Introduction

Wildfires have become an increasingly severe environmental issue in Canada, with recent years seeing record-breaking burn areas across the country. The 2023 wildfire season alone burned over 18 million hectares, more than double any previous year on record. These fires release enormous quantities of fine particulate matter (PM2.5) into the atmosphere, which can travel vast distances carried by prevailing winds.

PM2.5 refers to particulate matter with a diameter of 2.5 micrometers or less—about 30 times smaller than a human hair. These particles are particularly dangerous because they can penetrate deep into the lungs and even enter the bloodstream, causing respiratory issues, cardiovascular problems, and other serious health effects. The World Health Organization recommends that average daily PM2.5 concentrations should not exceed 15 µg/m³.

Toronto, Canada's largest city with over 6 million residents in its metropolitan area, has experienced notable increases in "bad air" days during severe wildfire seasons. This study investigates the relationship between total wildfire burn area across Canada and the number of days Toronto experiences unhealthy PM2.5 levels, using 20 years of data from 2003 to 2023.