EPA AQI categories explained — what 0–50 Good through 301–500 Hazardous mean

The six AQI categories at a glance

0–50 — Good (Green): Air quality is satisfactory and air pollution poses little or no risk.

51–100 — Moderate (Yellow): Air quality is acceptable. There may be a risk for unusually sensitive people, who should consider limiting prolonged outdoor exertion.

101–150 — Unhealthy for Sensitive Groups (Orange): Members of sensitive groups may experience health effects. The general public is less likely to be affected.

151–200 — Unhealthy (Red): Some members of the general public may experience health effects; members of sensitive groups may experience more serious effects.

201–300 — Very Unhealthy (Purple): Health alert — the risk of health effects is increased for everyone.

301–500 — Hazardous (Maroon): Health warning of emergency conditions; everyone is more likely to be affected.

What the AQI actually measures

The AQI is calculated from five major pollutants the EPA regulates under the Clean Air Act: ground-level ozone, particle pollution (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide.

Each pollutant gets its own sub-index on the 0–500 scale, and the reported AQI is simply the highest of those sub-indices for that hour or day. So an AQI of 120 means at least one pollutant reached the 101–150 range — most often fine particles (PM2.5) or ozone, the two that drive elevated readings across most of the US.

Because PM2.5 dominates during wildfire-smoke events and ozone peaks on hot, sunny afternoons, the same location can swing across several categories within a single day.

"Unhealthy for Sensitive Groups" — who that actually means

The 101–150 Orange category is the one most people misread. "Sensitive groups" is a specific list: children and teenagers, adults over 65, people with asthma or other lung disease, people with heart disease, and pregnant people. Outdoor workers and anyone exercising hard outdoors also fall into the at-risk set because they breathe more air, faster.

Children are singled out because their lungs are still developing and they breathe more air per pound of body weight than adults — which is why school recess and youth-sports decisions often key off this exact threshold.

At Orange, sensitive groups should cut back on prolonged or heavy outdoor exertion; at Red (151–200) and above, everyone should start limiting it.

What to do at each level

Good / Moderate (0–100): No action for most people. Unusually sensitive individuals can watch for symptoms toward the high end of Moderate.

Unhealthy for Sensitive Groups (101–150): Sensitive groups move activity indoors or shorten it; everyone else is generally fine.

Unhealthy (151–200): Everyone limits prolonged outdoor exertion and sensitive groups avoid it — a good time to close windows and run an air purifier.

Very Unhealthy / Hazardous (201+): Stay indoors with windows closed and filtration running, and reschedule outdoor activity. During wildfire smoke, a well-fitted N95/KN95 helps if you must go out.

What zipradar shows vs. AirNow

zipradar federates the EPA's AirNow program — the same data that powers the official air-quality maps — and reports the current AQI category and the nearest reporting monitor for a given US ZIP code, alongside multi-year averages.

For the live, hour-by-hour reading and active air-quality alerts, AirNow.gov is the official real-time source. zipradar is informational and station-based; readings can vary block to block, so treat the ZIP-level number as a neighborhood indicator rather than a measurement at your exact address.