You’ve seen the numbers — “AQI 85” on your weather app, “AQI 157” during wildfire season. But what do they actually mean for whether you should open your windows, go for a run, or turn on your air purifier? Here’s a practical guide to interpreting and acting on air quality data.
The AQI Scale: 0-500 in Six Categories
The EPA’s Air Quality Index translates raw pollutant concentrations into a 0-500 scale with six color-coded health concern categories:
| AQI Range | Color | Health Concern | What to Do |
|---|---|---|---|
| 0-50 | Green | Good | No restrictions. Open windows, exercise outdoors. |
| 51-100 | Yellow | Moderate | Acceptable for most. Unusually sensitive: reduce prolonged exertion. |
| 101-150 | Orange | Unhealthy for Sensitive Groups | Sensitive groups: limit outdoor activity. Run purifiers indoors. |
| 151-200 | Red | Unhealthy | Everyone may feel effects. Avoid outdoor exertion. Seal windows. |
| 201-300 | Purple | Very Unhealthy | Health alert: everyone avoid outdoor activity. Purifiers on high. |
| 301-500 | Maroon | Hazardous | Emergency: stay indoors, windows sealed, purifiers maxed. |
The “sensitive groups” in the orange category include people with asthma, COPD, or heart disease; children; and adults over 65. If you’re in any of these groups, treat orange as your personal red.
What Goes Into AQI
The AQI is calculated from five criteria pollutants: ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. The overall AQI reported is the highest individual pollutant value — if ozone is 120 (orange) but PM2.5 is 80 (moderate), the reported AQI is 120. This means AQI can be “good” overall while one pollutant is elevated — check the pollutant breakdown if you’re sensitive to a specific one.
PM2.5 is usually the dominant pollutant during wildfire events and in urban areas with traffic. Ozone dominates on hot, sunny summer days.
Indoor Decision-Making Guide
Use outdoor AQI to make indoor air decisions:
- AQI 0-50: Open windows for ventilation. No need to run purifiers unless you have indoor sources (cooking, cleaning).
- AQI 51-100: Ventilation is still fine for most. Those with respiratory conditions should start running purifiers.
- AQI 101-150: Close windows, run purifiers on auto. Sensitive groups should minimize outdoor time.
- AQI 151+: Seal windows, run purifiers on medium-high, minimize all outdoor exposure.
- AQI 200+: Purifiers on high continuously. If you must go outside, wear an N95 mask.
Where to Check AQI
- AirNow.gov: EPA’s official site and mobile app with real-time local AQI and pollutant breakdowns.
- IQAir AirVisual: Global air quality data with 7-day forecasts.
- Apple Weather / Weather.com: Built-in AQI in most weather apps, though with less detail than dedicated sources.
The Color-Coded Guide That Lies to You
The AQI scale is color-coded for quick reading, but the thresholds conceal risk. “Moderate” (yellow, 51-100) sounds fine — but at AQI 99, sensitive groups should already limit outdoor activity. “Unhealthy for Sensitive Groups” (orange, 101-150) sounds like it doesn’t apply to you — but if you have undiagnosed asthma, mild allergies, or are elderly, it applies.
For purifier operation: AQI under 50 (green) means you can open windows freely. AQI 51-100 means run the purifier on auto and limit window opening. AQI 101+ means close windows, run purifier(s) on high, and minimize outdoor time. AQI 200+ (purple/maroon) is where having a properly-sized purifier in every occupied room transitions from helpful to genuinely protective.
See also: Air Purifier vs Opening Windows for Ventilation, Wildfire Smoke Air Purifier Guide, Best Air Quality Monitors 2026.
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