Protect Yourself from the Health Effects of Air Pollution Exposure: The Air Quality Index

This page last reviewed September 30, 2016

Air Quality Index

The air quality index (AQI) is based on the National Ambient Air Quality Standards (NAAQS), and was developed as a means of communicating to the public the state of air quality each day. The NAAQS define clean air, and the AQI tells you how clean or polluted the air is relative to the NAAQS, as well as what associated health effects might be a concern for you. The AQI is calculated for each of five major air pollutants regulated by the Clean Air Act: ground-level ozone, particle pollution (also known as particulate matter), carbon monoxide, sulfur dioxide, and nitrogen dioxide, and whichever one has the highest AQI for the day is used to represent daily air quality. In California, the AQI is usually based on ozone during warm months and on particulate matter less than 2.5 microns in diameter (PM2.5) during cool months.

The AQI, shown in the chart below, is presented on a numerical scale. The AQI chart includes a qualitative description of predicted ranges of air quality, for example, good or unhealthy for sensitive groups. When AQI values are above 100, air quality is considered to be unhealthy-at first for certain sensitive groups of people, then for everyone as AQI values get higher. The AQI chart includes descriptions of who is at risk of adverse health effects at each level of the AQI scale. The chart also includes health cautionary statements that can be used to guide decisions about the duration and intensity of outdoor activity that is healthy with different levels of air pollution.

Index Values Air Quality Description (Color) Health Cautionary Statement
0 - 50 Good
No limitations
51 - 100 Moderate
(Light Yellow)
Extremely sensitive children and adults, especially with respiratory diseases such as asthma, should consider limiting outdoor exertion.
101 - 150 Unhealthy for Sensitive Groups
Sensitive children, adults and especially those with respiratory diseases, such as asthma, should limit prolonged outdoor exertion.
151 - 200 Unhealthy
Sensitive children and adults should avoid outdoor exertion and everyone else should limit prolonged outdoor exertion during peak ozone periods.
201 - 300 Very Unhealthy
Sensitive children and adults should avoid outdoor activities and remain indoors.  Everyone else should avoid outdoor exertion.
Over 300 Hazardous
(Deep Purple)
Everyone, especially children, should avoid outdoor activities and remain indoors.

For more information on the AQI, click on the following links:

For more information contact ARB’s Public Information Office at (800) 242-4450.