EPA Air Quality System daily AQI for every US county since 2015. Per county × day: 0-500 Air Quality Index, category (Good/Moderate/Unhealthy/Hazardous), defining pollutant (PM2.5/Ozone/NO2/SO2/CO/PM10), defining monitor site, count of reporting monitors. Joins with epa_pollutant_emissions on county FIPS to cross-reference daily air quality against annual stationary-source releases. Powers /county-aqi/{fips} canonical pages and daily-history queries.
Every record in this dataset can be traced back to its primary source at https://aqs.epa.gov/aqsweb/airdata/download_files.html. Underlying content is a US federal government work (public domain under 17 USC §105); our derived data is licensed CC0 1.0.