Top 10 US Counties with Most Unhealthy Days in 2024
Explore the 10 counties like San Bernardino, California, with up to 42 unhealthy days, based on EPA AQS data for 977 counties, revealing key AQI trends and pollutant impacts on public health.
Research
Original data research from the PlainAirData editorial team. Every page derives at least one statistic from our own database, cites primary sources inline, and documents its methodology and limits.
What does PlainAir 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 39.5, trended over the monitored period using EPA data, including pollutant days for ozone and PM2.5, affecting over 977 counties.
National Trends in Good Days Percentage from 2020 to 2024
Analyze nationwide good days trends across 977 counties and 496 metros, with Alabama averaging 77.17% good days and Alaska at 89.09%, drawn from EPA AQS data on AQI metrics.