What do the AQI categories (Good, Moderate, Unhealthy) mean?
Quick Answer: The US EPA Air Quality Index has six bands: Good (0-50, green), Moderate (51-100, yellow), Unhealthy for Sensitive Groups (101-150, orange), Unhealthy (151-200, red), Very Unhealthy (201-300, purple), and Hazardous (301-500, maroon). Lower numbers mean cleaner, safer air.
The AQI converts pollutant concentrations into one easy number from 0 to 500. Good means air quality is satisfactory and poses little or no risk. Moderate air is acceptable, though a small number of unusually sensitive people may notice mild effects. The colour and band rise together, so a quick glance tells you how careful to be.
From orange (Unhealthy for Sensitive Groups) upward, sensitive people such as children, older adults, and those with heart or lung conditions should limit prolonged or strenuous outdoor activity. From red (Unhealthy) upward, everyone should cut back, and at purple and maroon levels the whole population can feel serious effects, so outdoor exertion is best avoided.