The EPA's Air Quality Index can tell you whether breathing outside air can be harmful. Here's how to read the scale to plan ...

AirNow and PurpleAir use the Air Quality Index, or A.Q.I., a measure of air quality from 0 to 500 that is composed of six categories, from “good” to “hazardous.” ...

The EPA's AQI ranges from 0 to 500 and represents the measurement of these five different major pollutants regulated by the Clean Air Act.