Attribution of fine particulate matter and ozone health impacts in Canada to domestic and US emission sources

Abstract

Exposure to ambient air pollution is associated with a wide range of adverse health effects such as respiratory symptoms, cardiovascular events, and premature mortality. Canada and the United States (US) have worked collaboratively for decades to address transboundary air pollution and its impacts across their shared border. To inform transboundary air quality considerations, we conducted modelling to attribute health impacts from ambient PM_2.5 and O₃ exposure in Canada to Canadian and US emission sources. We employed emissions, chemical transport, and health impacts modelling for 2015, 2025, and 2035 using a brute-force modelling approach whereby anthropogenic domestic and US emissions were reduced separately by 20 % or 100 %, and the resulting changes in health impacts were estimated across Canada. We find that transboundary PM_2.5 and O₃ related health impacts vary widely by region, with >80 % of impacts occurring in Central Canada, and most health impacts occurring within 200-300 km of the Canada-US border. The relative contribution of US sources to O₃ in Canada is larger than for PM_2.5, yet we find that the health impacts from transboundary PM_2.5 exceeded those from transboundary O₃. Nationally, we estimate that roughly one in five PM_2.5 deaths in Canada is attributable to US sources (2000 deaths in 2015) and more than one in two O₃ deaths are attributable to US sources (roughly 800 to 1200 deaths in 2015). We project health impacts from domestic and US sources to increase from 2025 to 2035 in Canada. Our results suggest that there are substantial benefits to be gained by domestic and international strategies to reduce PM_2.5 in the Canada-US transboundary region.

Keywords: Air pollution; Air quality agreement; Cross-border; Mortality; PM(2.5); Transboundary.