SmogRank / World / Canada / Ontario / August 2025

Most polluted cities in Ontario, Canada, August 2025

29 cities in Ontario, Canada ranked by their average PM2.5 over the 31 days. Worst: Nepean at 15.4 µg/m³, #310 of 2,841 cities in the world. Cleanest: North Bay at 10.2. A city needs measurements on 70% of the days to be ranked.

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.

← Nepean 13% 10%0
Every city in Ontario, Canada ranked for August 2025, 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. 1Nepeanlow-cost sensors
    13%
  2. 2Newmarket
    13%
  3. 3Sarnia
    13%
  4. 4Kingston
    13%
  5. 5Barrie
    10%
  6. 5Kitchener
    10%
  7. 7Brampton
    10%
  8. 8St. Catharines
    10%
  9. 9Milton
    10%
  10. 10Brantford
    10%
  11. 11London
    10%
  12. 12Sault Ste. Marie
    10%
  13. 13Peterborough
    7%
  14. 14Hamilton
    7%
  15. 15Guelph
    7%
  16. 16Cornwall
    7%
  17. 17Greater Sudbury
    7%
  18. 18North Bay
    7%
  19. 19Toronto
    6%
  20. 20Windsor
    6%
  21. 21Oakville
    6%
  22. 22Parry Sound
    6%
  23. 23Mississauga
    6%
  24. 24Waterdown
    6%
  25. 25Alliston
    6%
  26. 26Belleville
    6%
  27. 27Ottawa
    6%
  28. 28Chatham
    3%
  29. 29St. Thomas
    3%

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

All cities in the world, August 2025 · All cities in Canada, August 2025 · Ontario now

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SmogRank, most polluted cities in Ontario, Canada, PM2.5, August 2025, smogrank.com/ca/region/ontario/history/2025-08 (accessed 6 Oct 2026).

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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.