartling et al., 1998; Berry et al., 2008). Power outages can also lead to the failure of mechanical ventilation systems, resulting in under-ventilated homes and buildings and, consequently, the build-up of air pollutants generated indoors (IOM, 2011). As the climate continues to change, extreme weather events are expected to increase the risk of impacts to energy infrastructure across Canada (CEA, 2018), indicating a need for adaptive measures to mitigate the adverse health effects of deteriorating indoor air quality as a result of power outages.

Climate change is also projected to result in a higher frequency of heavy precipitation events, as well as increased storm surges and coastal flooding in Canada (Bush & Lemmen, 2019) (see Chapter 3: Natural Hazards and Chapter 7: Water Quality, Quantity, and Security). Such events can result in flooding and water intruding into indoor space, creating conditions favourable to the growth of bacteria and fungi such as mould (Health Canada, 2007). For example, Hurricane Katrina in 2005 resulted in extensive proliferation of mould inside many homes in the affected regions of Louisiana (Solomon et al., 2005). Exposure to mould has been associated with eye, nose, and throat irritation; coughing and phlegm build-up; wheezing and shortness of breath; as well as increased prevalence of asthma symptoms (Health Canada, 2007). Furthermore, intrusion of water indoors can result in water damage to building materials and higher air pollutant emission rates from damp building materials (Korpi et al., 1998; Wolkoff, 1998; Huang et al., 2016). In 2013, a major flood in Alberta resulted in the evacuation of 100,000 residences and caused insured property damage that exceeded $1.74 billion (Insurance Bureau of Canada, 2013). Finally, renovations or repairs that follow interior water damage can result in higher VOC exposures due to emissions from new building materials (Weschler, 2009; ECA, 2013).

Wildfire events may also increase in severity and frequency under a changing climate, which could worsen both indoor and outdoor air pollution in affected areas (see section 5.6 Climate Change and Air Pollution from Wildfires). The impact of wildfire smoke on the indoor environment depends on a variety of housing characteristics; therefore, certain residences may be more affected than others. As wildfires become more prevalent, adaptive housing measures, such as air conditioners and stand-alone air cleaners, should be employed to protect occupants against the adverse health effects of wildfire smoke. For example, the infiltration of PM has been shown to be higher in parts of Canada with moderate climates where air conditioning use is less prevalent and houses are often less airtight (Clark et al., 2010). Moreover, infiltration of PM is lower in newer homes and residences with air cleaners (Barn et al., 2008; Hystad et al., 2009; Clark et al., 2010; MacNeill et al., 2012; Kearney et al., 2014; MacNeill et al., 2014; Wheeler et al., 2014).

In an emergency situation, people may need to congregate in a cleaner-air shelter. However, such shelters can pose unique indoor air quality challenges related to elevated CO₂ levels, indoor temperatures, and relative humidity (Barn, 2014; Keefe, 2014; US EPA, 2016) due to high occupancy. Maintaining clean air in a shelter during periods of high ambient air pollution can require specific strategies such as filtration, air cleaning, and air conditioning (Health Canada, 2020). Public health risks from infectious diseases are also a concern when sheltering people during emergencies. Public health protection during the 2020 COVID-19 pandemic has required approaches to cleaner-air shelters to be adjusted, including guidance regarding screening of symptomatic individuals, implementation of physical distancing, and provision of prevention supplies (US CDC, 2020).

People with pre-existing health conditions are especially susceptible to the health impacts of poor indoor air quality (Dales et al., 2008a; To et al., 2009; Potera, 2011; Fann et al., 2016). Children have also been shown to be more susceptible to environmental pollutants (Faustman et al., 2000), and aging can lead to the deterioration of immune defences and lung function, as well as a predisposition to respiratory infections (Viegi et al., 2009).

Other factors, such as an individual’s ability to adapt to or mitigate the adverse effects of climate change on their indoor environment can also influence their vulnerability to related health impacts. People who live in multi-family dwellings and/or rent their home may not be able to control temperature or humidity levels, which can influence indoor air quality by increasing pollutant emissions from building materials or enhancing the growth of mould, respectively. Renters may not be able to make modifications to their home to protect against water intrusion and infiltration of wildfire smoke (IOM, 2011; Romero-Lankao et al., 2014). In addition, those who lack financial resources or knowledge may not be able to take the necessary protective actions when faced with changes to their indoor air quality as a result of climate change (IOM, 2011). Poorly designed and poorly maintained dwellings can cause increased exposure to chemicals, moulds, and pathogens; poorly vented combustion apparatus contributes to acute and chronic disease; and exposure to environmental tobacco smoke is a significant health risk to adults and children (Sequel et al., 2017). Geographic location can also increase an individual’s vulnerability to health impacts, as certain areas will be more prone to extreme climate-related events such as flooding or wildfires (see Chapter 3: Natural Hazards).

