Air Quality Guides
EPA AQS monitors report from 1,002 U.S. counties and 504 metros, these guides explain how to read that annual record.
PlainAirData's guides translate EPA AQS annual summaries into plain language: what each AQI band means, which pollutants drive unhealthy days, how wildfire smoke appears in the record, and how to compare counties and metros. Every figure comes from the same monitored-county database behind the rankings - 53 states and territories with at least one active monitor.
Data source EPA Air Quality System (AQS)
- Monitored counties
- Monitored counties: 1,002
- Metros in DB
- Metros in DB: 504
- Guides
- Guides: 7
Educational resources to help you understand air quality data, pollutant types, and regional patterns: the Air Quality Index (AQI), PM2.5 and ozone, wildfire smoke, and how to read county-level monitor readings. Data from the EPA Air Quality System (AQS) and AirNow, covering all 50 states; see our methodology.
All guides reference EPA Air Quality System (AQS) data covering 977 counties and 496 metro areas.
Which air quality guide should you read first?
Understanding the Air Quality Index (AQI)
What each AQI level means, who is most at risk, and how to use AQI data to protect your health.
The 5 Most Common Air Pollutants Explained
Ground-level ozone, PM2.5, PM10, NO2, and CO, where they come from, why they matter, and how the EPA tracks them.
Best and Worst Air Quality in the US
Which counties and metros have the cleanest and most polluted air, and what drives the differences.
Cities with the Best and Worst Air Quality
Metro area air quality rankings, what drives urban pollution, and how to check your city's air.
How Air Pollution Affects Your Health
The health effects of PM2.5 and ozone exposure, who is most at risk, and practical steps to reduce exposure.
How Wildfire Smoke Affects Air Quality
Wildfire smoke has become the leading driver of unhealthy days in many US counties. What the EPA data shows and how to prepare.
Using Air Quality Data to Choose Where to Live
Air quality varies enormously across US counties. How to evaluate a potential new home using EPA AQI data.
Methodology
Our guides are based on publicly available data from authoritative government sources. All statistics, ratings, and figures cited in these guides are drawn directly from official datasets and publications, with sources clearly referenced throughout.
We aim to present complex government data in plain language that is accessible to general audiences. When methodologies differ between data sources or change over time, we note these variations inline. Our editorial process includes regular reviews to ensure accuracy and timeliness of the information presented.