SmogRank / World / Canada / Ontario / May 2025

Most polluted cities in Ontario, Canada, May 2025

29 cities in Ontario, Canada ranked by their average PM2.5 over the 31 days. Worst: Hamilton at 6.2 µg/m³, #1,819 of 2,771 cities in the world. Cleanest: Peterborough at 3.3. 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.

← Hamilton 0% 0.75%0
Every city in Ontario, Canada ranked for May 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.

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, May 2025 · All cities in Canada, May 2025 · Ontario now

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SmogRank, most polluted cities in Ontario, Canada, PM2.5, May 2025, smogrank.com/ca/region/ontario/history/2025-05 (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.