First Nations, Inuit, and Métis people may experience disproportionate health impacts from poor indoor air quality, given the existing unequal burden of illness in some Indigenous communities. For example, First Nations and Inuit children have been shown to have increased rates of severe lower respiratory tract infections requiring hospitalization (Kovesi, 2012; McCuskee et al., 2014) and an increased prevalence of bronchiectasis was reported in Inuit children (Das & Kovesi, 2015). Smoking is more prevalent among Indigenous populations, with 27% of off-reserve First Nations, 26% of Métis, and 49% of Inuit people aged 12 and older smoking daily, compared to 15% of non-Indigenous people (Statistics Canada, 2015). Importantly, while high-risk behaviours, such as smoking, have adverse health impacts, their prevalence is symptomatic of “deeper social and economic issues, as well as the legacy of colonialism” (ITK, 2014).

In Canada, tuberculosis rates are four times higher among Métis people, 57 times higher among First Nations people living on reserve, 24 times higher among First Nations people living off reserve, and 284 times higher among Inuit compared to Canadian-born non-Indigenous people (PHAC, 2018; Vachon et al., 2018). In addition, heart disease has been found to be 1.5 times higher among First Nations adults living on reserve compared with the general Canadian population (Indigenous Services Canada, 2018). These existing health inequities can compound the health risks related to climate change impacts on indoor air quality.

First Nations, Inuit, and Métis people commonly experience higher rates of poverty, overcrowding in homes, and poor housing quality (Adelson, 2005; NCCAH, 2017; Statistics Canada, 2017), which can increase the risk of health impacts from poor indoor air quality. For example, 27% of First Nations people with registered or “Treaty Indian” status and 26% of Inuit lived in a dwelling in need of major repairs in 2016 (Statistics Canada, 2017), and more than half of adults in First Nations communities reported the presence of mould or mildew in their homes (Health Canada, 2014). Similarly, the 2011 census indicated that, among Inuit, one-third of all dwellings were in need of major repairs in comparison to 14% of Métis homes and 7% for the overall Canadian population (NCCAH, 2017). Climate change impacts on indoor air quality are expected to exacerbate health risks related to poor quality and overcrowded housing.

Adaptation strategies to address indoor air quality in Canada require a multi-faceted risk management approach that incorporates pollution source control, ventilation, and filtration (Poulin et al., 2016). Increasing the airtightness of building envelopes can contribute to a build-up of indoor air pollutants in the absence of adequate ventilation. This can be addressed through the installation and proper maintenance of mechanical ventilation systems (IOM, 2011; Poulin et al., 2016; Health Canada, 2018) as well as by reducing or eliminating indoor sources of air pollutants, for example, through the use of low-VOC products (Poulin et al., 2016). In addition, the influence of deteriorating ambient conditions can be reduced by ensuring a tightly sealed building envelope, using air conditioning and stand-alone air cleaning devices, as well as by installing high-efficiency filters on furnaces. Many of these measures may be inaccessible to some individuals and subpopulations across the country, including Indigenous communities, due to social inequities that result in inadequate housing, low socio-economic status, and insufficient resources to implement protective measures. Government actions to improve ambient air quality will also help to improve air quality in the indoor environment (Poulin et al., 2016).

Flood-prevention measures can also mitigate or reduce home damage from flood and other water infiltration events (Warren & Lemmen, 2014). Furthermore, installation of CO alarms in every residence can help prevent deaths from CO poisoning during power outages. Finally, targeted actions such as increased poison control and other medical services, as well as support for building and infrastructure improvements, provided to rural, geographically affected, or low-income communities can help protect those who may be at increased risk.

