The Air Quality Index (AQI) is an indicator developed by government agencies to communicate to the public how polluted the air currently is or how polluted it is forecast to become. The AQI is a yardstick that runs from 0 to 500, and the higher the AQI value, the greater the level of air pollution and the greater the health concern. The AQI is calculated based on the concentrations of five pollutants: ozone, nitrogen dioxide, sulfur dioxide, PM2.5 (particles with an aerodynamic diameter less than 2.5 μm), and PM10. The index is divided into six categories, each of which has a name, an associated color, and advice to go along with it. AQI values at or below 100 are considered satisfactory for almost everyone, while AQI values above 100 indicate unhealthy air quality. The AQI is used to warn the public when air pollution is dangerous and to help people take steps to protect themselves from unhealthy levels of air pollution.