Air Pollution Index Calculator

An air quality index is a translation device, not a physical measurement. Pollutants are reported in incompatible units - micro-g/m3 for particulates, ppb for the gases, ppm for carbon monoxide - and each has its own dose-response curve, so the index maps every concentration onto a common 0 to 500 scale anchored to health-based standards. Within each segment the mapping is straight-line interpolation, I = I_low + (C - C_low) times (I_high - I_low)/(C_high - C_low), with breakpoints chosen so that 100 corresponds to the national ambient standard for that pollutant. The reported index is the maximum sub-index rather than a sum, so it flags the pollutant of greatest concern. Cutoffs at 50, 100, 150, 200, and 300 mark Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy, and Hazardous. Breakpoints are tied to the standards and change when the standards change. After the 2024 revision of the annual fine particulate standard from 12.0 to 9.0 micro-g/m3, the PM2.5 concentration corresponding to an index of 50 was lowered to 9.0 micro-g/m3, so historical values are not directly comparable with current ones. Three limitations matter. Taking the maximum means moderate simultaneous exposure to several pollutants reads no worse than one alone, though health effects are broadly additive. Each sub-index assumes its own averaging window - 24 hours for particulates, 8 hours for carbon monoxide, 1 hour for the gases - so an instantaneous reading misstates the result. And national scales differ, so 150 in India does not denote the same concentrations as 150 in the United States. World Health Organization 2021 guidelines, at 5 micro-g/m3 annual PM2.5, are far stricter than an index of 50.

Before standardization, American cities published incompatible indices. Through the early 1970s dozens of local and state agencies ran their own pollution scales with different pollutants, different weightings, and different verbal categories, so a reported figure meant nothing outside the jurisdiction that issued it. The Environmental Protection Agency and the Council on Environmental Quality resolved this in 1976 by publishing a uniform Pollutant Standards Index, which fixed the 0 to 500 range, set 100 at the national ambient air quality standard, and specified the piecewise linear interpolation still used today. The scheme was substantially revised in 1999 and renamed the Air Quality Index. That revision added fine particulate matter as a reported pollutant following the 1997 introduction of a PM2.5 standard, introduced the six color-coded categories, and created the Unhealthy for Sensitive Groups band between 101 and 150 to give asthmatics, children, and older adults a threshold distinct from the general population. The AirNow system was built out around the same time to distribute forecasts and real-time maps. Because breakpoints derive from the underlying standards, the table has been re-cut repeatedly as those standards tightened, including ozone revisions in 1997, 2008, and 2015 and particulate revisions in 2012 and 2024. Many countries adapted the framework with their own breakpoints, among them the Indian national index launched in 2015 and the European Common Air Quality Index.