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Air is almost always on the move, and pollution can easily drift from one reporting zone to another. This is particularly true in the Bay Area, which forms a single air basin, and where winds can blow pollution from one location into neighboring valleys.
That is why a Spare the Air Alert is called for the entire Bay Area when air quality is forecast to be unhealthy in any one of the five reporting zones: North Counties, Coast and Central Bay, Eastern Zone, South Central Bay, and Santa Clara Valley.
Reporting zones are different from county lines: they are defined by air current patterns and geographic features that lead to shared air quality within certain areas.
The Air District issues a daily forecast that predicts air pollution levels for the coming five-day period. The forecast uses the U.S. EPA's Air Quality Index to indicate how healthy or unhealthy those air pollution levels are expected to be.
A Spare the Air Alert is called when air quality is forecast to be unhealthy, or above 100 in the AQI, in any one of the reporting zones. An alert may span over two days if air quality is expected to remain unhealthy for prolonged periods.
The daily forecast is available by signing up for email AirAlerts or calling the Air District’s 24-hour, toll-free information line (1-800-HELP AIR).