The U.S. Environmental Protection Agency and the South Coast Air Quality Management District of El Monte, California, devised the Pollutant Standard Index to monitor concentrations of pollutants in the air and inform the public concerning related health effects. The scale measures the amount of pollution in parts per million, and has been in use nationwide since 1978.
||Alert: elderly or ill should stay indoors and reduce physical activity.
||Warning: General population should stay indoors and reduce physical activity.
||Emergency: all people remain indoors windows shut, no physical exertion.
||Significant harm; same as above.