Methodology
How we turn EPA’s annual county AQI files into the pages on this site, and what those pages can and can’t tell you.
The data
We use EPA’s AirData “Annual AQI by County” files, one per year. For each county and year, EPA reports how many days had an AQI value, how many of those days fell in each AQI category, the maximum, 90th-percentile and median daily AQI, and how many days each pollutant was the main pollutant. We do not change EPA’s values. See Data sources for the exact files and dates used.
The AQI categories
| Category | AQI values |
|---|---|
| Good | 0–50 |
| Moderate | 51–100 |
| Unhealthy for Sensitive Groups | 101–150 |
| Unhealthy | 151–200 |
| Very Unhealthy | 201–300 |
| Hazardous | 301 and higher |
The AQI is EPA’s index for reporting daily air quality. On a given day the index reflects whichever measured pollutant scores highest. AirNow explains the AQI in more detail.
What we calculate
- Share of Good days = Good days ÷ days with an AQI, for the same county and year.
- State and national comparisons use the median of county median-AQI values for that year, counting only county-years with at least 120 monitored days.
- Trend statements compare the average share of Good days in a county’s first three and last three qualifying years, and only appear when it has at least six qualifying years. A change under 3 percentage points is described as roughly steady.
- Similar counties are other counties in the same state whose median AQI in the same year was closest.
Which pages are published for search
We build a page for every county in EPA’s files, but ask search engines to index it only when it has at least 3 years with 120+ monitored days and its latest such year is within 5 years of the newest data. In this build, 994 of 1,536 county pages meet that bar. The others remain available from state pages.
Limits
- Counties without regulatory monitors have no AQI record, so they don’t appear. That is a gap in monitoring, not evidence of clean air.
- A county’s record reflects where its monitors are, which may not represent every neighborhood.
- Some monitors run only part of the year. Each page states how many days were monitored.
- EPA may revise past data. Our figures match the file dates listed on each page.
- AQI breakpoints have changed over time (most recently for fine particles in 2024).
- Nothing here is medical, legal or real-estate advice, and we do not rate places as safe or unsafe.