Air quality index
The air quality index, or AQI, translates outdoor air pollution levels into color-coded health guidance so people can adjust activity and reduce exposure.
What the AQI is
The air quality index is a public health scale for reporting outdoor air pollution. Instead of asking people to interpret pollutant concentrations directly, the AQI turns measured or forecast pollution into a number, a color, and a short health message. A higher AQI means greater pollution and greater health concern.
How the scale works
The U.S. AQI uses six color-coded categories: Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous. Values at or below 100 are generally considered satisfactory for most people. Above 100, the message changes from caution for sensitive groups to broader health concern as values rise.
Pollutants behind the number
AQI values are calculated for major pollutants regulated under the Clean Air Act, including ground-level ozone, particle pollution, carbon monoxide, nitrogen dioxide, and sulfur dioxide. In many everyday AQI forecasts, ozone and fine particle pollution are the pollutants people notice most often, especially during heat, stagnant weather, and wildfire smoke events.
Sensitive groups
Sensitive groups can include children, older adults, people with asthma or other lung disease, people with heart disease, pregnant people, outdoor workers, and anyone doing prolonged or heavy outdoor activity. These groups may need to reduce exertion or move activities indoors before the general public feels obvious symptoms.
Daily AQI and NowCast
A daily AQI summarizes air quality for a health-relevant averaging period, which varies by pollutant. The NowCast AQI is designed for current conditions, using recent monitoring data to show what air quality is like now. NowCast can respond more quickly when pollution changes rapidly, such as during wildfire smoke.
Forecasts and planning
Air quality forecasts help people choose when and where to exercise, work outside, commute, open windows, or use filtration. Forecasters use monitors, weather models, satellite imagery, emissions information, and knowledge of local pollution patterns. Like weather forecasts, AQI forecasts can change and are worth checking again during active events.
What the AQI does not show
The AQI is a useful summary, but it is not a complete picture of every air hazard. It may not capture indoor air, very local sources, odors, dust, ash, pollen, or all toxic compounds from fires and industrial events. People should combine AQI guidance with local alerts, health advice, visibility, smell, and their own symptoms.
Why it matters
The AQI matters because air pollution is often invisible until it becomes severe. A simple color and number can help families, schools, outdoor workers, event organizers, and people with health conditions make quicker decisions about exposure, masks, filtration, activity level, and timing.