AQI scale
The AQI scale has six categories. A lower number means less pollution, while a higher number indicates greater potential health concern.
| AQI | Category | What it means |
|---|---|---|
| 0-50 | Good | Air quality is satisfactory and the pollution risk is low. |
| 51-100 | Moderate | Most people are unlikely to be affected. People who are unusually sensitive may notice effects sooner. |
| 101-150 | Unhealthy for sensitive groups | Children, older adults, and people with heart or lung conditions should pay more attention to prolonged or strenuous outdoor activity. |
| 151-200 | Unhealthy | Some effects are possible in the general population. Sensitive groups face a greater risk. |
| 201-300 | Very unhealthy | The risk is increased for everyone. Reducing prolonged and strenuous outdoor activity is sensible. |
| 301-500 | Hazardous | Pollution is very high. Follow official advice and avoid unnecessary exposure. |
These ranges match the official U.S. EPA AQI categories.
What AirQuality.city includes in the overall AQI
The website calculates a separate AQI for PM2.5 and PM10. The higher of the two becomes the station's overall AQI.
This means the overall website value does not include NO₂, O₃, SO₂, or CO, even when an official station reports them.
A category boundary is not a wall
The difference between an AQI of 100 and 101 is not a sudden change from acceptable to dangerous air. Categories make a continuous scale easier to read.
When a value is near a boundary, the trend and the concentration of the dominant pollutant are more useful than the colour alone.
Who may be more sensitive
Sensitive groups commonly include:
- children
- older adults
- people with asthma or other lung conditions
- people with heart conditions
- people exercising or working hard outdoors
Individual responses differ. Health guidance on the website is general information, not a substitute for medical advice.
The official scale is available on the AirNow AQI Basics page.