Air Quality Rankings for California (2026)

Cities Tracked

State Avg AQI

Improving

Worsening

How California Compares

California has 53 cities tracked by EPA air-quality monitors, with a state-wide 5-year median AQI of 49 — 8 points more polluted than the national average of AQI 41. Humboldt, California ranks #1 with the cleanest air (AQI 28, Grade A), while San Bernardino, California sits at the bottom (AQI 82, Grade F). The rankings below are computed from the EPA Air Quality System (AQS), which aggregates daily AQI readings from federally certified monitors into annual averages. Cities are sorted by 5-year median AQI (lowest = cleanest = #1). The 5-year window smooths out year-to-year volatility from weather and wildfire events.

California is on an improving trajectory: 38 of 53 monitored cities show measurably cleaner air over the past decade, against just 9 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.

The dominant pollutant across 27 of 53 California cities is Ground-Level Ozone. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California. Other California cities report Fine Particulate Matter (PM2.5) (26) as their dominant concern.

The fastest-improving city in California is Napa, California, with median AQI falling by 2.3 points per year. Steady improvement at that pace usually reflects fleet turnover (older diesels retiring), upwind power-plant retirements, or tighter regional emissions controls.

The city with the steepest decline is Mono, California, where median AQI is rising by 1.9 points per year. Rapid deterioration in a single city usually points to either wildfire-smoke exposure (in the West) or a new local emissions source — a power plant, port, or freight corridor coming online.

Full California Ranking

#	City	5yr Avg AQI	Current AQI	Worst Pollutant	Trend	Grade
1	Humboldt, California	28	32	Ozone	Improving	A
2	Del Norte, California	32	32	PM2.5	Worsening	C
3	Lake, California	32	32	Ozone	Improving	B
4	Trinity, California	32	31	PM2.5	Worsening	C
5	Mono, California	33	24	PM2.5	Worsening	D
6	Sonoma, California	36	33	Ozone	Improving	B
7	San Mateo, California	36	31	PM2.5	Improving	B
8	Marin, California	36	34	PM2.5	Improving	B
9	Amador, California	37	36	Ozone	Improving	B
10	Santa Cruz, California	37	36	PM2.5	Stable	B
11	San Francisco, California	38	32	PM2.5	Improving	B
12	Napa, California	39	41	PM2.5	Improving	B
13	Monterey, California	40	42	Ozone	Stable	C
14	San Benito, California	41	40	Ozone	Stable	C
15	Siskiyou, California	41	43	Ozone	Worsening	D
16	Tuolumne, California	41	43	Ozone	Improving	B
17	Glenn, California	43	41	Ozone	Improving	C
18	Yolo, California	43	43	Ozone	Improving	B
19	El Dorado, California	43	44	Ozone	Stable	C
20	Calaveras, California	44	40	Ozone	Improving	B
21	Shasta, California	44	45	Ozone	Stable	C
22	Solano, California	45	40	PM2.5	Improving	B
23	Tehama, California	45	44	Ozone	Stable	C
24	Mendocino, California	46	53	PM2.5	Worsening	C
25	Santa Barbara, California	46	45	PM2.5	Improving	B
26	Nevada, California	47	42	Ozone	Stable	C
27	Colusa, California	47	55	PM2.5	Stable	C
28	Contra Costa, California	48	51	PM2.5	Stable	C
29	Mariposa, California	49	47	Ozone	Improving	C
30	Santa Clara, California	49	43	PM2.5	Stable	C
31	San Joaquin, California	50	47	PM2.5	Improving	C
32	Alameda, California	50	48	PM2.5	Improving	C
33	Ventura, California	50	46	PM2.5	Improving	C
34	San Luis Obispo, California	51	47	PM2.5	Improving	C
35	Butte, California	52	50	PM2.5	Improving	C
36	Sutter, California	52	49	PM2.5	Improving	C
37	Plumas, California	52	50	PM2.5	Worsening	D
38	Sacramento, California	53	51	Ozone	Improving	C
39	Merced, California	54	51	Ozone	Improving	C
40	Orange, California	54	52	PM2.5	Improving	C
41	Placer, California	54	52	Ozone	Worsening	D
42	Madera, California	56	52	Ozone	Improving	C
43	Inyo, California	57	61	Ozone	Worsening	F
44	Stanislaus, California	57	54	PM2.5	Improving	D
45	Imperial, California	61	61	PM2.5	Improving	D
46	Kings, California	64	58	Ozone	Improving	D
47	San Diego, California	67	67	PM2.5	Stable	D
48	Fresno, California	68	64	Ozone	Improving	D
49	Tulare, California	75	69	Ozone	Improving	D
50	Los Angeles, California	75	67	PM2.5	Improving	D
51	Kern, California	77	69	Ozone	Improving	D
52	Riverside, California	82	79	Ozone	Improving	D
53	San Bernardino, California	82	71	Ozone	Improving	F

Air quality data for California is sourced from the EPA Air Quality System (AQS), which monitors outdoor air quality at thousands of stations nationwide.
, EPA Air Quality System

Frequently Asked Questions

Humboldt, California has the best air quality in California with a 5-year average AQI of 28 and a Grade A (85/100). Its dominant pollutant is Ground-Level Ozone and the long-run trend is improving.

San Bernardino, California has the worst air quality in California with a 5-year average AQI of 82 and a Grade F (33/100). Its dominant pollutant is Ground-Level Ozone.

California has 53 cities with EPA air quality monitoring data, covering 2014-2023 of daily AQI measurements aggregated into annual averages.

California's state-wide 5-year median AQI is 49, 8 points more polluted than the national average of AQI 41. California is on an improving trajectory: 38 of 53 monitored cities show measurably cleaner air over the past decade, against just 9 that are getting worse. That mirrors the broader national pattern of falling particulate and ozone pollution as cleaner vehicles, cleaner power generation, and tighter industrial standards take effect.

Ground-Level Ozone is the dominant pollutant in 27 of 53 monitored California cities. Ground-level ozone forms when sunlight reacts with vehicle and industrial emissions. It is worst on hot, sunny, stagnant summer days and is the leading air quality concern across much of the Sun Belt and California.

California cities log an average of 25 days per year at "Unhealthy for Sensitive Groups" or worse, based on EPA monitor data over the last five years. Across all 53 California cities tracked, that totals 6,581 unhealthy days over the period.

Sources: EPA Air Quality System (AQS)

Cities ranked by 5-year average AQI (lower is better). Grades factor in average AQI, trend direction, unhealthy days, and dominant pollutant.

Cleanest Cities Nationwide →All Air Quality in California →Worst Air Quality Cities →Methodology →

The this entity category groups every U.S. air quality and pollution monitoring entity sharing this attribute. The list above is the data; the paragraphs below explain what the grouping means against the broader the EPA Air Quality System (AQS) distribution and how to read the relative rankings within the category.

For readers using this category as a starting point, the per-entity detail pages linked from the table above carry the underlying the EPA Air Quality System (AQS) data in full. The category-level view is the filter; the per-entity pages are the actual answer.

Source: EPA Outdoor Air Quality Data, 2026.