The Risks to Human Health of Air Toxics, PM _2.5 , and Ozone from the 2023 Canadian Wildfires

The 2023 wildfires in Canada resulted in substantial emissions to air. In this work, we used predicted concentrations of hazardous air pollutants (HAPs), fine particulate matter (PM_2.5), and ozone across the U.S. and Canada to estimate the potential impacts of 2023 Canadian wildfire smoke on human health. HAPs from Canadian wildfires were estimated to increase lifetime population-weighted cancer inhalation risk by 1-in-1 million (500 excess cancer cases) and noncancer risk by a hazard index (HI) of 0.05 across the domain. The additional risk from HAPs in smoke was predicted to exceed a cancer risk of 100-in-1 million for 1,300 people and a HI of 1.0 for 110,000 people, all in Canada. In addition, 360,000 people in Canada were predicted to experience an increase in annual-mean PM_2.5 of at least 5 <i>μ</i>g m^-3. Nearly 63 million people, 76% of which were in the U.S., were predicted to experience an increase in seasonal maximum daily 8-hour average ozone greater than 1 ppb. In Canada, ozone and PM_2.5 associated with the 2023 fires were estimated to result in over 3x more attributable deaths per year than wildfire smoke in years 2013 through 2018.