SmogRank / World / United States / California / September 2023

Most polluted cities in California, United States, September 2023

105 cities in California, United States ranked by their average PM2.5 over the 30 days. Worst: Yreka at 20.7 µg/m³, #89 of 2,128 cities in the world. Cleanest: Big Bear City at 4.3. A city needs measurements on 70% of the days to be ranked.

About 33.2K people in these 13 cities breathed unhealthy air on an average day of September 2023.

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.

← San Jose 33.2K 30K0
Every city in California, United States ranked for September 2023 with monitors in two or more places, one column each, ranked by people breathing unhealthy air: height is people on an average day, colour its average. Point at it to read a city; tap to open it.
  1. 1San Jose
    33.2K
  2. 2Compton
    0
  3. 3Los Angeles
    0
  4. 4La Crescenta-Montroselow-cost sensors
    0
  5. 5Bakersfield
    0
  6. 6Fresno
    0
  7. 7Agouralow-cost sensors
    0
  8. 8Carsonlow-cost sensors
    0
  9. 8Torrancelow-cost sensors
    0
  10. 10Inglewoodlow-cost sensors
    0
  11. 10Santa Monicalow-cost sensors
    0
  12. 12Banning
    0
  13. 13Merced
    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, September 2023 · All cities in United States, September 2023 · California now

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SmogRank, most polluted cities in California, United States, PM2.5, September 2023, smogrank.com/us/region/california/history/2023-09 (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.