Top 10 US Counties with Most Unhealthy Days in 2024
Explore the 10 counties like San Bernardino, California, with up to 43 unhealthy days, based on EPA AQS data for 984 counties, revealing key AQI trends and pollutant impacts on public health.
Research
Original data research from PlainAirData. Every page derives at least one statistic from our own database, cites primary sources inline, and documents its methodology and limits.
Research coverage
PlainAirData’s research collection connects 984 monitored counties and 498 metros in the latest 2025 release, so readers can move from national air-quality patterns to the underlying local evidence.
- 984
- monitored counties in the latest release
- 498
- metros available for comparison
- 2025
- latest county AQI data year
Each linked analysis cites EPA AQS source data and explains the period and coverage it uses.
What does PlainAirData research cover?
State Averages of Median AQI Across 2020-2024 (50 states + DC + US territories)
Discover how 50 states + DC + US territories (53 jurisdictions) like Alabama, with an average median AQI of 41.1, trended over the monitored period using EPA data, including pollutant days for ozone and PM2.5, affecting over 984 counties.
National Trends in Good Days Percentage from 2020 to 2024
Analyze nationwide good days trends across 984 counties and 498 metros, with Alabama rated Good on 69.2% of its monitored days across 2020-2024 and Alaska at 85.9%, drawn from EPA AQS data on AQI metrics.
Download the EPA AQS annual county AQI extract cited on this page: annual-aqi-by-county.csv or the JSON distribution (public domain / CC0). Method notes live on /methodology and /data.