Air Quality Index Calculator
The Air Quality Index (AQI) converts pollutant concentrations into a single number that tells you how clean or polluted the air is. This calculator implements the EPA's piecewise linear interpolation formula for two of the most commonly monitored pollutants: PM2.5 (24-hour average, in micrograms per cubic metre) and ozone (8-hour maximum average, in parts per million). Breakpoints are sourced from the EPA Air Quality Index Technical Assistance Document (2024) and 40 CFR Part 58. Enter a concentration and select the pollutant to see the AQI value and health category.
EPA AQI piecewise linear formula
AQI = ((Ihi - Ilo) / (Chi - Clo)) × (C - Clo) + Ilo
Where C is the pollutant concentration, C(lo) and C(hi) are the breakpoint concentrations that bracket C, and I(lo) and I(hi) are the AQI values corresponding to those breakpoints. Source: EPA Technical Assistance Document for AQI (EPA-454/B-24-001).
PM2.5 AQI breakpoints (24-hour, ug/m3)
- 0.0 to 9.0: AQI 0 to 50 (Good)
- 9.1 to 35.4: AQI 51 to 100 (Moderate)
- 35.5 to 55.4: AQI 101 to 150 (Unhealthy for Sensitive Groups)
- 55.5 to 125.4: AQI 151 to 200 (Unhealthy)
- 125.5 to 225.4: AQI 201 to 300 (Very Unhealthy)
- 225.5 to 325.4: AQI 301 to 400 (Hazardous)
- 325.5 to 500.4: AQI 401 to 500 (Hazardous)
Air quality index: frequently asked questions
What is the Air Quality Index (AQI)?
The AQI is a standardised index developed by the US EPA to communicate daily air quality to the public. It is calculated for five major air pollutants regulated under the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide. AQI values range from 0 to 500. An AQI of 100 corresponds to the National Ambient Air Quality Standard (NAAQS) for each pollutant.
What are the AQI health categories?
EPA AQI categories: 0 to 50 is Good (satisfactory, air quality poses little or no risk); 51 to 100 is Moderate (acceptable, but some pollutants may be a moderate concern for a very small number of people); 101 to 150 is Unhealthy for Sensitive Groups (people with heart or lung disease, older adults and children should reduce prolonged outdoor exertion); 151 to 200 is Unhealthy (everyone may begin to experience health effects); 201 to 300 is Very Unhealthy (health warnings of emergency conditions); 301 to 500 is Hazardous (health alert; everyone is more likely to be affected).
What PM2.5 concentration corresponds to AQI 100?
AQI 100 for PM2.5 corresponds to a 24-hour average concentration of 35.4 micrograms per cubic metre (ug/m3), which is the EPA 24-hour NAAQS for PM2.5 (revised to 9 ug/m3 annual mean in 2024, but the 24-hour standard of 35 ug/m3 still underpins the AQI calculation per EPA AQI Technical Assistance Document, 2024).
What ozone concentration corresponds to AQI 100?
For the 8-hour ozone standard, AQI 100 corresponds to 0.070 parts per million (ppm), which is the 2015 EPA NAAQS for ground-level ozone (70 ppb). The ozone AQI calculation is based on the maximum 8-hour average for the day, using the breakpoints in EPA Technical Assistance Document for AQI (EPA-454/B-24-001).
Where can I find the official AQI breakpoints?
The official AQI breakpoints are published in the EPA Air Quality Index Technical Assistance Document (EPA-454/B-24-001, revised 2024) and are codified in 40 CFR Part 58, Appendix G. The AirNow website (airnow.gov) provides real-time AQI values for monitoring stations across the United States.
Official sources
- EPA AQI Technical Assistance Document (EPA-454/B-24-001, 2024): airnow.gov.
- EPA AirNow real-time AQI: airnow.gov.
- 40 CFR Part 58, Appendix G, Air Quality Index: ecfr.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.