Air Quality Index
The AQI scale, explained
The U.S. Air Quality Index (AQI), set by the EPA, runs from 0 to 500 and has six color-coded categories. At 100 or below, air quality is considered satisfactory or acceptable; above 100 it becomes unhealthy, first for sensitive groups and then for everyone. Each area's AQI is the highest of the pollutants measured there.
The six categories
| AQI | Category | Health effects | What to do | Areas now |
|---|---|---|---|---|
| 0–50 | Good | Air pollution is low and is not expected to affect health. | A good day to be active outside. | 547 |
| 51–100 | Moderate | Air quality is acceptable for most people. A small number of people who are unusually sensitive to this pollutant may notice effects. | If you are unusually sensitive, consider making long or intense outdoor activity shorter or less intense. | 173 |
| 101–150 | Unhealthy for Sensitive Groups | People in sensitive groups may have health effects. Most other people are unlikely to be affected. | Sensitive groups should cut back on long or heavy outdoor exertion and take more breaks. People with asthma should keep quick-relief medicine handy. | 3 |
| 151–200 | Unhealthy | Anyone may begin to feel health effects. People in sensitive groups may have more serious effects. | Sensitive groups should avoid long or heavy outdoor exertion. Everyone else should cut back on it, take more breaks and do less intense activities. | 0 |
| 201–300 | Very Unhealthy | A health alert level: the risk of health effects is higher for everyone. | Sensitive groups should avoid all physical activity outdoors and move activities indoors. Everyone else should avoid long or heavy outdoor exertion. | 0 |
| 301–500 | Hazardous | An emergency-level condition: everyone is more likely to be affected. | Everyone should avoid all physical activity outdoors. Sensitive groups should stay indoors and keep activity levels low. | 0 |
Category descriptions summarize EPA's AQI guidance in our own words. "Areas now" counts U.S. reporting areas by current AQI in the AirNow file published Oct 28, 2026, 3:15 AM EDT.
Who is in a sensitive group?
For particle pollution (PM2.5 and PM10): people with heart or lung disease (including asthma), older adults, children and teenagers.
For ozone: people with asthma or other lung disease, children and teenagers, older adults, and people who are active or work outdoors. Ozone is usually highest on sunny, hot afternoons, so early-morning outdoor activity is often a better choice on high-ozone days.
If you are not sure whether you are in a sensitive group, ask your doctor. This page is general information, not medical advice.
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Pollutant levels behind each range
The AQI converts each pollutant's concentration into the same 0–500 scale using breakpoints set by EPA. In 2024 EPA lowered the PM2.5 breakpoints after it strengthened the annual PM2.5 standard, so the same particle level now produces a higher AQI than before.
| Category | PM2.5, 24-hour (µg/m³) | PM10, 24-hour (µg/m³) | Ozone, 8-hour (ppm) |
|---|---|---|---|
| Good | 0.0–9.0 | 0–54 | 0.000–0.054 |
| Moderate | 9.1–35.4 | 55–154 | 0.055–0.070 |
| Unhealthy for Sensitive Groups | 35.5–55.4 | 155–254 | 0.071–0.085 |
| Unhealthy | 55.5–125.4 | 255–354 | 0.086–0.105 |
| Very Unhealthy | 125.5–225.4 | 355–424 | 0.106–0.200 |
| Hazardous | 225.5 and up | 425 and up | 1-hour values used |
Breakpoints from EPA's AQI technical guidance (PM2.5 values as revised in 2024). Hourly readings on this site use EPA's NowCast method, which estimates the AQI from recent hours so it responds quickly when air quality changes.
AQI forecasts and Action Days
State and local air agencies issue daily AQI forecasts for many areas. When they expect unhealthy air, some declare an Air Quality Action Day and ask people to cut pollution, for example by driving less, refueling after dark or skipping gas-powered lawn equipment. Area pages on OpenAir show these flags as received from AirNow.
Learn more at AirNow: AQI Basics.
Preliminary data. AirNow readings and forecasts come from federal, state, local and tribal air quality agencies. They are not fully verified, may change, and must not be used for regulatory or official decisions; official data are in EPA's Air Quality System. For the latest official readings and advisories, check AirNow.gov and your state or local air agency.
This page is general information, not medical advice. If you have symptoms or a health condition, follow your doctor's guidance.