SmogRank / World / North America / United States / California

Most polluted cities in California, United States

123 measured cities, last 30 days; 0 above the WHO 24-hour guideline. Worst: Hanford at 13.7 µg/m³, #390 in the world.

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.

← Hanford 0% 0.75%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.

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

Estimated: cities without a monitor

California cities we can't measure, with model estimates for the last 7 days. Listed by population, not ranked: estimates aren't precise enough to rank. Model estimates from the Copernicus Atmosphere Monitoring Service (CAMS), adjusted to measured cities nearby where there are any. Adjusted, a single day lands within ×1.5 of what a monitor would read about two times in three; unadjusted, it can be off by half or double. Treat them as a guide, not a measurement.

City	7-day estimate	Band
Santa Ana	≈ 9 µg/m³	Good
Irvine	≈ 9 µg/m³	Good
San Bernardino	≈ 7 µg/m³	Good
Oxnard	≈ 6 µg/m³	Good
Moreno Valley	≈ 7 µg/m³	Good
Huntington Beach	≈ 9 µg/m³	Good
Glendale	≈ 13 µg/m³	Moderate
Garden Grove	≈ 8 µg/m³	Good
Rancho Cucamonga	≈ 10 µg/m³	Moderate
Corona	≈ 10 µg/m³	Moderate
Palmdale	≈ 6 µg/m³	Good
Hayward	≈ 10 µg/m³	Moderate
Pomona	≈ 9 µg/m³	Good
Escondido	≈ 10 µg/m³	Moderate
Valencia	≈ 6 µg/m³	Good
Pasadena	≈ 11 µg/m³	Moderate
Orange	≈ 9 µg/m³	Good
Fullerton	≈ 11 µg/m³	Moderate
East Los Angeles	≈ 11 µg/m³	Moderate
Santa Clara	≈ 10 µg/m³	Moderate
El Monte	≈ 11 µg/m³	Moderate
Carlsbad	≈ 10 µg/m³	Moderate
Downey	≈ 11 µg/m³	Moderate
Costa Mesa	≈ 9 µg/m³	Good
Fairfield	≈ 5 µg/m³	Good
Antioch	≈ 10 µg/m³	Moderate
Murrieta	≈ 9 µg/m³	Good
West Covina	≈ 11 µg/m³	Moderate
Norwalk	≈ 11 µg/m³	Moderate
Daly City	≈ 7 µg/m³	Good
Santa Maria	≈ 8 µg/m³	Good
Rialto	≈ 7 µg/m³	Good
Vista	≈ 10 µg/m³	Moderate
Ventura	≈ 5 µg/m³	Good
South Gate	≈ 13 µg/m³	Moderate
Hesperia	≈ 4 µg/m³	Good
San Marcos	≈ 10 µg/m³	Moderate
Westminster	≈ 8 µg/m³	Good
Santa Barbara	≈ 5 µg/m³	Good
Menifee	≈ 9 µg/m³	Good

Bad-air episodes in California

• North-western United States and Canada, 4 to 22 August 202619 days151 cities204 µg/m³peak, Wenatchee
• North-western United States and Canada, July and August 202616 July to 4 August 2026, 20 days118 cities168 µg/m³peak, Prineville
• California, United States, May 202617 to 18 May 2026, 2 days11 cities40 µg/m³peak, Torrance
• California, United States, 30 January to 6 February 20268 days21 cities44 µg/m³peak, Bakersfield
• California, United States, 26 to 27 January 20262 days9 cities44 µg/m³peak, Bakersfield
• California, United States, 21 to 22 January 20262 days18 cities54 µg/m³peak, Riverside

Times when many cities around here had PM2.5 at twice their usual or more on the same days.

Use this board

SmogRank, most polluted cities in California, United States, PM2.5, last 30 days to 6 Oct 2026, 22:00 UTC, smogrank.com/us/region/california?w=30d.

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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.

Updated 2026-10-06 22:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index.