Key facts
- The US AQI uses six categories from Good to Hazardous.
- An AQI above 100 becomes unhealthy first for sensitive groups and then for everyone as values rise.
- The category describes short-term public-health concern, not a personal medical diagnosis.
US AQI categories
| AQI | Category | Color | General meaning |
|---|---|---|---|
| 0–50 | Good | Green | Little or no expected health risk. |
| 51–100 | Moderate | Yellow | Acceptable for most people; unusually sensitive people may be affected. |
| 101–150 | Unhealthy for Sensitive Groups | Orange | Sensitive groups face greater risk. |
| 151–200 | Unhealthy | Red | Some effects are possible for everyone; sensitive groups may experience more serious effects. |
| 201–300 | Very Unhealthy | Purple | Health alert with increased risk for everyone. |
| 301–500 | Hazardous | Maroon | Emergency conditions; everyone is more likely to be affected. |
Who may need to act earlier?
The pollutant matters. People with heart or lung disease, people with asthma, children, older adults and people who are active outdoors may be more susceptible to particle or ozone pollution.
At orange levels, the general population may not be affected in the same way as sensitive groups. By red, purple and maroon levels, guidance progressively applies to more people and recommends stronger exposure reduction.
The scale is not universal
India’s National AQI uses Good, Satisfactory, Moderate, Poor, Very Poor and Severe categories with different pollutant breakpoints. A numeric value should therefore always be read together with the standard that produced it.
Frequently asked questions
Is AQI 100 safe?
On the US scale, 100 is the top of the Moderate category. Most people can continue normal activities, while unusually sensitive people may need to reduce prolonged or heavy exertion.
What AQI is considered unhealthy?
On the US scale, 101–150 is unhealthy for sensitive groups and 151–200 is unhealthy for the general population.
Can I compare US AQI directly with India AQI?
The numbers share a 0–500 format but use different pollutant breakpoints and category labels, so direct comparison can be misleading.
Sources
This reference was prepared from the following primary public-health and regulatory sources:
- Patient Exposure and the Air Quality IndexUnited States Environmental Protection Agency