Skip to content
lung.com

EPA Air Quality System

What the air outside is doing

The EPA scores each day in each county on the Air Quality Index, a 0-500 scale set by whichever pollutant was worst that day. These pages report the annual summaries: how many days landed in each category, how bad the bad days got, and how that has moved over five years.

1,012 of 3,143 US counties have a monitor that reports. The rest are unmeasured rather than clean, and no page here will pretend otherwise.

1,012
Counties monitored
2,131
Counties unmonitored
444
Clean-record counties, 2025
No day above the moderate category
5
Years of history

The scale

What the AQI categories mean

CategoryIndexWhat it means
Good0-50Air quality is satisfactory and air pollution poses little or no risk.
Moderate51-100Acceptable for most, but there may be a risk for people unusually sensitive to air pollution.
Unhealthy for sensitive groups101-150People with lung disease, older adults and children are at greater risk from ozone; people with lung or heart disease, older adults and children are at greater risk from particles.
Unhealthy151-200Some members of the general public may experience health effects; sensitive groups may experience more serious effects.
Very unhealthy201-300Health alert: the risk of health effects is increased for everyone.
Hazardous301-500Health warning of emergency conditions: everyone is more likely to be affected.

2025

The 25 counties with the most days above moderate

Days when the index passed 100, meaning the air was at least unhealthy for sensitive groups. Western counties dominate this list in wildfire years, and the ranking moves a long way from one year to the next for that reason.

GoodModerateUnhealthy for sensitive groupsUnhealthyVery unhealthyHazardous
CountyAbove moderate90th %ileMax AQIDays by category
San Bernardino County, CA110156207
Riverside County, CA109150365
Los Angeles County, CA98147201
Kern County, CA65112200
El Paso County, TX631232,122
Doña Ana County, NM48115528
Imperial County, CA44108426
Tulare County, CA43105147
Maricopa County, AZ38101366
Harris County, TX38101172
Pinal County, AZ341001,215
Fresno County, CA34100164
San Diego County, CA33100161
Tarrant County, TX3097162
Clark County, NV2694138
Cook County, IL2584168
Eddy County, NM2590168
St. Louis County, MN2379216
Collin County, TX2390156
Fairfield County, CT2174174
Bernalillo County, NM2188204
Fairbanks North Star Borough, AK2085190
Denton County, TX1987179
Kenosha County, WI1977167
Luna County, NM1868321

EPA Air Quality System annual summaries by county, 2025. Only counties with a reporting monitor appear. A high rank here is a statement about one year in one county, not a durable ranking.

By state

Air quality by state