SmogRank / World / Canada / Ontario / 2025
Most polluted cities in Ontario, Canada, 2025
29 cities in Ontario, Canada ranked by their average PM2.5 over the 365 days; 29 above the WHO annual guideline (5.0 µg/m³). Worst: Windsor at 10.5 µg/m³, #813 of 2,624 cities in the world. Cleanest: North Bay at 5.9. A city needs measurements on 70% of the days to be ranked.
About 85.7K people in these 4 cities breathed unhealthy air on an average day of 2025.
Ranked by people: each city's population times its share of unhealthy days, those with a daily mean over 35.4 µg/m³ (US AQI 101+). That is how many people living there breathed unhealthy air on an average day. Only cities measured at 2 or more places at least 3 km apart are ranked: one monitor shouldn't speak for a whole city. Population is the city itself, not its wider metro area.
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.
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, 2025 · All cities in Canada, 2025 · Ontario now
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SmogRank, most polluted cities in Ontario, Canada, PM2.5, 2025, smogrank.com/ca/region/ontario/history/2025 (accessed 6 Oct 2026).
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.