How is the Air Quality Index calculated?

The US EPA AQI uses a piecewise-linear formula. Each pollutant has concentration breakpoints that map onto the same 0 to 500 health scale. The AQI is interpolated linearly between the breakpoints that bracket your concentration, so any reading lands on the common scale.

What is a good AQI value?

An AQI of 0 to 50 is Good (green): air quality is satisfactory and poses little or no risk. From 51 to 100 is Moderate, and 101 to 150 is Unhealthy for Sensitive Groups. Values above 150 are Unhealthy or worse for the general population.

What is the difference between PM2.5 and PM10?

PM2.5 is fine particulate matter 2.5 microns or smaller, which penetrates deep into the lungs and bloodstream; PM10 includes coarser particles up to 10 microns. They have separate breakpoint tables, so the same concentration gives different AQI values for each.

Why did PM2.5 AQI breakpoints change?

In 2024 the US EPA tightened the PM2.5 standard, lowering the concentration thresholds for each AQI category. This tool uses the updated breakpoints, so a given PM2.5 reading now corresponds to a higher AQI than under the older table, reflecting stricter health protection.

How is the overall AQI for a city decided?

Each measured pollutant (PM2.5, PM10, ozone, CO, SO2, NO2) gets its own sub-index. The overall reported AQI is simply the highest of those sub-indices, because air quality is judged by whichever pollutant is most harmful at that moment.

Air Quality Index (AQI) Reference

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Reading the US EPA Air Quality Index

This reference explains the US EPA Air Quality Index, which converts pollutant concentrations into a single 0-to-500 number with six colour-coded health categories. It lists each category from Good to Hazardous with its standard colour and health message, and includes a converter from PM2.5 or PM10 concentration to AQI using the official breakpoint tables.

How it works

AQI is piecewise-linear. Every pollutant has a table of breakpoints, each a band [Clow, Chigh] of concentration mapped to a band [Ilow, Ihigh] of AQI. To convert a concentration C, find the band containing it and interpolate:

AQI = (Ihigh - Ilow) / (Chigh - Clow) * (C - Clow) + Ilow

The result is rounded to a whole number and looked up in the category table. PM2.5 and PM10 have different breakpoints, so the same microgram concentration yields a different AQI for each. The PM2.5 thresholds here follow the 2024 EPA update.

Tips and notes

The categories are: Good (0-50), Moderate (51-100), Unhealthy for Sensitive Groups (101-150), Unhealthy (151-200), Very Unhealthy (201-300) and Hazardous (301+).
AQI uses 24-hour average concentrations for particulates; short spikes are smoothed out.
The headline AQI for a location is the worst sub-index among all pollutants.
Sensitive groups (children, older adults, and people with heart or lung conditions) should act one category earlier than the general public.