Tested Air shows how often the air is unhealthy in each US county, what usually causes it and which months it happens. It's published by Rudderberry, LLC, a small Florida company that also publishes Tested Water, a county-by-county guide to private well water.
We aren't a government agency, a doctor or an air purifier company. Every number on the site comes from public data the EPA and the CDC have already published.
How we count
A bad-air day is a day when a county's Air Quality Index (AQI) goes over 100. At that level the EPA says children, older adults, people who are active outdoors and people with asthma, heart disease or lung disease should cut back on long or hard outdoor activity. Days over 150 are unhealthy for everyone.
County pages use the EPA's AirData summaries. Each day, the EPA takes the highest AQI among a county's monitors and pollutants, and records which pollutant set it. We average the days over 100 from 2021 to 2025, and count a year only if the county's monitors reported on at least 200 days. A county gets numbers when it has at least 3 such years, which leaves 923 counties. We publish a page for 689 of them: every county whose monitors cover both ozone and fine particles (571), counties that measure only one of them when 100,000 or more people live there (the page says which one is missing), and the 3 biggest monitored counties in every state. The cleanest and worst lists rank only counties that measure both, since a county that measures one can't record the other's bad days.
The map and tool pages use the CDC's Environmental Public Health Tracking Network, which estimates daily fine particles (PM2.5) and ozone for every county in the lower 48 from monitors, satellites and weather models, 2001 to 2022. We count a day as bad when the worst spot in a county goes over 35.4 µg/m³ of fine particles or 70 ppb of ozone, the points where the AQI passes 100. Using the worst spot matches how the EPA counts a county by its worst monitor.
The main cause on a county page comes from the pollutant that set each bad day since 2016. If ozone set 60% or more, we call the county an ozone county. If fine particles set 60% or more, we split those by season: mostly November to February means winter smoke and soot trapped by cold air; mostly June to October means wildfire smoke season.
Where the two sources disagree
For most counties, monitor counts and CDC estimates line up within a few days. In a few, the monitors record far more. Maricopa County, Arizona, is the biggest gap: monitors averaged about 100 bad-air days a year from 2021 to 2025, mostly ozone, while the CDC estimate for 2018 to 2022 is about 38. Monitors are direct measurements, so county pages lead with them and say so when the gap is large.
What the numbers can't tell you
- Your street. A county is large. Air beside a highway, a port or a valley floor can be worse than the county number, and a hilltop can be better.
- Today. We describe the pattern over years. For today's reading and tomorrow's forecast, use AirNow.
- Days a monitor missed. Some monitors run only in ozone season. Each county page says how many days its monitors covered.
- Anything after the data ends. EPA monitor data here runs through 2025. The CDC estimates end in 2022, so they miss the 2023 Canadian wildfire smoke.
More about us
- Editorial standards: how we choose sources, check claims and fix mistakes
- How we make money
- Sources: every data file and its limits
- Download the data
Corrections and questions: hello@testedair.com.