Air quality highlights

  • Overall, air quality in Ontario has improved over time as both ambient concentrations of common air pollutants and emissions to air have decreased. Over the last 10 years, nitrogen dioxide concentrations have decreased by 25%, fine particulate matter concentrations by 17%, maximum ground-level ozone concentrations by 13%, and sulphur dioxide concentrations by 50% on average across the province (see table below).
  • In 2020, Ontario reported air quality in the low risk category 96.0% of the time, based on the Air Quality Health Index (AQHI).
  • Ozone and fine particulate matter, the main components of smog, remain as pollutants of concern, however the maximum measured concentrations of ozone continue to decrease indicating reduced emissions in Ontario and the United States.
  • In 2020, there were exceedances of the provincial Ambient Air Quality Criteria and/or Canadian Ambient Air Quality Standard for ground-level ozone, fine particulate matter and sulphur dioxide in some communities.
  • Air quality in Ontario can vary from year to year due to a variety of factors including pollutant emissions, weather, natural events such as forest fires, and the long-range transport of air pollutants from the United States and elsewhere. Long term trends are thus a better reflection of any improvements or a deterioration in air quality over time versus year over year changes.
Trends in common air pollutants, 2011-2020
Nitrogen dioxide/ Nitrogen oxides (NO2/NOx)-25%-36%
Fine particulate matter (PM2.5)-17%-18%
Ground-level ozone (O3)-13%N/A
Sulphur dioxide (SO2)-50%-57%

Note: Trends for pollutant concentrations are based on annual means except for ground-level ozone based on 1-hour maximums.