There are several key uncertainties regarding the magnitude of climate change impacts on indoor air quality in Canada. For example, climate change may affect patterns of activity, resulting in people spending more time indoors in some conditions and more time outdoors in others. The Canadian housing stock may change as environmental sustainability and climate resilience are increasingly considered. This might include more construction that meets standards such as net zero (buildings that generate as much on-site renewable energy as they consume [Singh et al., 2019]) or passive homes (ultralow-energy building design [Wright & Klingenberg, 2015]), or result in the development of novel building materials engineered to withstand water damage or increase fire resistance. It also remains unclear whether the new adaptive strategies available to homeowners will be adopted and implemented, what forces will drive them (e.g., costs, insurance requirements, etc.), and whether they will be sufficient to mitigate the indoor air quality issues associated with climate change.

The levels of aeroallergens, including tree pollen, grass pollen, ragweed pollen, and fungal spores are increasing in specific regions of the world and in Canada, and some of this increase has been linked to climate change (Ariano et al., 2010; Sierra-Heredia et al., 2018; Ziska et al., 2019). The timing and seasonal length of aeroallergens, as well as the production and allergenic content of pollen grains, will continue to be affected by climate change (see Figure 5.6) (Ariano et al., 2010; Ziska & Beggs, 2012; Bonofiglio et al., 2013).

Due to climate change and related increases in CO₂ emissions, temperatures will increase and, as a result, aeroallergen seasons will start earlier and end later (Traidl-Hoffmann et al., 2003; D’Amato et al., 2014; Rice et al., 2014; D’Amato et al., 2016). In North America, the ragweed pollen season was found to have increased by 27 days between 1995 and 2009 due to warming temperatures (Takaro et al., 2013). In addition, increases in the atmospheric concentrations of CO₂ can affect the reproductive processes of plants, which will increase the production of pollen (Taylor et al., 2007; Shea et al., 2008; Ariano et al., 2010; Ariano et al., 2015; Bjerg et al., 2016). Moreover, there is evidence that higher growing temperatures and increased CO₂ emissions can increase the allergenicity (ability to induce an allergic response) of pollen, and climate change will enhance this effect (Beggs, 2004; Stach et al., 2007). Climate change will also affect regional changes in meteorological variables (e.g., humidity, precipitation, and temperature) linked to pollen dispersal and deposition (D’Amato et al., 2015). These regional climate changes will also affect plant distribution, as species that could not survive in previously hostile environments can potentially thrive because of changes in temperature and precipitation (Stach et al., 2007). Changes in the dispersal patterns of aeroallergens, longer pollen seasons, greater production of pollen grains, and increased allergenicity will lead to changes in human exposure and possibly in sensitization of individuals to allergens (Breton et al., 2006; Reid & Gamble, 2009; Bonofiglio et al., 2013). The magnitude of the impact on aeroallergens and related health effects depends, in part, on the effectiveness of adaptation actions. Adaptation strategies can help to reduce exposure to aeroallergens; for instance, research conducted in the province of Quebec showed that reducing pollen-producing plants in an area could reduce exposure to aeroallergens (Demers & Gosselin, 2019).

Approximately 20% to 25% of the Canadian population is affected by allergic rhinitis, which is most commonly due to pollen allergy (Vaitla & Drewe, 2011; Keith et al., 2012). Asthma affects between 12% to 25% of Canadian children and is estimated to affect about 3 million Canadians overall (Asher et al., 2006; Gershon et al., 2010; Ismaila et al., 2013; Sierra-Heredia et al., 2018). About two-thirds of asthmatic individuals are allergic to aeroallergens, which act as triggers for asthma exacerbations (Lafeuille et al., 2013). A number of studies in Canada have evaluated the health effects of day-to-day fluctuations in aeroallergens. Ambient aeroallergens, including tree pollen, grass pollen, ragweed pollen, and fungal spores, have been associated with increased risk of asthma-related and allergic rhinitis-related emergency department visits and hospitalizations in cities across Canada (Dales et al., 2000; Cakmak et al., 2002; Dales et al., 2004; Dales et al., 2008b; Heguy et al., 2008), as well as an increased risk of myocardial infarction among the elderly (Weichenthal et al., 2016), and earlier delivery among term pregnancies (Lavigne et al., 2017). In addition, high aeroallergen counts during the gestational period have been associated with increased risk of atopic disease in the child later in life (Lowe et al., 2012). As climate change continues to intensify, it is anticipated that allergy sufferers will experience increased exposure to aeroallergens in Canada. The incidence and prevalence of respiratory allergies and asthma are therefore projected to increase, which will be associated with an increase in health care expenses to treat these conditions (Sierra-Heredia et al., 2018).

