The Air Quality Index (AQI) is a representation of air pollution concentration levels. It assigns numbers on a scale between 0 and 500 and is used to help determine when air quality is expected to be unhealthy.
Based on federal air quality standards, the AQI includes measures for six major air pollutants: ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and two sizes of particulate matter. In the Bay Area, the pollutants most likely to prompt a Spare the Air Alert or Winter Spare the Air Alert are ozone, between April and October, and particulate matter, between November and February.
Each AQI number refers to specific amounts of pollution in the air. For most of the six pollutants represented by the AQI chart, the federal standard corresponds with a number of 100. If the concentration of a pollutant rises above 100, air quality can be unhealthy for the public.
Readings below 100 on the AQI should not affect health of the general public, though readings in the moderate range of 50 to 100 may affect unusually sensitive people. Levels above 300 rarely occur in the United States.
When the Air District prepares the daily AQI forecast, it measures the anticipated concentration for each of the six major pollutants included in the index, converts the readings into AQI numbers, and reports the highest AQI number for each reporting zone. A Spare the Air Alert or Winter Spare the Air Alert is called for the Bay Area the day before air quality is expected to be unhealthy in any of the region’s five reporting zones.