SmogRank / World / North America / Canada / Ontario

Most polluted cities in Ontario, Canada

29 measured cities, last 30 days; 0 above the WHO 24-hour guideline. Worst: Cornwall at 5.4 µg/m³, #2,057 in the world.

Ranked by unhealthy days: the share of each city's measured days with a daily mean over 35.4 µg/m³ (US AQI 101+), Unhealthy for Sensitive Groups or worse on the US EPA's scale. Ties go to the higher average.

Quebec: 17 measured citiesNunavutManitobaOntario: 29 measured cities#29 Sault Ste. Marie: 3.1 µg/m³#28 Parry Sound: 3.5 µg/m³#27 Peterborough: 3.6 µg/m³#26 Alliston: 3.8 µg/m³#25 North Bay: 3.9 µg/m³#24 Kingston: 4.1 µg/m³#20 Milton: 4.2 µg/m³#21 Mississauga: 4.2 µg/m³#22 Ottawa: 4.2 µg/m³#23 Sarnia: 4.2 µg/m³#19 Belleville: 4.3 µg/m³#17 Chatham: 4.4 µg/m³#18 Greater Sudbury: 4.4 µg/m³#15 Brampton: 4.5 µg/m³#16 Waterdown: 4.5 µg/m³#12 Guelph: 4.6 µg/m³#13 Oakville: 4.6 µg/m³#14 St. Catharines: 4.6 µg/m³#10 Kitchener: 4.8 µg/m³#11 St. Thomas: 4.8 µg/m³#7 Hamilton: 5.0 µg/m³#8 London: 5.0 µg/m³#9 Windsor: 5.0 µg/m³#2 Barrie: 5.1 µg/m³#3 Brantford: 5.1 µg/m³#4 Nepean: 5.1 µg/m³#5 Newmarket: 5.1 µg/m³#6 Toronto: 5.1 µg/m³#1 Cornwall: 5.4 µg/m³
Each dot is a measured city, coloured by its PM2.5; tap a dot to open the city.
← Cornwall 0% 0.75%0
Every measured city in Ontario, Canada, one column each, ranked by unhealthy days: height is the share of its measured days that were unhealthy, colour its average. Point at it to read a city; tap to open it.
  1. 1Cornwall 0%
  2. 2Barrie 0%
  3. 2Brantford 0%
  4. 2Nepeanlow-cost sensors 0%
  5. 2Newmarket 0%
  6. 2Toronto 0%
  7. 7Hamilton 0%
  8. 7London 0%
  9. 7Windsor 0%
  10. 10Kitchener 0%
  11. 10St. Thomas 0%
  12. 12Guelph 0%
  13. 12Oakville 0%
  14. 12St. Catharines 0%
  15. 15Brampton 0%
  16. 15Waterdown 0%
  17. 17Chatham 0%
  18. 17Greater Sudbury 0%
  19. 19Belleville 0%
  20. 20Milton 0%
  21. 20Mississauga 0%
  22. 20Ottawa 0%
  23. 20Sarnia 0%
  24. 24Kingston 0%
  25. 25North Bay 0%
  26. 26Alliston 0%
  27. 27Peterborough 0%
  28. 28Parry Sound 0%
  29. 29Sault Ste. Marie 0%

Low-cost sensors: consumer units, counted only in cities with no government-grade monitor; they read close to reference monitors on the whole, but 15 to 25% high in humid air. How we use them.

Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales

Estimated: cities without a monitor

Ontario cities we can't measure, with model estimates for the last 7 days. Listed by population, not ranked: estimates aren't precise enough to rank. Model estimates from the Copernicus Atmosphere Monitoring Service (CAMS), adjusted to measured cities nearby where there are any. Adjusted, a single day lands within ×1.5 of what a monitor would read about two times in three; unadjusted, it can be off by half or double. Treat them as a guide, not a measurement.

City7-day estimateBand
Etobicoke≈ 5 µg/m³Good
Markham≈ 5 µg/m³Good
Vaughan≈ 5 µg/m³Good
Richmond Hill≈ 5 µg/m³Good
Burlington≈ 5 µg/m³Good
Oshawa≈ 5 µg/m³Good
Gloucester≈ 4 µg/m³Good
Whitby≈ 5 µg/m³Good
Cambridge≈ 5 µg/m³Good
Orléans≈ 4 µg/m³Good
Ottawa South≈ 4 µg/m³Good
Ajax≈ 5 µg/m³Good
Thornhill≈ 5 µg/m³Good
Thunder Bay≈ 4 µg/m³Good
Waterloo≈ 5 µg/m³Good
Niagara Falls≈ 6 µg/m³Good
Pickering≈ 5 µg/m³Good
Kanata≈ 4 µg/m³Good
Willowdale≈ 6 µg/m³Good
Caledon≈ 4 µg/m³Good
Stoney Creek≈ 5 µg/m³Good
Halton Hills≈ 5 µg/m³Good
Norfolk County≈ 5 µg/m³Good
Aurora≈ 5 µg/m³Good
Welland≈ 5 µg/m³Good
Whitchurch-Stouffville≈ 5 µg/m³Good
Georgetown≈ 5 µg/m³Good
Quinte West≈ 5 µg/m³Good
Innisfil≈ 4 µg/m³Good
Timmins≈ 4 µg/m³Good
Stittsville≈ 4 µg/m³Good
Ancaster≈ 5 µg/m³Good
Woodstock≈ 5 µg/m³Good
Bowmanville≈ 5 µg/m³Good
Stouffville≈ 5 µg/m³Good
Leamington≈ 4 µg/m³Good
Brant≈ 5 µg/m³Good
LaSalle≈ 8 µg/m³Good
Stratford≈ 5 µg/m³Good
Orillia≈ 3 µg/m³Good

Bad-air episodes in Ontario

Times when many cities around here had PM2.5 at twice their usual or more on the same days.

Use this board

SmogRank, most polluted cities in Ontario, Canada, PM2.5, last 30 days to 6 Oct 2026, 22:00 UTC, smogrank.com/ca/region/ontario?w=30d.

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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.

Updated 2026-10-06 22:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index.