For individuals suffering from seasonal aeroallergens, aeroallergen alert systems that provide knowledge of current or forecast pollen levels may aid in efforts to choose the right medications to treat symptoms (Lougheed et al., 2010; D’Amato et al., 2015). Health care providers can discuss optimal therapies for allergic rhinitis with their patients. For example, there is evidence that efficacies of prescribed and over-the-counter medications for allergic rhinitis are enhanced if they are taken consistently or before symptom onset (Kim et al., 2008; Keith et al., 2012). In addition, reminders and warnings that the aeroallergen season is approaching may allow Canadians to ensure that they have visited a health care provider, refilled prescriptions, and begun taking preventive medications according to their management plan (Johnston et al., 2018).

An aeroallergen alert system is also a beneficial communication strategy to advise people at risk to control their exposure when aeroallergen levels are high (Sierra-Heredia et al., 2018). In Canada, daily aeroallergen forecasts are provided by Aerobiology Research Laboratories (ARL) through the Weather Network (Aerobiology Research Laboratories, 2019a; The Weather Network, 2019). A free application provided by ARL can also be downloaded on a smart phone in order to obtain forecasts of pollen and fungal spores (Aerobiology Research Laboratories, 2019b). Other potential risk-mitigation strategies at a population level include the greening of cities with trees and species that minimize allergenicity for people exposed (Fuertes et al., 2016; Carinanos et al., 2017; Fong et al., 2018). Strategies in the province of Quebec have shown that engaging various partners at the municipal level in synchronizing their ragweed-control actions can reduce exposure to ragweed pollen (Demers & Gosselin, 2019). This includes mowing regularly, applying low-impact herbicides, and planting a competitive plant cover to prevent the spread of ragweed.

Although several studies have identified health effects associated with exposure to aeroallergens, key uncertainties remain. A better understanding of the spatial coverage of aeroallergens and interactions with air pollutants and urban green spaces is needed to provide information that at-risk populations can use to modify their exposures and to shape urban greening initiatives (D’Amato et al., 2015; Sierra-Heredia et al., 2018). Additional information on the health benefits and effectiveness of adaptation strategies, such as the use of alert systems and aeroallergen control actions, is also needed. Further studies examining long-term impacts of climate change on aeroallergens would help to explain regional differences and increase understanding of the key characteristics of climate change on pollen load and seasonal length (Ziska et al., 2019). Projections of the impacts of climate change on future levels of aeroallergens are also required.

Climate change and air quality are intimately linked; a warming climate can worsen air pollution (i.e., the climate penalty), and some air pollutants, including ozone and components of PM_2.5, can affect the climate and enhance warming. GHGs and air pollutants are also derived from common sources linked to fossil fuel combustion; therefore, strategies to address one may have important co-benefits of reducing emissions of the other.

Air pollution is the leading environmental cause of death in Canada, contributing to an estimated 15,300 deaths annually, along with many non-fatal outcomes (Health Canada, 2021). Recent research indicates that the adverse effects of air pollution extend beyond cardiorespiratory impacts, linking exposure to varied outcomes such as diabetes, dementia, and reproductive health. Even small increases in exposure to air pollution are associated with an increased risk of adverse health impacts. The broad range of adverse health effects caused by air pollution, along with the ubiquitous and involuntary nature of air pollution exposure, highlight the importance of air quality management as a key public health issue.

Studies that have quantified the climate penalty in terms of air pollution health impacts under various climate projections have focused primarily on ozone, with some addressing PM_2.5 as well. Analyses undertaken for this assessment estimate hundreds of excess annual deaths due to air pollution in 2050, associated with the impact of climate change on air pollution, primarily in Ontario and Quebec, with a net social value of $2.7 billion. Similarly, studies in the United States have reported hundreds to thousands of excess annual air pollution deaths later this century associated with a warming climate.

