Who Issues Air Quality Alerts
Alerts originate with the state or local air quality agency that operates the monitors in your area, such as a state department of environmental quality or a regional air district. Those agencies forecast the next day's AQI and, when it is expected to reach an unhealthy category, issue an alert. The National Weather Service relays many of these as an “Air Quality Alert” product, which is why they arrive through the same weather apps and phone notifications as severe weather. The EPA's AirNow site publishes the same forecasts and current readings nationally, and the free EnviroFlash service emails alerts for your ZIP code. Our active alerts page lists US cities whose latest observation is in an unhealthy range.
Alert Levels and Color Codes
Alerts follow the six EPA Air Quality Index categories defined in the agency's AQI basics. Most agencies issue a formal alert at code orange and above, though some announce “Moderate” particle advisories during wildfire season. The guidance below is the EPA's cautionary statement for each band.
| AQI | Category / color | Alert typically issued? | EPA guidance |
|---|---|---|---|
| 0 to 50 | Good (green) | No | No restrictions. |
| 51 to 100 | Moderate (yellow) | Rarely; some smoke advisories | Unusually sensitive people consider reducing prolonged or heavy exertion. |
| 101 to 150 | Unhealthy for Sensitive Groups (orange) | Yes, in most states | Sensitive groups reduce prolonged or heavy outdoor exertion; take more breaks. |
| 151 to 200 | Unhealthy (red) | Yes | Everyone reduces prolonged or heavy exertion; sensitive groups avoid it. |
| 201 to 300 | Very Unhealthy (purple) | Yes, often a health warning | Everyone avoids prolonged or heavy exertion; sensitive groups stay indoors. |
| 301 and up | Hazardous (maroon) | Yes, emergency conditions | Everyone avoids all physical activity outdoors. |
Who Counts as a “Sensitive Group”
The phrase appears in nearly every alert, and it is broader than most people assume. For ozone, the EPA lists people with asthma or other lung disease, children and teenagers (whose lungs are still developing and who spend more time active outdoors), older adults, and outdoor workers. For particle pollution it adds people with heart disease, pregnant people, and anyone with diabetes. If you are in one of these groups, an orange alert is written for you. See our guides for asthma, children and other sensitive groups.
Alert vs. Action Day vs. Advisory
- Air Quality Alert. Pollution is at or forecast to reach an unhealthy category. Focus: protect your health.
- Air Quality Action Day / Ozone Action Day. Weather favors ozone or particle formation tomorrow. Focus: reduce emissions by carpooling, refueling after dusk, skipping gas-powered yard equipment and not burning wood. Many regions declare action days at a forecast of code orange.
- Advisory or health warning. Terminology varies by state; a “warning” usually signals red or purple conditions, and wildfire smoke advisories may be issued at Moderate if conditions are worsening.
- Burn ban. A regulatory order prohibiting wood burning or open burning during a particle episode, common in winter in valley cities.
What to Do During an Alert
If the pollutant is ozone
Ozone peaks in the afternoon and early evening and drops after sunset, so shift runs, practices and yard work to the morning. Indoor ozone is typically much lower than outdoors, so being inside with windows closed is protective. A HEPA air purifier does not remove ozone; the timing shift is the tool.
If the pollutant is fine particles (PM2.5)
Particle alerts, driven by wildfire smoke or winter inversions, can last all day and for days at a time. Close windows, set HVAC to recirculate, avoid indoor sources like candles and frying, and run a HEPA air cleaner in the rooms you use most. The EPA's Guide to Air Cleaners in the Home recommends a smoke CADR of at least two-thirds of the room's floor area in square feet. If you must be outside in red or worse conditions, a well-fitted N95 respirator reduces particle exposure; see our wildfire smoke guide.