AQI Basics

What is the U.S. Air Quality Index (AQI)?

The U.S. AQI is EPA’s index for reporting air quality.

How does the AQI work?

Think of the AQI as a yardstick that runs from 0 to 500. The higher the AQI value, the greater the level of air pollution and the greater the health concern. For example, an AQI value of 50 or below represents good air quality, while an AQI value over 300 represents hazardous air quality.

For each pollutant an AQI value of 100 generally corresponds to an ambient air concentration that equals the level of the short-term national ambient air quality standard for protection of public health. AQI values at or below 100 are generally thought of as satisfactory. When AQI values are above 100, air quality is unhealthy: at first for certain sensitive groups of people, then for everyone as AQI values get higher.

The AQI is divided into six categories. Each category corresponds to a different level of health concern. Each category also has a specific color. The color makes it easy for people to quickly determine whether air quality is reaching unhealthy levels in their communities.

AQI Basics for Ozone and Particle Pollution

Daily AQI Color 

Values of Index

Description of Air Quality

Good

Green
0 to 50 Air quality is satisfactory, and air pollution poses little or no risk.

Moderate
 

Yellow

51 to 100

Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.

Unhealthy for Sensitive Groups

Orange

101 to 150

Members of sensitive groups may experience health effects. The general public is less likely to be affected.

Unhealthy

Red

151 to 200

Some members of the general public may experience health effects; members of sensitive groups may experience more serious health effects.

 Very Unhealthy

Purple

201 to 300

Health alert: The risk of health effects is increased for everyone.

Hazardous
 

Maroon

301 and higher

Health warning of emergency conditions: everyone is more likely to be affected.

Note: Values above 500 are considered beyond the AQI. Follow recommendations for the "Hazardous category." Additional information on reducing exposure to extremely high levels of particle pollution is available here.


Five major pollutants

EPA establishes an AQI for five major air pollutants regulated by the Clean Air Act. Each of these pollutants has a national air quality standard set by EPA to protect public health:

  • ground-level ozone
  • particle pollution (also known as particulate matter, including PM2.5 and PM10)
  • carbon monoxide
  • sulfur dioxide
  • nitrogen dioxide

Using the Air Quality Index
Technical Assistance Document for the Reporting of Daily Air Quality – the Air Quality Index (AQI) (PDF, 23 pp., 7 M, about PDF)