What do the AQI numbers and color categories mean?

One number on the air quality index stands in for several pollutants at once. The AQI tracks five major ones: ground-level ozone, particle pollution, carbon monoxide, sulfur dioxide, and nitrogen dioxide. Whatever pollutant is worst that day sets the number you see. That single number is sorted into six color-coded bands. The color does more work than the exact figure: it's telling you who should pay attention today.

Green is Good, an AQI of 0 to 50, when the air is satisfactory and poses little or no risk. Yellow is Moderate, 51 to 100, when the air is acceptable, though a few people who are unusually sensitive may notice symptoms. The line sits at 100: at or below it the air counts as satisfactory, and above it the air turns unhealthy, first for sensitive groups and then for everyone as it climbs.

Orange, 101 to 150, is Unhealthy for Sensitive Groups, who may feel effects while the general public usually won't. Those sensitive groups include people with heart or lung disease, older adults, and children. Red, 151 to 200, is Unhealthy: some healthy people start to feel it too, and sensitive groups feel it more.

The top of the scale is rare but serious. Purple, 201 to 300, is Very Unhealthy, where the risk of health effects rises for everyone. Maroon, 301 and higher, is Hazardous, a warning of emergency conditions where everyone is more likely to be affected. On those days, treat the color as the instruction. Match it to who you are, and let it set your plans.