How BadAirDays Counts Bad Air Days, County by County
The source
EPA publishes AirData, a record of every regulatory monitor. Two files feed this site: the annual AQI summary by county (days in each AQI category, median and 90th percentile AQI, days each pollutant led) from 2000, and the daily AQI by county from 2016 to 2025, which gives the month of every bad day, the pollutant behind it, and the worst days on file.
One number per county per day
Where a county has several monitors, EPA's county file takes the highest AQI reported that day. So a county figure describes its worst-measured spot, not its average. A county with one monitor beside a highway and one in the hills reports the highway on a bad day.
Days a monitor did not run
Many monitors, especially for ozone, run only part of the year, or every third or sixth day. A county monitored 120 days a year and one monitored 365 cannot be compared raw, so bad days are scaled to a full year: bad days per monitored day, times 365. 666 counties are monitored on 95% or more of days and need almost no scaling; 330 are monitored on between half and 95% of days, among them Trinity County, California (81% of days), Sheboygan County, Wisconsin (58% of days), Racine County, Wisconsin (57% of days).
Counties monitored on fewer than half of days across 2016-2025 get no yearly figure or tier. There are 93 of them; their pages say so and show what was recorded. Counties with no monitor at all are not listed.
Recent years and the comparison
The headline figure averages 2016-2025. The "better or worse" comparison is against 2000-2009, and is only shown when both windows meet the coverage bar. 2025 may be a partial year in EPA's file when this site was last built.
Smoke or ozone
A county is called particle-driven when 60% or more of its bad days were led by PM2.5 or PM10, ozone-driven at 40% or less, mixed in between. The purifier advice uses absolute days rather than the share: 3 or more particle days a year for a bedroom HEPA purifier, 10 or more for a MERV 13 furnace filter as well, and 3 or more ozone days for the timing advice. See will an air purifier help.
Known limits
Monitors sit where agencies put them, often in cities, so a large rural county can be described by one station far from most of its homes. Smoke events that monitors miss do not count. And a history is not a forecast: for today's air, use AirNow.