Unhealthy Air Days by ZIP Code

Source period December 2025. Recomputed by ZipCheckup from the cited federal source.

In 2024, 560 of the 975 U.S. counties with air-quality monitors recorded at least one day when the air was unhealthy for sensitive groups or worse, and 208 had at least one day rated unhealthy for everyone, as of December 2025.

By state

We report two independent facts side by side and do not rank states. A larger count reflects the size of a state's inventory and its reporting activity, not a judgment about water safety.

How we compute this

We use the EPA's annual Air Quality Index summary by county for 2024. The AQI combines ground-level ozone, fine particles (PM2.5 and PM10), carbon monoxide, and nitrogen dioxide into one daily number; a value above 100 is unhealthy for sensitive groups, and above 150 is unhealthy for everyone. Of the 975 counties with a monitor, 560 had at least one day above 100 and 208 at least one day above 150, totaling 3,125 and 677 such days respectively.

Only about a thousand of the roughly 3,140 U.S. counties operate an air-quality monitor, so every figure here is a share of MONITORED counties, never of all counties. A county that is absent from the EPA file is unmonitored, not clean — we omit it rather than record it as having zero unhealthy days. A monitored county with no unhealthy days is a real measured zero and is kept.

Counts are grouped by state and recompute on every build, and each monitored county maps to the ZIP codes within it. The 560 affected counties span 51 states and jurisdictions.

Frequently asked questions

@misc{zipcheckup-findings-unhealthy-air-days-by-zip,
  author = {{ZipCheckup}},
  title  = {{Unhealthy Air Days by ZIP Code}},
  year   = {2025},
  url    = {https://zipcheckup.com/findings/unhealthy-air-days-by-zip/}
}