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Unhealthy Air Quality Days by County (2024)
Which counties had the most days with air unhealthy to breathe in 2024, from EPA monitors.
Riverside County, California has the most, 147 unhealthy air days in 2024, followed by San Bernardino County, California (144) and Los Angeles County, California (133).
Open the live map →🌫️Counties with the most unhealthy air days in 2024
🔎About this data
Number of days in 2024 when the county's Air Quality Index was above 100 (unhealthy for sensitive groups or worse), from EPA's air monitors. The popup adds EPA's official 3-year design values for fine particles and ozone. Counties without a monitor are left grey, not counted as clean.
Source: U.S. EPA AirData, annual AQI by county 2024; EPA Air Quality Design Values 2023-2025 (June 2026)
Coverage: About 990 counties with EPA air monitors (others grey)
How it was checked: Compared with EPA's separate 2023-2025 ozone design values: counties with more unhealthy days have higher ozone values (rank correlation 0.63 over 574 counties), and 99% of the 96 counties over the ozone standard had at least one unhealthy day in 2024.
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💬Questions and answers
Which county has the most unhealthy air days in 2024?
Riverside County, California has the most, 147 unhealthy air days in 2024, followed by San Bernardino County, California (144) and Los Angeles County, California (133).
Where does this data come from?
U.S. EPA AirData, annual AQI by county 2024; EPA Air Quality Design Values 2023-2025 (June 2026). Coverage: About 990 counties with EPA air monitors (others grey).
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Numbers computed from the live map data on October 18, 2026. Free to quote with credit to “Map the Block (maptheblock.com)”; see reuse terms.