Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
Detailed Explanation
The Air Quality Index is the primary tool the EPA uses to communicate air pollution levels to the public. The scale runs from 0 to 500, divided into six color-coded categories: Good (0-50, green), Moderate (51-100, yellow), Unhealthy for Sensitive Groups (101-150, orange), Unhealthy (151-200, red), Very Unhealthy (201-300, purple), and Hazardous (301-500, maroon). Each category corresponds to a different level of health concern. The AQI is calculated separately for five major pollutants — ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide — and the highest individual pollutant AQI becomes the overall AQI for that day. A city with an AQI consistently below 50 has excellent air quality, while readings above 100 indicate conditions that may affect sensitive populations. AirHistory tracks annual median AQI for over 1,000 US cities using 10 years of EPA monitoring data, revealing long-term trends that daily readings cannot show.
Related Terms
AQI Breakpoints
The specific pollutant concentration values that correspond to each AQI level, used to convert raw measurements to the 0-500 scale.
AQI Categories
The six color-coded health concern levels — Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous — used by the EPA to describe air quality.
Good Air Quality
Air quality conditions with an AQI of 0-50, posing little or no health risk for any population group.
Criteria Pollutants
The six common air pollutants regulated by the EPA under the Clean Air Act: ozone, PM2.5, PM10, CO, NO2, and SO2.
Frequently Asked Questions
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
The Air Quality Index is the primary tool the EPA uses to communicate air pollution levels to the public. The scale runs from 0 to 500, divided into six color-coded categories: Good (0-50, green), Moderate (51-100, yellow), Unhealthy for Sensitive Groups (101-150, orange), Unhealthy (151-200, red), Very Unhealthy (201-300, purple), and Hazardous (301-500, maroon). Each category corresponds to a different level of health concern.