EPA Air Quality Index (AQI) Explained — PM2.5, Ozone, NO₂, SO₂, CO

EPA Air Quality Index (AQI) Explained — PM2.5, Ozone, NO₂, SO₂, COThe 0–500 AQI scale, six health categories (Good through Hazardous), and per-pollutant breakpoints for PM2.5, PM10, ozone, NO₂, SO₂, and CO.The U.S. Air Quality Index (AQI) is EPA's standard for reporting air quality on a 0–500 scale. Lower is cleaner. Each value maps to a color and a health-impact category.Six health categories

0–50 Good: Air quality is satisfactory; little or no risk.

51–100 Moderate: Acceptable for most; unusually sensitive may have minor effects.

101–150 Unhealthy for Sensitive Groups: Sensitive groups should limit exertion.

151–200 Unhealthy: Everyone may experience effects.

201–300 Very Unhealthy: Health alert; everyone should avoid prolonged exertion.

301+ Hazardous: Emergency conditions; avoid all outdoor exertion.

The pollutants

PM2.5: Fine particulate matter (soot, smoke). The most-tracked pollutant; what wildfire smoke loads on you. Penetrates deep into lungs and bloodstream.

PM10: Coarser particles (dust, pollen). Less harmful per-microgram than PM2.5 but still problematic at high levels.

Ozone (O₃): Ground-level ozone — formed by sunlight + NOₓ reactions. Primary smog component on hot, sunny days.

NO₂: Nitrogen dioxide — combustion byproduct, mostly traffic and power plants.

SO₂: Sulfur dioxide — coal/oil combustion, refineries, volcanic activity.

CO: Carbon monoxide — vehicle exhaust and incomplete combustion.

The dashboard shows per-pollutant breakdowns so the operator can see which pollutant is driving the overall AQI on a given day.SourceThe dashboard uses Open-Meteo's CAMS-based AQI feed. Same EPA scale as AirNow, but Open-Meteo provides global coverage where AirNow has US-only ground stations. Breakpoints from EPA AQI Technical Assistance Document.Part of the BloomWX learn library — beginner-friendly explainers covering every surface of the BloomWX weather dashboard. Open BloomWX to see live data for any U.S. county.