What Does the Air Quality Index Mean?

The EPA says the Air Quality Index, or AQI, tells the public how clean or polluted the air is and how to avoid related health effects, using a scale from 0 to 500 where a higher value means more pollution and greater health concern. It sets an AQI for five major pollutants regulated by the Clean Air Act, namely ozone, particle pollution, carbon monoxide, nitrogen dioxide and sulfur dioxide.

The dividing line is 100. The EPA says an AQI value of 100 generally corresponds to the level of the short-term national air quality standard, values at or below 100 are generally thought of as satisfactory, and values above 100 mean air quality is unhealthy, at first for certain sensitive groups and then for everyone as values rise.

The six categories are 0 to 50 Good (green), 51 to 100 Moderate (yellow), 101 to 150 Unhealthy for Sensitive Groups (orange), 151 to 200 Unhealthy (red), 201 to 300 Very Unhealthy (purple), and 301 and above Hazardous (maroon). The EPA says Good poses little or no risk, Moderate may be a concern for a small number of unusually sensitive people, and in the Unhealthy for Sensitive Groups range people in sensitive groups may feel effects when active outdoors. In the Unhealthy range everyone active outdoors may experience effects, Very Unhealthy means widespread effects are expected, and Hazardous triggers health warnings of emergency conditions, with the entire population more likely to be affected by serious health effects.