The Air Quality Index (AQI) is the U.S. Environmental Protection Agency's (EPA) color-coded scale for reporting how polluted the air is. Higher numbers mean more pollution and greater health concern, and a value of 100 generally matches the level of the short-term national health standard for that pollutant.
Categories and pollutants
EPA groups AQI values into six categories: Good (0 to 50, green), Moderate (51 to 100, yellow), Unhealthy for Sensitive Groups (101 to 150, orange), Unhealthy (151 to 200, red), Very Unhealthy (201 to 300, purple) and Hazardous (301 and higher, maroon).
The scale is often described as ending at 500, but EPA's current guidance places every value above 300 in Hazardous, including values above 500.
EPA sets an AQI for five major pollutants regulated by the Clean Air Act: ozone, particle pollution, carbon monoxide, nitrogen dioxide and sulfur dioxide. The AQI reported for an area is the highest of those values, and that pollutant is named as the main one.
Metropolitan areas with more than 350,000 people are required to report the AQI daily.
Hourly readings and the NowCast
The fine particle AQI is based on a 24-hour average, which responds slowly when smoke moves in or clears out. For real-time reporting, AirNow uses EPA's NowCast, which relates the past 12 hours of particle data to the AQI.
When air quality is changing, the NowCast weights the most recent hours more heavily, approaching a 3-hour average.
The 2024 change for fine particles
In February 2024, EPA set the annual health-based standard for fine particles (PM2.5) at 9.0 micrograms per cubic meter and revised the PM2.5 AQI, effective 05/06/24. The Good range now ends at 9.0, and Hazardous now begins at 225.5 micrograms per cubic meter.
The 2024 annual standard remains under legal challenge as of October 2026, so details tied to it could change.
What it does not tell you
An AQI value summarizes monitors across a reporting area, so it can miss a smoky valley or a sharp local change. Use the category guidance as a starting point, not a measurement of the air at your exact location.