Pollutant Concentration to AQI Converter
Convert pollutant concentrations to the US EPA Air Quality Index (AQI) using official breakpoint tables. Supports PM2.5, PM10, O₃, CO, SO₂, and NO₂.
Enter a concentration value and click Calculate.
Formula
The EPA AQI is calculated using piecewise linear interpolation within concentration breakpoint ranges:
AQI = ((IHi − ILo) / (CHi − CLo)) × (Cp − CLo) + ILo
- Cp – Truncated pollutant concentration
- CLo, CHi – Concentration breakpoints bracketing Cp
- ILo, IHi – AQI values corresponding to CLo and CHi
Concentrations are truncated (not rounded) before calculation: PM2.5 to 1 decimal, PM10/SO₂/NO₂ to integers, O₃ to 3 decimals, CO to 1 decimal.
Assumptions & References
- Breakpoint tables follow the US EPA Technical Assistance Document for the Reporting of Daily Air Quality – AQI (2018).
- O₃ 8-hour breakpoints are only valid up to AQI 300 (concentration ≤ 0.200 ppm); higher values require 1-hour breakpoints.
- O₃ 1-hour breakpoints are only valid for AQI ≥ 100 (concentration ≥ 0.125 ppm).
- PM2.5 and PM10 concentrations are in µg/m³; O₃ and CO in ppm; SO₂ and NO₂ in ppb.
- AQI values above 500 are reported as 500+ and are beyond the standard scale.
- This calculator computes the sub-index for a single pollutant. The overall AQI for an air quality report is the maximum sub-index across all measured pollutants.
- Reference: EPA AQI Technical Assistance Document (2018)