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US Air Quality: Worst Counties Revealed
15 Jun
Summary
- Imperial and San Bernardino counties had the most unhealthy air days.
- US air quality has significantly improved since 1980.
- AQI measures pollutants like ozone and particulate matter.
In 2025, Imperial County and San Bernardino County, California, recorded the highest frequency of days with unhealthy to hazardous air quality. These counties experienced significant percentages of days where the Air Quality Index (AQI) signaled serious health risks for the general population.
Nationwide, U.S. air quality has seen substantial improvements over recent decades. Between 1980 and 2024, concentrations of pollutants such as carbon monoxide, nitrogen dioxide, sulfur dioxide, and ozone decreased by 29% to 95%. These gains are attributed to government regulations and have positively impacted public health and environmental protection.
The AQI is a composite score reflecting pollution levels, with good air quality rated 0-50. Scores from 101-150 are unhealthy for sensitive groups, while levels above 150 indicate unhealthy or very unhealthy conditions for everyone. An AQI of 301 or higher signifies hazardous air quality, posing risks of stroke and COPD even from short-term exposure.
Despite an average U.S. AQI of 33 in 2025, specific counties like those listed — including Cass County, North Dakota, and El Paso County, Texas — regularly experience poor air quality. Many of these counties also report higher incidences of stroke and COPD, as noted by the CDC.
