Data Source

All air quality data on PlainAir comes from the U.S. Environmental Protection Agency (EPA) Air Quality System (AQS), which collects ambient air quality measurements from thousands of monitoring stations operated by state, local, and tribal agencies across the United States.

Source URL: aqs.epa.gov/aqsweb/airdata/download_files.html

Data Vintage & Coverage

PlainAir currently uses EPA AQS annual summary data covering 2020 through 2025, representing six years of air quality monitoring. The data includes AQI summaries for nearly 1,000 monitored U.S. counties and 500+ metropolitan statistical areas (CBSAs). State-level summaries are computed by aggregating county-level data. Not all counties in the United States have EPA air quality monitors, approximately two-thirds of counties lack AQS monitoring stations, particularly in rural and less populated areas. Our coverage reflects only counties where the EPA has active monitoring infrastructure.

How We Process the Data

1. Download annual AQI summary files from EPA AQS for each year from 2020–2025
2. Parse county-level AQI readings including good, moderate, unhealthy, and very unhealthy day counts
3. Extract primary pollutant breakdowns (ozone, PM2.5, PM10, NO2, CO) for each county-year
4. Aggregate county data to metro (CBSA) level by averaging monitored counties within each metro
5. Aggregate county data to state level in the same way
6. Compute 5-year trends and rankings (based on median AQI)
7. Load into our search-optimized database

Rankings Methodology

Rankings are based on median AQI, a metric that reflects the typical day rather than only the worst episodes. A lower median AQI indicates better typical air quality. Counties and metros with fewer than 90 monitored days in a year are excluded from rankings to avoid skewing results from incomplete monitoring.

How the Source Agency Collects Data

The EPA Air Quality System (AQS) collects data from a nationwide network of ambient air monitoring stations operated by state, local, and tribal agencies. These stations use federally approved reference or equivalent monitoring methods to measure concentrations of criteria pollutants including ozone (O3), particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), and lead. Monitors record readings continuously or at regular intervals (typically hourly), and the data is transmitted electronically to the AQS database.

The EPA computes the Air Quality Index (AQI) from these pollutant concentrations using standardized breakpoint tables defined in 40 CFR Part 58. The AQI translates pollutant concentrations into a 0-500 scale where higher values indicate worse air quality. The pollutant with the highest AQI on any given day determines the overall AQI for that location.

Data Accuracy Commitment

PlainAir presents EPA data without modification or interpolation. We do not estimate air quality for unmonitored counties or fill gaps where data is missing. All aggregations from county to metro and state level use only monitored data points and are transparently computed. Our processing pipeline validates data integrity during each import, checking for consistent units and valid AQI ranges. If you believe any information is displayed incorrectly, please contact us and we will verify against the source data.

Limitations

• Not all counties have EPA monitoring stations, data reflects only monitored areas; approximately two-thirds of U.S. counties have no AQS monitor
• AQI data represents annual summaries, not real-time readings, for current air quality, visit AirNow.gov
• State and metro aggregations use only monitored counties and may not represent the entire geographic area
• EPA publishes data with a delay of several months; our most recent year is 2025
• For AQI calculation rules and breakpoint tables, see the EPA Outdoor Air Quality Data reference and the CDC Air Quality public-health guidance