https://www.mercurynews.com/2022/09/02/bay-area-heat-wave-prompts-spare-the-air-alert-for-saturday/

The potentially record-setting heat wave heading for the Bay Area over Labor Day weekend caused regulators to issue a Spare the Air alert for Saturday, as extreme high pressure was expected to degrade the region’s air quality.

The Spare the Air alert means that light winds and vehicle exhaust were expected to work together and bring ozone pollution to unhealthy levels, according to the Bay Area Air Quality Management District. Residents should only perform outdoor exercise in the early morning hours when ozone concentrations are lower.