The Air Quality Index, or AQI, is an indicator developed by the U.S. Environmental Protection Agency to report air quality and to help you understand what it means to your health. It is used to report the five most common air pollutants that are regulated under the Clean Air Act: ground-level ozone (O3), particulate matter (PM), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2).
The AQI uses a scale from 0 to 500. The higher the AQI value, the greater the level of pollution and the greater the health concern. An AQI value of 100 generally corresponds to the national ambient air quality standard set for each pollutant to protect public health. AQI values below 100 are generally considered to be satisfactory, and values above 100 represent unhealthy air pollution levels — at first for members of sensitive populations, then for everyone as AQI values go above 150.
To make it easier to understand, the AQI is divided into six color-coded categories. Each category corresponds to a different level of health concern. The six AQI categories and what they mean are:
| AQI Value | Air Quality | |
|---|---|---|
| Green 0 to 50 |
Good | Air quality is considered satisfactory, and air pollution poses little or no risk. |
| Yellow 51 to 100 |
Moderate | Air quality is acceptable; however, for a very small number of people, there may be a moderate health concern. |
| Orange 101 to 150 |
Unhealthy for Sensitive Groups | Members of sensitive groups may experience health effects. The general public is not likely to be affected when the AQI is in this range. |
| Red 151 to 200 |
Unhealthy | Everyone may begin to experience health effects. Members of sensitive groups may experience more serious health effects. |
| Purple 201 to 300 |
Very Unhealthy | Pollution levels trigger a health alert. Everyone may experience more serious health effects. |
| Maroon Above 300 |
Hazardous | Pollution levels trigger health warnings of emergency conditions. The entire population is more likely to be affected. |
For more information please see www.epa.gov/airnow/aqi_cl.pdf.