Importantly, many studies have reported the considerable potential air pollution health co-benefits of pursuing climate change mitigation strategies that target either long-lived climate pollutants (e.g., CO₂) or short-lived climate forcers (e.g., black carbon or methane). Although climate benefits of near-term GHG emission reductions may only be realized in the mid- to long-term and generally occur globally, the public health benefits of the associated reductions in air pollutant emissions would be realized immediately and locally where the emission reductions are implemented. In addition, these benefits can have a large social value, which can offset a portion of the GHG mitigation costs. Incorporating air quality health co-benefits into climate change mitigation policy provides additional justification for pursuing more stringent or accelerated reduction measures to address climate change. Joint consideration of options to reduce both GHG and air pollution emissions would contribute to the strategic development of policies that optimize both and help to avoid unintended negative consequences, such as inadvertent increases in air pollution emissions from implementation of GHG mitigation strategies.

Wildfires have become an important source of air pollution in Canada, and increasing wildfire emissions due to climate change represent one of the most significant risks to air quality. It is expected that wildfire frequency, severity, and distribution will change under a warming climate, resulting in higher emissions, and the evidence suggests that some of the current increased wildfire activity can already be attributed to climate change. Exposure to wildfire smoke affects respiratory health, while the evidence for cardiovascular outcomes remains inconclusive. A Canadian analysis estimates 620 to 2700 annual deaths attributable to wildfire emissions during the 2013 to 2018 period, with an annual net social value ranging from $4.7 billion to $21 billion, as well as many non-fatal cardiorespiratory effects. Overall, the greatest impacts were noted in British Columbia and Alberta, although during the 2013 fire season the greatest impacts were reported in Ontario and Quebec. Although the highest air pollution concentrations are expected to occur closer to the fires, smoke plumes can spread over vast areas of the country, affecting population centres far from the sources. For example, 20% to 30% of the Canadian population was exposed to average wildfire PM_2.5 concentrations of 1 μg/m³ or more during the May to September season in 2014, 2015, 2017, and 2018. The current air pollution health impacts of wildfire smoke are expected to increase under climate change, making this an important public health issue.

Canadians spend 90% of their time indoors, and climate change is expected to impact indoor air quality in a variety of ways. It is anticipated that climate change will include more frequent extreme weather events, which raise the risk of health effects from mould due to flooding, smoke from wildfires, and indoor air pollution from the inappropriate use of combustion sources, among other impacts. In addition, increased home airtightness associated with improved energy efficiency requires sufficient ventilation to ensure pollutants do not accumulate indoors. Conversely, under conditions of poor outdoor air pollution, impacts on indoor air quality can be mitigated through increased home airtightness and the use of air conditioning and filtration. Economic resources are typically needed to take protective measures, such as increasing home airtightness and ventilation. Thus, low-income households and socially disadvantaged communities may face challenges with implementing such measures.

Ambient levels of aeroallergens, including pollen and fungal spores, have been associated with increased risk of asthma-related and allergic rhinitis-related hospital visits in Canada, as well as other adverse effects. Warming temperatures, changing weather conditions, and rising atmospheric CO₂ levels result in increases in pollen counts, season length, and allergenicity, as well as changes to species distribution. Asthma affects about 3 million Canadians, including 12% to 25% of children, while approximately one-quarter of Canadians suffer from allergic rhinitis. Both of these conditions are commonly triggered by aeroallergens. The incidence and prevalence of asthma and allergic rhinitis are projected to increase with climate change.

The scientific evidence indicates that multiple subpopulations are at increased risk from the adverse health effects of air pollution and aeroallergens and would therefore likely be more affected by worsening outdoor air pollution, wildfire smoke, indoor air pollution, and airborne allergen concentrations resulting from a changing climate.

Some groups are more susceptible to air pollutants due to age (children and seniors), pre-existing disease (e.g., asthma, COPD, or heart disease) or genetic predispositions. Growing evidence of air pollution effects on reproductive outcomes, diabetes, and progression of cardiac disease, among other health effects, suggests that a large proportion of the population may be at elevated risk. Other groups may be at higher risk due to elevated exposures because of where they live or how much time they spend outdoors, including for occupational purposes. A recent study estimates that one-third of the Canadian population has a least one risk factor making them at greater risk than the general population to the adverse effects of ozone and PM_2.5, highlighting the need for risk communication and interventions to manage air quality to target both highly exposed and more susceptible populations (Stieb et al., 2019).

Key populations at higher risk from the increasing pollen exposure and allergenicity expected under climate change include those with asthma and allergic rhinitis, and recent evidence suggests that other populations, including those with heart disease, may also be affected.

