Understanding the Air Quality Index

The Air Quality Index (AQI) is the U.S. EPA's scale for reporting daily air quality. It runs from 0 upward: the higher the number, the more pollution and the greater the health concern.

The six categories

Every AQI value belongs to one of six official categories, each with its own color. This site uses EPA's names and colors exactly and always writes the category name or value beside the color.

U.S. AQI categories, colors and ranges
Category	AQI range	Color	What it means
Good	0–50	Green	Air quality is satisfactory, and air pollution poses little or no risk.
Moderate	51–100	Yellow	Air quality is acceptable. There may be a risk for some people, particularly those who are unusually sensitive to air pollution.
Unhealthy for Sensitive Groups	101–150	Orange	Members of sensitive groups may experience health effects. The general public is less likely to be affected.
Unhealthy	151–200	Red	Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.
Very Unhealthy	201–300	Purple	Health alert: the risk of health effects is increased for everyone.
Hazardous	301 and higher	Maroon	Health warning of emergency conditions: everyone is more likely to be affected.
No data	None	Grey, hatched	No reading was reported. This is unknown air quality and can hide any category.
Out of date	None	Grey, cross-hatched	The latest reading is too old to show as current.

Source: EPA Technical Assistance Document for the Reporting of Daily Air Quality (May 2024) and AirNow AQI basics.

Since 2024, the Hazardous category has no upper limit: values above 500 occur during severe smoke and are shown as published.

The pollutants

The AQI is reported separately for each pollutant, and the day's AQI is the highest of them; that pollutant is called the main pollutant.

Fine particles (PM2.5): tiny particles from smoke, vehicles, industry and chemical reactions in the air. Wildfire smoke is mostly PM2.5. EPA: particulate matter basics
Ground-level ozone: forms when sunlight acts on emissions, so it is usually highest on hot, sunny afternoons. EPA: ground-level ozone basics
Coarse particles (PM10): larger particles such as dust.
Nitrogen dioxide (NO2), Carbon monoxide (CO) and Sulfur dioxide (SO2): gases from burning fuel, measured at fewer sites. EPA: nitrogen dioxide basics

NowCast: the hourly AQI

The official AQI for particles and ozone is based on 24-hour and 8-hour averages, which would react slowly to a sudden change such as arriving smoke. AirNow therefore reports current conditions with the NowCast, which weights the most recent hours more heavily when air quality is changing quickly. Current readings on this site are NowCast values as AirNow publishes them. AirNow: how the NowCast works

Four kinds of information

Measured: A reading from a monitor, or a reporting area's current AQI built from its monitors. Preliminary until the agency completes its quality checks.
Interpolated: AirNow's estimate for places between monitors, drawn as shaded areas on the map. It shows the general pattern and can differ from conditions at a specific place.
Forecast: A state or local agency's prediction of the AQI category for a reporting area and day, with the date it was issued.
Official alert: A National Weather Service message such as an Air Quality Alert or Dense Smoke Advisory, shown with its issuing office and expiry time.

This site keeps the four apart and never combines them into a single score.

Sensitive groups and health guidance

Who counts as sensitive depends on the pollutant: for particles, people with heart or lung disease, older adults, children and teenagers, and others named in EPA guidance; for ozone, people with lung disease such as asthma, children, older adults and people who are active outdoors. EPA's guides describe what each group can do at each AQI level.

This site does not give medical advice. For questions about your own health, talk to a health professional.