On long, hot days - generally between April and October - pollutants emitted primarily from vehicle exhaust combine in the presence of sunlight to form ozone,
which is the main constituent of smog. The Air District issues
Spare the Air Alerts on days when ozone is forecast to be unhealthy and urges residents to drive
less and curtail other polluting activities. Residents who are sensitive to unhealthy air are advised to limit their time outdoors, especially in the afternoon
when temperatures are highest and ozone levels rise.
During colder months, concentrations of fine particulate matter pollution can rise to unhealthy levels in the Bay Area. On cold, still days, temperature inversions trap residential wood smoke close to the ground to create unhealthy air quality. On days when fine particulate pollution is expected to exceed the Air District's
Wood Burning Rule threshold, the Air District issues a Spare the Air Alert for fine particulate pollution, prohibiting the use of all wood-burning devices both indoors and outdoors throughout the Bay Area. On these days, residents are also advised to limit their time outdoors, especially those sensitive to unhealthy air.
The wood burning ban has been extended to include any days year-round when a Spare the Air Alert is in effect due to high levels of fine particulate pollution, such as during a wildfire.