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.
Detailed Explanation
The AQI is divided into six categories, each associated with a color and a level of health concern. "Good" (0-50, green) means air quality is satisfactory with little or no risk. "Moderate" (51-100, yellow) means air quality is acceptable but some pollutants may concern unusually sensitive individuals. "Unhealthy for Sensitive Groups" (101-150, orange) means members of sensitive groups — including children, older adults, and people with heart or lung disease — may experience health effects, while the general public is less likely to be affected. "Unhealthy" (151-200, red) means everyone may begin to experience health effects, and sensitive groups may experience more serious effects. "Very Unhealthy" (201-300, purple) triggers health warnings of emergency conditions, where the entire population is more likely to be affected. "Hazardous" (301-500, maroon) represents health emergency conditions where everyone is likely to be affected. AirHistory tracks how many days per year each city spends in each category, revealing patterns that are invisible in single-day readings. For example, some cities rarely reach "Unhealthy" but spend 100+ days in "Moderate," while others have few moderate days but spike into dangerous territory during wildfire season.
Related Terms
Air Quality Index (AQI)
A standardized EPA scale from 0 to 500 that communicates daily air quality and associated health risks.
AQI Breakpoints
The specific pollutant concentration values that correspond to each AQI level, used to convert raw measurements to the 0-500 scale.
Good Air Quality
Air quality conditions with an AQI of 0-50, posing little or no health risk for any population group.
Unhealthy Air Days
Days when the AQI exceeds 100, meaning air quality may pose health risks for sensitive groups or the general population.
Frequently Asked Questions
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.
The AQI is divided into six categories, each associated with a color and a level of health concern. "Good" (0-50, green) means air quality is satisfactory with little or no risk. "Moderate" (51-100, yellow) means air quality is acceptable but some pollutants may concern unusually sensitive individuals.