SmogRank / World / Canada / Ontario / March 2025

Most polluted cities in Ontario, Canada, March 2025

28 cities in Ontario, Canada ranked by their average PM2.5 over the 31 days. Worst: Nepean at 8.0 µg/m³, #1,898 of 2,809 cities in the world. Cleanest: Sault Ste. Marie at 4.1. 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 0% 0.75%0

Every city in Ontario, Canada ranked for March 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.

1Nepeanlow-cost sensors
0%
2Windsor
0%
3Toronto
0%
4Ottawa
0%
5Hamilton
0%
6Brantford
0%
7London
0%
7Milton
0%
9Kitchener
0%
10Barrie
0%
10St. Thomas
0%
12Brampton
0%
13Newmarket
0%
13Sarnia
0%
15Oakville
0%
16St. Catharines
0%
16Waterdown
0%
18Chatham
0%
18Guelph
0%
20Peterborough
0%
21Mississauga
0%
22Alliston
0%
23Belleville
0%
23Kingston
0%
25Greater Sudbury
0%
25North Bay
0%
27Parry Sound
0%
27Sault 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

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

Use this board

SmogRank, most polluted cities in Ontario, Canada, PM2.5, March 2025, smogrank.com/ca/region/ontario/history/2025-03 (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.