Air Quality Index (AQI) Calculator
Calculate the AQI for major air pollutants using the US EPA linear interpolation method. Enter the measured concentration for one or more pollutants to find the overall AQI and health category.
Formula
The US EPA uses linear interpolation between breakpoints to convert a pollutant concentration into a sub-index AQI value:
AQIp = [(IHi − ILo) / (CHi − CLo)] × (Cp − CLo) + ILo
- Cp – truncated pollutant concentration
- CLo, CHi – concentration breakpoints bracketing Cp
- ILo, IHi – AQI values corresponding to those breakpoints
The overall AQI is the maximum sub-index across all pollutants entered.
Concentration truncation rules (per EPA): PM2.5 → 1 decimal; PM10, SO₂, NO₂ → integer; O₃ → 3 decimals; CO → 1 decimal.
Assumptions & References
- Breakpoints follow the US EPA Technical Assistance Document for the Reporting of Daily Air Quality – AQI (May 2024).
- O₃ 8-hour breakpoints are used for AQI ≤ 300; concentrations above 0.200 ppm use the 1-hour O₃ scale (not included here).
- SO₂ breakpoints above 304 ppb use 24-hour averages in the EPA document; this calculator uses the 1-hour scale for all SO₂ inputs.
- Averaging periods: PM2.5 & PM10 = 24-hr; O₃ = 8-hr; CO = 8-hr; SO₂ = 1-hr; NO₂ = 1-hr.
- This tool is for educational purposes. Official AQI reporting requires calibrated monitoring equipment and certified data.
- Reference: EPA AQI Technical Assistance Document