Counties in California by median Air Quality Index (US EPA)
San Bernardino County had the highest median Air Quality Index (US EPA) of 53 counties in 2018, at 89. Mono County had the lowest, at 27. The median was 50. Source: the U.S. Environmental Protection Agency.
Map of counties, 2018
- Under 42
- 42 to 45
- 45 to 50
- 50 to 57
- 57 to 61
- 61 and over
- No data
All counties, 2018
Full ranking
Other periods
About this ranking
Median of the daily Air Quality Index (AQI) values of a US county over the days of the year with an AQI, from the EPA AirData annual summaries.
More on how it is measured
- The daily county AQI is the highest AQI among the monitors and pollutants reported that day (ozone, PM2.5, PM10, carbon monoxide and nitrogen dioxide).
- Lower is cleaner: 0 to 50 is good, 51 to 100 moderate and above 100 unhealthy for sensitive groups.
United States counties with air quality monitors only, about 1,000 of some 3,100 in recent years; a county without monitors has no value. The number of days with an AQI differs between counties because some monitors run only in the ozone season.
Every value in the ranking comes from the same publisher and the same period, so the counties are compared on one definition. 5 counties with data in other periods have no value for 2018 and are left out rather than filled from another source. Equal values share a rank.
How to cite
U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.Citation styles
APA 7
U.S. Environmental Protection Agency. (2026). EPA AirData annual AQI by county: median daily AQI (Counties in California by median Air Quality Index (US EPA), 2018) [Data set]. Retrieved October 12, 2026, from https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
MLA 9
EPA AirData annual AQI by county: median daily AQI, Counties in California by median Air Quality Index (US EPA), 2018. U.S. Environmental Protection Agency, 13 July 2026, https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Invokely, https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018. Accessed 12 Oct. 2026.
Chicago (author-date)
U.S. Environmental Protection Agency. 2026. EPA AirData annual AQI by county: median daily AQI. Counties in California by median Air Quality Index (US EPA), 2018. Dataset. Accessed October 12, 2026. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Also distributed by Invokely, https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018.
Harvard
U.S. Environmental Protection Agency (2026) EPA AirData annual AQI by county: median daily AQI, Counties in California by median Air Quality Index (US EPA), 2018 [Dataset]. Available at: https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual, via Invokely: https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018 (Accessed: 12 October 2026).
BibTeX
@misc{environmental_counties_in_california_by_2018,
author = {{U.S. Environmental Protection Agency}},
title = {{EPA AirData annual AQI by county: median daily AQI: Counties in California by median Air Quality Index (US EPA), 2018}},
year = {2026},
howpublished = {Data set},
url = {https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual},
urldate = {2026-10-12},
note = {Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018. Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download\_files.html\#Annual. Retrieved 2026-10-12.},
}
RIS
TY - DATA
AU - U.S. Environmental Protection Agency
TI - EPA AirData annual AQI by county: median daily AQI: Counties in California by median Air Quality Index (US EPA), 2018
PY - 2026
DA - 2026/07/13
PB - U.S. Environmental Protection Agency
DB - Invokely
UR - https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual
L2 - https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018
Y2 - 2026/10/12
N1 - Accessed via Invokely, https://invokely.ai/rankings/median-air-quality-index/california-counties?period=2018.
N1 - Publisher attribution: U.S. Environmental Protection Agency, Air Quality System, AirData. EPA AirData annual AQI by county: median daily AQI. https://aqs.epa.gov/aqsweb/airdata/download_files.html#Annual. Retrieved 2026-10-12.
ER -
Use this data in your AI agent
Invokely serves this figure to AI assistants through MCP and to code through a REST API, with the source and citation attached. Ask your assistant:
Rank Counties in California by median Air Quality Index (US EPA) using official data, and cite the source.
rank_places {"indicator":"aqi_median","level":"province","within":"California","limit":10}
GET https://invokely.ai/api/v1/rank?indicator=aqi_median&level=province&within=California&limit=10Questions
Which county has the highest median Air Quality Index (US EPA)?
San Bernardino County, with 89 in 2018, according to the U.S. Environmental Protection Agency.
Which county has the lowest median Air Quality Index (US EPA)?
Mono County, with 27 in 2018.
Where does this ranking come from?
Every value comes from one source and one period, so the places are comparable: U.S. Environmental Protection Agency, EPA AirData annual AQI by county: median daily AQI. Retrieved 2026-10-12. 5 counties have no value for 2018 and are not ranked.