The AQI is a scale from 0 to 500 used by the EPA to communicate daily air quality levels. Lower is better. 0–50 is 'Good', 51–100 is 'Moderate', 101–150 is 'Unhealthy for Sensitive Groups', 151–200 is 'Unhealthy', 201–300 is 'Very Unhealthy', and 301+ is 'Hazardous'.

All data comes from the EPA's Air Quality System (AQS), which aggregates measurements from thousands of monitoring stations operated by state, local, and tribal agencies across the United States. PlainAir uses annual AQI summary files covering 2020–2025.

PlainAir includes air quality data for nearly 1,000 U.S. counties that have EPA AQS monitoring stations. Not all counties are monitored, approximately two-thirds of U.S. counties have no AQS monitor.

If your county has no EPA air quality monitoring station, it won't appear in our data. PlainAir only shows data for counties with AQS monitors. For areas without monitors, EPA's interpolated maps at AirNow.gov provide estimated readings.

No. PlainAir presents annual AQI summaries, aggregate data for full calendar years (2020–2025). For current real-time air quality, visit AirNow.gov, which shows live readings from EPA monitors.

The primary pollutant is the specific air pollutant that drove the AQI reading on a given day. Common primary pollutants are ozone (O3), fine particulate matter (PM2.5), coarse particulate matter (PM10), nitrogen dioxide (NO2), and carbon monoxide (CO). The primary pollutant breakdown shows which pollutants are most responsible for unhealthy days in your area.

No. PlainAir is an independent data portal presenting publicly available EPA data. We are not affiliated with, endorsed by, or connected to the U.S. Environmental Protection Agency.