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.

What is the Air Quality Index (AQI) Reference?

Reference for the US EPA Air Quality Index from Good to Hazardous with colour codes and health guidance, plus a converter that turns PM2.5 or PM10 concentration in micrograms per cubic metre into an AQI value. It runs free in your browser on Gera Tools, with nothing uploaded.

Air Quality Index (AQI) Reference

Name: Air Quality Index (AQI) Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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, which tightened the annual standard and shifted the 24-hour breakpoints downward.

The six AQI categories

Category	AQI range	Colour	Who is at risk
Good	0–50	Green	Air is satisfactory; little to no risk
Moderate	51–100	Yellow	Sensitive individuals may experience minor effects
Unhealthy for Sensitive Groups	101–150	Orange	Sensitive people should reduce prolonged exertion
Unhealthy	151–200	Red	Everyone may experience health effects
Very Unhealthy	201–300	Purple	Health alert; serious effects possible for all
Hazardous	301–500	Maroon	Emergency conditions; entire population at risk

Worked example: converting a PM2.5 reading

For example, suppose you have a PM2.5 24-hour average of 18 µg/m³. Under the 2024 breakpoints the 12.1–35.4 µg/m³ band maps to an AQI of 51–100. Plugging into the formula:

AQI = (100 - 51) / (35.4 - 12.1) * (18 - 12.1) + 51
    = (49 / 23.3) * 5.9 + 51
    ≈ 64

An AQI of 64 falls in the Moderate (yellow) category. Healthy adults can continue normal activities, but unusually sensitive people might want to limit prolonged outdoor exertion.

Practical guidance

PM2.5 vs PM10 — which matters more in your area? In urban areas dominated by vehicle exhaust and combustion, PM2.5 is usually the binding pollutant because it penetrates deepest into lung tissue. In agricultural or desert regions, windblown dust raises PM10 without proportionally raising PM2.5, so both sub-indexes are worth watching.

Short spikes are smoothed. The AQI uses 24-hour averages for particulates, so a two-hour spike during rush hour is diluted by clean air the rest of the day. Real-time monitors (now common in air-quality apps) often report “NowCast” AQI, which weights recent hours more heavily to reflect what you are actually breathing right now.

Sensitive groups should act one category earlier. Children, older adults, pregnant people, and anyone with asthma, heart disease, or chronic lung conditions experience effects at lower AQI values than the general public. If the headline says Moderate, sensitive groups should already treat it as Unhealthy for Sensitive Groups.

The headline AQI is the worst sub-index. Each measured pollutant — PM2.5, PM10, ozone, carbon monoxide, sulfur dioxide, and nitrogen dioxide — gets its own sub-index. The single number reported for a city is whichever sub-index is highest. On wildfire days that is almost always PM2.5.