Toronto, where IQAir's historical data typically places air quality in the 'Good' category, briefly became the world's most polluted major city this week as wildfire smoke engulfed Canada's largest city during a heatwave.
IQAir's historical data shows Toronto typically records an Air Quality Index (AQI) of around 28 and an annual PM2.5 concentration of about 7.4 micrograms per cubic metre. Those figures generally place Toronto among cities with relatively clean air by international standards, making the dramatic deterioration during the wildfire smoke event particularly unusual for residents accustomed to good air quality.
