AQI Calculator

The EPA Air Quality Index (AQI) translates a raw pollutant measurement into a single 0-500 scale using pollutant-specific breakpoint tables from 40 CFR Part 58, Appendix G, published in machine-readable form as EPA's Air Quality System (AQS) breakpoints code table. This calculator implements six of them -- PM2.5, PM10, ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen dioxide (NO2) -- and linearly interpolates the AQI Value for whichever pollutant and concentration you enter, plus the six-tier Category it falls into (Good through Hazardous). The PM2.5 table reflects EPA's 2024 PM NAAQS reconsideration, which tightened the "Good" ceiling from 12.0 to 9.0 ug/m3 and rescaled the breakpoints above it: a reading of 10 ug/m3 is now Moderate (AQI 53) where the pre-2024 table called the same reading Good. Each pollutant carries its own unit and averaging period -- PM2.5/PM10/CO use 24- or 8-hour averages, SO2/NO2 use 1-hour averages -- and because those scales differ by orders of magnitude (a Concentration of 25 sits mid-Moderate for PM2.5 but deep in Good for NO2), switching Pollutant at a fixed Concentration can move the AQI Value at least as much as a realistic change to Concentration itself. Enter Concentration in the unit the selected Pollutant expects, not a generic number.

Before looking up the AQI, the entered Concentration is truncated to the selected pollutant's official EPA reporting precision -- PM2.5 and CO to the nearest 0.1, PM10, SO2, and NO2 to the nearest whole unit, ozone to the nearest 0.001 -- matching 40 CFR Part 58 Appendix G's own truncation convention, which is what makes the published breakpoint table contiguous with no numeric gaps between rows. Concentration Used reports the truncated value actually looked up, which may differ slightly from what you typed. What this calculator doesn't do: it does not average repeated readings, apply EPA's NowCast smoothing used for real-time public reporting, or combine pollutants into the composite daily AQI an agency publishes, which is the WORST of all measured pollutants' individual AQI values for that day, never a blend of them. Ozone is also a known simplification here -- EPA's real methodology switches from an 8-hour to a 1-hour concentration scale above AQI 300, and this calculator does not model that switch, so treat an Ozone AQI Value above roughly 300 as unreliable and consult your local air quality agency's reported figure instead.