Importantly, in Canada, Indigenous populations bear a disproportionately higher burden of respiratory diseases, including asthma and COPD, than the general population, increasing their susceptibility to outdoor air pollution, wildfire smoke, indoor air pollution, and aeroallergens. Multiple, compounding health inequities (see Chapter 9: Climate Change and Health Equity) may further contribute to higher vulnerability. Indigenous Peoples living in remote communities may also be more likely to experience high levels of wildfire smoke.

A key goal of air quality management in Canada is continuous improvement, through reductions in air pollutant emissions across multiple sectors (CCME, 2019). However, additional programs have been developed to address the health risks of air pollution by reducing exposure, and these programs will become increasingly important to counterbalance the expected negative impacts of climate change.

The AQHI serves as a health-protection tool designed to help Canadians make decisions to protect their health by limiting short-term exposure to outdoor air pollution. The AQHI reports current and forecast conditions daily for communities across Canada and provides specific advice related to air quality levels associated with low, moderate, high, and very high health risks, including for susceptible individuals. In addition, local authorities use air quality alerts or advisories under high-risk conditions. The AQHI does not measure effects from heat, and there is currently only limited integration of heat and air quality messages for the public in Canada. The Government of Canada also provides forecasts of wildfire PM_2.5 levels for the next 48 hours, which include mapping of predicted smoke paths, and fire smoke air quality alerts are issued when necessary. The British Columbia Centre for Disease Control has implemented an alternative version of the AQHI during the wildfire season to better reflect risks during wildfire smoke events, and multiple provinces and territories have information for protecting community health from wildfire smoke.

Enhancing urban green space can have multiple co-benefits for health, including direct positive health impacts as well as a role in attenuating exposure to air pollution and contributing to the reduction of the urban heat island effect.

Adaptation strategies to address indoor air quality in Canada under a changing climate will require a multi-faceted risk management approach that incorporates control of indoor sources of air pollutants, adequate ventilation under increasing home airtightness, and air filtration. Flood-prevention strategies would help to mitigate mould caused by water damage, and increased use of CO detection would help to prevent deaths from CO poisoning during power outages. In addition, targeting actions to rural, geographically affected, or low-income communities can help to protect populations at higher risk.

Daily aeroallergen forecasts, which can aid in reducing exposure to aeroallergens and optimization of pharmacotherapy for allergy sufferers, are currently provided in Canada by ARL through the Weather Network. Community-based adaptation strategies to reduce pollen levels could include urban greening with low-allergenicity species. A project in Quebec has shown that municipal-level partnerships may be effective in coordinating multiple control strategies to address ragweed, a common allergen species.

Several important knowledge gaps remain in understanding of how the health of Canadians will be affected by air quality under a changing climate. Integrated modelling of climate change and air quality, including the effects of climate parameters on PM_2.5 levels, is needed to improve understanding of both the population health impacts associated with the climate penalty and the potentially large air quality health co-benefits of GHG mitigation measures. The impact of changing climatic conditions on biogenic emissions needs to be better understood and incorporated into air quality models. In addition, an overall synthesis and comparison of the potential air quality co-benefits of multiple IPCC climate mitigation pathways remain difficult because studies to date have used different approaches, assumptions, and modelling methodologies. Aligning future study methodologies would provide improved information for pursuing climate change mitigation policies.

Improved capacity to model wildfire smoke exposure and to understand interactions between climate and wildfire risk is required to inform projections of wildfire smoke health impacts under climate change. Improved understanding of the range of adverse health effects associated with air pollution exposure, including whether the health effects of wildfire smoke are different from those of ambient air pollution, would provide a more comprehensive assessment of the population health impacts and identify populations at higher risk.

Although it is well known that Canadians spend 90% of their time indoors, characterizing pollutant exposures indoors and the associated health risks is challenging. Changing climate conditions, as well as mitigation and adaptation measures, increase the complexity of this problem. There is a need for research that supports the assessment of the health implications of changing environmental conditions and which can guide healthy building design, energy saving, ventilation, and material selection.

Recent research suggests that the spectrum of health impacts associated with airborne allergen exposure may go beyond respiratory outcomes, which may help to identify new susceptible groups. A better understanding of aeroallergen distribution and interactions with air pollutants and green space would help to inform urban greening initiatives and adaptation measures to protect populations at higher risk.

Finally, further research is required to inform the development of effective adaptation and risk mitigation strategies for wildfire smoke, indoor air quality, and aeroallergens under a changing climate to better protect the health of Canadians.