Air Quality Index Table
| Color | Level | AQI | AirNow health description |
|---|---|---|---|
| Green | Good | 0-50 | Air quality is satisfactory, and air pollution poses little or no risk. |
| Yellow | Moderate | 51-100 | Air quality is acceptable; a risk remains for unusually sensitive people. |
| Orange | Unhealthy for Sensitive Groups | 101-150 | Sensitive groups may experience health effects; the general public is less likely to be affected. |
| Red | Unhealthy | 151-200 | Some general-public effects; sensitive groups more serious. |
| Purple | Very Unhealthy | 201-300 | Health alert: the risk of health effects is increased for everyone. |
| Maroon | Hazardous | 301+ | Emergency conditions: everyone is more likely to be affected. |
The Air Quality Index folds five pollutants - ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide and nitrogen dioxide - into one number from 0 to 500, and EPA's AirNow program colors it: green Good (0-50), yellow Moderate (51-100), orange Unhealthy for Sensitive Groups (101-150), red Unhealthy (151-200), purple Very Unhealthy (201-300) and maroon Hazardous (301 and up). The table below carries each band with AirNow's official description of what it means for your body, and the lookup reads any number back to its color and guidance.
Two habits make the number useful. First, compare it to your own sensitivity: the orange band is written for 'sensitive groups' - children, older adults, people with asthma or heart disease - which is most parents' real threshold, not the red line. Second, know what the number is made of: on any given day one pollutant drives the index, and wildfire smoke days are almost always PM2.5 - which is why the PM2.5-to-AQI converter linked below is the fastest way to interpret a raw particulate reading.
How to use
- Type any AQI number (a reading from an app, a monitor or the news) to see its color band and the official AirNow health description.
- Scan the table for the full six-band ladder with colors, ranges and guidance - the same scale every US air-quality forecast uses.
- On orange days, sensitive groups should shorten outdoor exertion; the band descriptions in the table say exactly who and how much.
Frequently asked questions
What do the AQI colors mean?
Each color is a health band on EPA's 0-500 scale: green Good (0-50, little or no risk), yellow Moderate (51-100, acceptable with a caveat for unusually sensitive people), orange Unhealthy for Sensitive Groups (101-150, the sensitive feel it first), red Unhealthy (151-200, some effects in the general public), purple Very Unhealthy (201-300, health alert for everyone) and maroon Hazardous (301+, emergency conditions). The table carries AirNow's exact wording for each band - the same scale behind every purple-dot map you have seen.
Why do different apps show different AQI numbers for the same moment?
Three honest reasons: they may sit on different scales - some apps report US EPA AQI while others use a European or raw-concentration scale; they update at different cadences, because EPA's official AQI runs on rolling nowcast averages (hourly for ozone, several-hour weighted for particles) that smooth spikes; and they may read different monitors. If two numbers disagree by a band, check which scale each app uses before trusting either - the bands on this page are the US EPA definitions.
Is PM2.5 the same thing as AQI?
No - PM2.5 is one pollutant (particles under 2.5 microns, the ones that reach deep into the lungs); AQI is the composite index built from five pollutants including PM2.5. On wildfire-smoke days PM2.5 is usually the driver, so a raw PM2.5 reading converts almost directly to the AQI you see reported - the linked converter does that math for any concentration. On a summer ozone day, though, particles can be fine while the AQI is orange, because ozone is the pollutant setting the index.
What should I do when the AQI is over 150?
At 151 and above (red), AirNow's guidance shifts from 'sensitive groups' to everyone: reduce prolonged or heavy outdoor exertion, and move outdoor workouts indoors. At 201+ (purple) it becomes a health alert - avoid outdoor exertion entirely, run air filtration if you have it, and watch for symptoms like coughing or shortness of breath even indoors on smoke days. The bands in the table are cumulative: each one assumes the advice of every band below it still applies.