Sources

Every data file behind Tested Air, who publishes it, the years it covers, and what it can and can't tell you about the air where you live.

EPA AirData: county AQI summaries

The EPA publishes a yearly file for every county with an air monitor: the number of days in each AQI category, the median and highest AQI, and how many days each pollutant set the AQI. We use the files for 2000 to 2024. Source: EPA pre-generated data files.

  • Covers: about 1,300 counties that have had a regulatory monitor since 2000, run by states, tribes and the EPA.
  • Limits: no data where there's no monitor. Some monitors run only part of the year. A county's daily value is its worst monitor, so one monitor near a highway or a fire can set it.

EPA AirData: daily county AQI

One row per county per day with the AQI and the pollutant that set it. We use 2016 to 2025 to find which months bring bad air and what causes it. Same source as above.

  • Limits: county averages cover 2021 to 2025, the 5 latest years of complete EPA files. We move the window forward when the EPA finishes a new year.

CDC Environmental Public Health Tracking Network: modeled PM2.5 and ozone

The CDC and the EPA estimate daily fine particles and ozone for every county in the lower 48, from 2001 to 2022, by combining monitor readings with satellite data and air quality models. We count days over 35.4 µg/m³ of fine particles and over 70 ppb of 8-hour ozone at the worst spot in each county. Sources: Daily County-Level PM2.5 Concentrations 2001-2022 and Daily County-Level Ozone Concentrations 2001-2022, built from the EPA's Downscaler model.

  • Covers: every county in the lower 48.
  • Limits: these are estimates. They can miss short, local peaks that a monitor catches. They end in 2022 and don't include Alaska or Hawaii.

Air purifier ratings

Health and safety guidance

Guides cite the specific pages they use from the EPA, AirNow, the CDC, the California Air Resources Board and peer-reviewed studies. Each guide lists its sources at the bottom.

County boundaries and population

The main cities on each county page are the Census Bureau's principal cities of metro and micro areas (July 2023), placed in a county by the Census city population estimates for 2024, plus other towns of 50,000 or more. ZIP code lookups use the Census Bureau's 2020 ZIP code area to county file, with each ZIP placed in the county that holds most of its land.

County shapes come from the US Census Bureau's cartographic boundary files, through the us-atlas project. Population counts are from the 2020 Census, summed from census blocks in the EPA's 2020 water source estimates.