A Spare the Air alert has been issued for Saturday by Bay Area air quality officials.
The alert, issued by the Bay Area Air Quality Management District, means that air quality is expected to be poor and residents are advised to reduce driving and avoid outdoor activities during the hottest parts of the day.
The alert is triggered in part by a forecast for sunny weather, officials said.
"This late summer hot spell means poor air quality is expected in the Bay Area, said Jack Broadbent, executive director for the district.
Spare the Air alerts are triggered when ozone pollutions is forecast to reach unhealthy levels.
Ozone can cause throat irritation, congestion, chest pain and inflammation of the lining of the lungs. Those with asthma, bronchitis and emphysema are especially vulnerable, as are young children, seniors and those with respiratory and heart conditions.
Residents are asked to avoid driving on Spare the Air days. There is no free transit offered, and no wood burning ban is in place.