AQI scale and the meaning of each category

AirQuality.city uses the U.S. EPA numerical scale and concentration breakpoints for PM2.5 and PM10. The scale runs from 0 to 500 and is divided into six categories designed to provide a quick health-oriented description of current pollution levels.^[1]

AQI	Category	General meaning
0-50	Good	Pollution is low and the expected risk is small.
51-100	Moderate	Air is acceptable for most people, although unusually sensitive individuals may notice effects sooner.
101-150	Unhealthy for Sensitive Groups	Children, older people and those with heart or lung conditions may be more affected by prolonged or intense exposure.
151-200	Unhealthy	Health effects become possible in the general population, with greater risk for sensitive groups.
201-300	Very Unhealthy	Risk is increased for everyone and outdoor exposure deserves more serious restriction.
301-500	Hazardous	Very high pollution levels with potential effects across the population.

The category boundaries are 50, 100, 150, 200 and 300. A value of 50 is still Good, while 100 is still Moderate.^[1]

The same AQI value represents different concentrations for PM2.5 and PM10

AQI is not a direct measurement. Each pollutant has its own concentration breakpoints that are mapped onto the common AQI categories.

For the current U.S. EPA particulate-matter breakpoints, the table is:^[3]

AQI category	PM2.5 µg/m³	PM10 µg/m³
Good	0.0-9.0	0-54
Moderate	9.1-35.4	55-154
Unhealthy for Sensitive Groups	35.5-55.4	155-254
Unhealthy	55.5-125.4	255-354
Very Unhealthy	125.5-225.4	355-424
Hazardous	225.5-325.4	425-604

In the official US system, these breakpoints are applied with defined averaging rules. AirQuality.city currently uses them to convert the latest processed PM value into a fast local indicator, so the result is not the same as the official daily AQI or the AirNow NowCast.^[2]

A colour boundary is not a wall

The categories are useful communication tools, but pollution changes continuously in the real atmosphere. AQI 100 and AQI 101 sit on opposite sides of a defined boundary even though the physical difference in the air is very small.

When values are close to a boundary, the direction of travel and the underlying concentration are usually more informative than treating a colour change as a sudden jump in risk.

The European scale uses different thresholds

The European Air Quality Index does not use the same six numerical bands. The EEA presents Good, Fair, Moderate, Poor, Very poor and Extremely poor, and the station result can be driven by PM2.5, PM10, NO₂, O₃ or SO₂.^[4]

The difference is especially visible at lower PM2.5 concentrations. The European Good band for PM2.5 ends at 5 µg/m³, whereas the U.S. EPA Good band extends to 9.0 µg/m³.^[4][3]

Colours from two services therefore should not be compared without first checking the methodology. The most solid common reference is the underlying concentration in µg/m³.

Health meaning also depends on the person and duration of exposure

An AQI category is a public-health guide, not a diagnosis. Individual response depends on health status, age, physical activity and how long exposure lasts. When pollution remains elevated for hours or days, the duration and trend become as important as any single reading.

During more serious episodes, health advice from competent public authorities should take priority, particularly for children, older people and those with cardiovascular or respiratory disease.