Air Quality Index Calculator

The Air Quality Index maps concentrations of several unrelated pollutants onto one dimensionless 0 to 500 scale anchored to health-based standards. For each pollutant a piecewise linear function interpolates the measured concentration between the breakpoint pair that brackets it, and index 100 is pinned to the corresponding National Ambient Air Quality Standard, so any sub-index above 100 signals an exceedance. The reported value is the maximum of the sub-indices rather than a sum, because the index is meant to report the single pollutant of greatest concern at that moment. Averaging periods are part of the definition: PM2.5 and PM10 use 24-hour averages, ozone and carbon monoxide 8-hour averages, and nitrogen dioxide and sulfur dioxide 1-hour averages. Under the breakpoints revised with the 2024 particulate standard, a 24-hour PM2.5 of 9.0 micro-g/m3 sits at index 50 and 35.4 micro-g/m3 at index 100. Categories run Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous. Two limits matter: the maximum rule ignores additive effects when several pollutants are simultaneously elevated, and substituting an instantaneous reading for the specified averaging window inflates the result.

Before the mid-1970s American cities published incompatible local pollution indices; a 1976 review by Wayne Ott and Gary Thom catalogued dozens of mutually inconsistent systems, several of which could rate the same air as clean or hazardous depending on which city published the number. The Environmental Protection Agency answered in 1976 with the Pollutant Standards Index, a uniform national scale that pinned the value 100 to the National Ambient Air Quality Standards created under the 1970 Clean Air Act and 500 to concentrations judged to represent significant harm. The Pollutant Standards Index was renamed the Air Quality Index in 1999, when the EPA added color coding, the Unhealthy for Sensitive Groups band, and the fine particle standard adopted in 1997. Because index 100 is tied to the standards themselves, the breakpoints are rewritten whenever a standard changes: ozone shifted with the 1997 move to an 8-hour metric and again with the 2008 and 2015 revisions to 75 and 70 parts per billion, while PM2.5 breakpoints were recalculated after the 2006 revision and again following the February 2024 rule that lowered the annual standard to 9.0 micro-g/m3. AirNow began national public distribution of AQI maps and forecasts in the late 1990s.