US Air Quality Index: Categories, Breakpoints and Health Guidance

At a glance

What matters most

01
The US AQI runs from 0 to 500 across six color-coded health categories.
02
EPA defines AQI calculations for ozone, particle pollution, carbon monoxide, sulfur dioxide and nitrogen dioxide.
03
The highest qualifying pollutant sub-index determines the overall AQI.
04
Current AirNow readings for ozone and particle pollution use NowCast calculations rather than a single raw hourly value.

US AQI categories and health meaning

AQI	Category	General meaning
0–50	Good	Air quality is satisfactory; little or no risk is expected.
51–100	Moderate	Acceptable overall, with possible risk for unusually sensitive people.
101–150	Unhealthy for Sensitive Groups	Sensitive groups may experience health effects.
151–200	Unhealthy	Some effects are possible for everyone; sensitive groups may have more serious effects.
201–300	Very Unhealthy	Health alert; risk is increased for everyone.
301–500	Hazardous	Emergency-level health warning; everyone is more likely to be affected.

How the reported AQI is selected

EPA defines pollutant-specific concentration breakpoints and averaging rules for ground-level ozone, PM2.5, PM10, carbon monoxide, sulfur dioxide and nitrogen dioxide. A concentration is converted to a pollutant sub-index through linear interpolation between the applicable breakpoints.

When several pollutants are available, the highest qualifying sub-index becomes the overall AQI and that pollutant is commonly described as the dominant or primary pollutant. Missing measurements can therefore affect which pollutants participate in the result.

Current US AQI breakpoints for PM2.5

AQI category	AQI range	24-hour PM2.5 concentration
Good	0–50	0.0–9.0 µg/m³
Moderate	51–100	9.1–35.4 µg/m³
Unhealthy for Sensitive Groups	101–150	35.5–55.4 µg/m³
Unhealthy	151–200	55.5–125.4 µg/m³
Very Unhealthy	201–300	125.5–225.4 µg/m³
Hazardous	301–500	225.5–325.4 µg/m³

Daily AQI, forecasts and current NowCast

The daily AQI summarizes air quality over the pollutant’s required averaging period. AQI forecasts estimate the next day’s conditions. For current ozone and particle pollution, AirNow uses NowCast algorithms that combine multiple recent hours and respond more quickly when concentrations are changing.

A current AQI, a daily final AQI and a raw concentration can therefore show different numbers without contradicting one another. Always check the pollutant, reporting period, location and data status before comparing services.

AQI versus a legal air-quality standard

The AQI is a health-communication index, not a direct concentration and not by itself a regulatory compliance determination. EPA notes that AQI 100 generally corresponds to the level of the relevant short-term national ambient air quality standard, but formal compliance uses prescribed monitoring, averaging and statistical rules.

Frequently asked questions

Which AQI scale does AQI.news use?

AQI.news labels its headline live city readings as US AQI and displays pollutant concentrations separately for context.

Is US AQI the same as India AQI?

No. Both use a 0–500 format, but category names, pollutant breakpoints and calculation rules differ, so equal numbers do not always represent equal concentrations.

Why can the current AQI change during the day?

Pollution concentrations change with emissions and weather. AirNow’s current ozone and particle readings also use adaptive NowCast calculations built from multiple recent hours.

Does an AQI of 100 mean 100 µg/m³?

No. AQI is a unitless index. The concentration represented by AQI 100 depends on the pollutant and its applicable breakpoint.

Primary sources

This reference was prepared from the following primary public-health and regulatory sources: