Air quality index (AQI)

The air quality index is a standardized scale, published by agencies such as the US EPA or European Environment Agency, that converts measured concentrations of pollutants like ground-level ozone, PM2.5, PM10, nitrogen dioxide, sulfur dioxide and carbon monoxide into a single number and category, from "good" to "hazardous."

Each pollutant is measured against its own health-based breakpoints, and the reported AQI is the highest of the individual pollutant sub-indices for that location and period, not an average across pollutants. Definitions differ by country: the scale, breakpoints and pollutant set used by the US AQI are not identical to China's or India's national indices, so raw AQI numbers are not directly comparable across systems.

Analysts combine ground-monitor readings with remote sensing and satellite imagery analysis to estimate AQI in areas without physical sensors, then map results using a geographic information system for public health and environmental-justice spatial analysis. A common misreading is treating a single day's AQI as representative of long-term exposure; chronic health effects are driven by sustained average exposure, not one high-pollution day.