Which county in California has the best air quality?

Based on 2024 EPA AQS data, Sonoma County had the best air quality in California with a median AQI of 19 and 348 good-air days.

2024 monitoring year Public dataset EPA AQS

California Air Quality Rankings

Compiled by PlainEnviro Editorial, Environmental Data Editorial Team · July 13, 2026

County-level AQI rollup using EPA Air Quality System monitor records. Refreshed as new EPA AQS data lands.

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County-by-county air quality data for California based on EPA AQS annual AQI readings. Data covers 53 monitored counties for 2024.

California (2024): 53 counties monitored. State average median AQI: 48 (national average: 38.8). 58% of monitored days rated "Good." Most unhealthy air: San Bernardino County (88 days above AQI 100).

California air quality on this page is drawn from the EPA Air Quality System (AQS), which aggregates daily readings from federal, state, tribal, and local monitors into annual summaries per county. The current dataset covers 53 California counties with at least ten days of valid AQI readings in 2024, out of the state's total monitored counties.

The state-average median AQI across those counties is 48, compared with a national average of 38.8 in the same reporting period. On average, 58% of monitored days fell into the "Good" AQI bucket (0–50). The heaviest burden of unhealthy days lands on San Bernardino County (88 days above AQI 100), while Sonoma County records the cleanest air with a median AQI of 19.

AQI values translate ambient concentrations of pollutants (ozone, PM2.5, PM10, NO₂, SO₂, CO) onto a single 0–500 scale with six health-based bands. A low median AQI reflects typical daily conditions; peak values matter separately because short-term exposure drives most acute health effects. Counties without monitors are not "clean" - they are simply unmeasured by EPA AQS. For site-specific exposure guidance, check AirNow.gov alongside the county detail links below.

Counties with Data

State Avg Median AQI

National Avg Median AQI

38.8

Avg Good Days %

58%

Which California counties have the worst air quality?

Ranked by median AQI (higher = worse). Click a county for full environmental data.

#	County	Median AQI	Good Days	Good %	Unhealthy Days	Peak AQI
1	San Bernardino	87	40	11%	88	593
2	Riverside	87	43	12%	67	213
3	Los Angeles	80	38	10%	55	230
4	Kern	72	76	21%	21	177
5	San Diego	71	47	13%	4	161
6	Tulare	71	76	21%	28	185
7	Fresno	67	71	19%	14	169
8	Imperial	63	83	23%	8	341
9	Inyo	61	90	25%	13	1322
10	Kings	60	100	27%	2	156
11	Orange	54	119	33%	3	172
12	Colusa	54	68	19%	0	90
13	Stanislaus	53	166	45%	0	150
14	Madera	52	163	45%	0	140
15	Merced	52	168	46%	1	151
16	San Joaquin	51	178	49%	0	140
17	Sacramento	51	182	50%	0	140
18	Ventura	50	191	52%	1	176
19	Placer	50	188	51%	0	140
20	Santa Barbara	49	193	53%	0	146
21	San Luis Obispo	49	193	53%	0	122
22	Alameda	49	194	53%	0	105
23	Sutter	49	195	53%	0	115
24	Contra Costa	48	193	53%	0	94
25	Plumas	48	188	51%	9	253
26	Santa Clara	47	201	55%	0	111
27	Butte	46	231	63%	2	213
28	Yolo	45	238	65%	0	118
29	Mariposa	45	255	70%	0	133
30	El Dorado	44	261	71%	0	115
31	Shasta	44	268	73%	1	232
32	San Benito	43	260	71%	0	105
33	Mendocino	43	244	67%	0	83
34	Tehama	43	252	69%	0	109
35	Calaveras	42	261	73%	0	129
36	Napa	41	108	77%	0	77
37	Siskiyou	41	284	78%	0	94
38	Tuolumne	40	286	81%	0	115
39	Amador	40	281	78%	0	143
40	Monterey	39	292	80%	0	71
41	Santa Cruz	39	293	80%	0	80
42	Glenn	39	292	80%	0	80
43	Humboldt	38	266	73%	0	86
44	Solano	36	343	94%	0	80
45	Nevada	36	313	86%	0	132
46	San Francisco	33	254	76%	0	94
47	Del Norte	32	232	80%	0	125
48	Lake	32	361	99%	0	67
49	Trinity	31	258	72%	0	113
50	Marin	28	308	86%	0	86
51	San Mateo	27	281	88%	0	73
52	Mono	19	341	93%	0	69
53	Sonoma	19	348	95%	0	72

Frequently Asked Questions

What is the air quality like in California?

In 2024, California had an average median AQI of 48 across 53 monitored counties, with 58% of monitored days rated "Good" (AQI 0–50). The national average median AQI is 38.8.

Which county in California has the worst air quality?

Based on 2024 EPA AQS data, San Bernardino County had the most unhealthy air days in California - 88 days above AQI 100, with a median AQI of 87.

Explore More

National AQI Rankings → California Full Profile → California Water Quality →

About This Data

Air quality data is from the EPA Air Quality System (AQS), Annual AQI by County dataset. Counties with fewer than 10 monitored days are excluded. "Unhealthy days" counts days with AQI above 100 (Unhealthy for Sensitive Groups or worse). All data is for 2024.

Source: U.S. Environmental Protection Agency, Air Quality System (AQS) U.S. Environmental Protection Agency, Air Quality System (AQS)

Read our methodology - how this data is sourced, computed, and verified.

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Data sourced from the U.S. Environmental Protection Agency (EPA). See our methodology for details. Retrieved and formatted by PlainEnviro Editorial