AQI Calculator
The EPA Air Quality Index (AQI) translates raw pollutant concentration measurements into a 0-to-500 scale with color-coded health categories. This calculator implements EPA's piecewise linear interpolation formula for PM2.5 (24-hour average, NowCast) and PM10 (24-hour average) using the breakpoints from EPA's Technical Assistance Document for AQI (2024). The AQI is the highest of all individual pollutant AQIs. Select a pollutant, enter the concentration, and the calculator returns the AQI value and health category.
AQI piecewise linear formula (EPA)
AQI = ((I_high - I_low) / (C_high - C_low)) * (C - C_low) + I_low
C = measured concentration (truncated per EPA rounding rules)
I = AQI index value at breakpoint
EPA PM2.5 24-hr breakpoints (ug/m3): 0/9.0/35.4/55.4/125.4/225.4/325.4 mapping to AQI 0/50/100/150/200/300/500. EPA PM10 breakpoints: 0/54/154/254/354/424/604 mapping to same AQI levels. Values above the highest breakpoint are AQI 500+.
AQI health categories
- 0-50 (Good, Green): Air quality is satisfactory. No health concerns.
- 51-100 (Moderate, Yellow): Acceptable air quality. Sensitive individuals may have minor concerns.
- 101-150 (Unhealthy for Sensitive Groups, Orange): People with heart or lung disease, older adults, and children should reduce prolonged outdoor exertion.
- 151-200 (Unhealthy, Red): Everyone may begin to experience health effects. Sensitive groups should avoid prolonged outdoor exertion.
- 201-300 (Very Unhealthy, Purple): Health alert. Everyone should avoid prolonged outdoor exertion.
- 301-500 (Hazardous, Maroon): Health emergency. Everyone should avoid all outdoor exertion.
Frequently asked questions
What is the Air Quality Index (AQI)?
The AQI is a standardized scale from 0 to 500 developed by EPA to report daily air quality to the public. It translates pollutant concentration into a color-coded category with health guidance: 0-50 (Good), 51-100 (Moderate), 101-150 (Unhealthy for Sensitive Groups), 151-200 (Unhealthy), 201-300 (Very Unhealthy), 301-500 (Hazardous).
What pollutants does EPA calculate AQI for?
EPA calculates AQI for six criteria pollutants: PM2.5 (fine particles), PM10 (coarse particles), ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2). The reported AQI for a location is based on the highest AQI among all measured pollutants.
How is the AQI piecewise linear formula applied?
The AQI formula maps pollutant concentration to the AQI scale using breakpoints defined by EPA. The formula is: AQI = ((AQI_high - AQI_low) / (Conc_high - Conc_low)) * (Conc - Conc_low) + AQI_low. Each pollutant has its own breakpoint table.
What is PM2.5 and why is it the most important AQI pollutant?
PM2.5 refers to fine particles with aerodynamic diameter 2.5 micrometers or smaller. They penetrate deep into the lungs and enter the bloodstream, causing cardiovascular and respiratory effects. PM2.5 is the most health-relevant criteria pollutant in most US locations and drives the AQI on many days.
Where can I find real-time AQI data for my location?
EPA's AirNow website (airnow.gov) provides real-time AQI data, forecasts, and fire and smoke maps for all US locations. The AirNow app provides location-specific alerts. Purple Air provides community sensor data (not official but useful for local reference).
Official sources
- EPA: AirNow AQI Basics.
- EPA: AQI Technical Assistance Document (2024).
- EPA: Criteria Air Pollutants.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.