Air Quality in California

California has 54 counties with EPA air quality monitoring. Air quality is stable across the state, with an average of 201 "Good" days per county per year. The best air quality is in Mono (median AQI 19) and the most polluted is Riverside (median AQI 87).

County	Median AQI	Max AQI	Good Days	Unhealthy Days	Trend	Top Pollutant
Alameda	49	105	194	1	→ stable	PM2.5
Alpine	29	69	19	0	↓ improving	PM2.5
Amador	40	143	281	6	→ stable	Ozone
Butte	46	213	231	2	→ stable	Ozone
Calaveras	42	129	261	8	↓ improving	Ozone
Colusa	54	90	68	0	→ stable	PM2.5
Contra Costa	48	94	193	0	→ stable	PM2.5
Del Norte	32	125	232	2	↑ worsening	PM2.5
El Dorado	44	115	261	8	→ stable	Ozone
Fresno	67	169	71	79	→ stable	Ozone
Glenn	39	80	292	0	→ stable	Ozone
Humboldt	38	86	266	0	↓ improving	PM2.5
Imperial	63	341	83	50	→ stable	Ozone
Inyo	61	1322	90	30	↑ worsening	Ozone
Kern	72	177	76	94	→ stable	Ozone
Kings	60	156	100	35	→ stable	Ozone
Lake	32	67	361	0	→ stable	Ozone
Los Angeles	80	230	38	133	→ stable	Ozone
Madera	52	140	163	25	→ stable	Ozone
Marin	28	86	308	0	↓ improving	PM2.5
Mariposa	45	133	255	6	→ stable	Ozone
Mendocino	43	83	244	0	↑ worsening	PM2.5
Merced	52	151	168	35	→ stable	Ozone
Mono	19	69	341	0	↑ worsening	PM2.5
Monterey	39	71	292	0	→ stable	Ozone
Napa	41	77	108	0	↓ improving	Ozone
Nevada	36	132	313	2	→ stable	PM2.5
Orange	54	172	119	16	→ stable	PM2.5
Placer	50	140	188	27	↑ worsening	Ozone
Plumas	48	253	188	16	↑ worsening	PM2.5
Riverside	87	213	43	147	→ stable	Ozone
Sacramento	51	140	182	15	→ stable	Ozone
San Benito	43	105	260	1	→ stable	Ozone
San Bernardino	87	593	40	144	→ stable	Ozone
San Diego	71	161	47	52	→ stable	Ozone
San Francisco	33	94	254	0	↓ improving	PM2.5
San Joaquin	51	140	178	11	→ stable	PM2.5
San Luis Obispo	49	122	193	10	→ stable	Ozone
San Mateo	27	73	281	0	↓ improving	PM2.5
Santa Barbara	49	146	193	2	→ stable	PM2.5
Santa Clara	47	111	201	1	→ stable	PM2.5
Santa Cruz	39	80	293	0	→ stable	PM2.5
Shasta	44	232	268	4	→ stable	Ozone
Siskiyou	41	94	284	0	↑ worsening	Ozone
Solano	36	80	343	0	↓ improving	Ozone
Sonoma	19	72	348	0	↓ improving	PM2.5
Stanislaus	53	150	166	38	→ stable	Ozone
Sutter	49	115	195	5	→ stable	Ozone
Tehama	43	109	252	2	→ stable	Ozone
Trinity	31	113	258	1	↓ improving	PM2.5
Tulare	71	185	76	111	→ stable	Ozone
Tuolumne	40	115	286	4	→ stable	Ozone
Ventura	50	176	191	15	→ stable	Ozone
Yolo	45	118	238	1	→ stable	Ozone