The AQI is a piecewise-linear conversion: each pollutant has a table of breakpoints that map a measured concentration (for example µg/m³ of fine particles) onto an index sub-range. PM2.5 of 12.0 µg/m³ sits at the top of “Good” (AQI 50); 35.4 µg/m³ is the top of “Moderate” (AQI 100). The formula interpolates linearly between the breakpoints, so the index rises smoothly as pollution climbs.
There are six categories, each with a color: 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). The category, not the raw number, is what tells you whether to change your plans.
Crucially, a location’s reported AQI is the maximum across all measured pollutants, never an average. If ozone is at AQI 70 but PM2.5 is at AQI 130, the city reports 130 (Unhealthy for Sensitive Groups) and names PM2.5 as the responsible pollutant. Averaging would hide exactly the pollutant you need to know about.
Because pollutants are averaged over different windows — PM2.5 and PM10 over rolling hours scaled to a 24-hour basis, ozone over 8 hours — a sudden spike (wildfire smoke, rush-hour traffic) can take time to fully register, and a clearing front can take time to fall. Treat the AQI as the recent state of the air, not an instantaneous one.