With temperatures expected to rise this weekend, the Bay Area Air Quality Management District has issued a "Spare the Air" alert for Friday.
The district is expecting unhealthy levels of ozone pollution in the Bay Area because of warm weather and low winds, combined with car exhaust.
Ozone is harmful to young children, seniors and anyone with respiratory and heart conditions, district officials said.
District officials are encouraging Bay Area residents to walk, bike, carpool or use public transit instead of driving on Friday.
There is no free public transit on Friday and no ban on wood burning, according to the district.
Anyone who plans to exercise outdoors is advised to do so in the early morning hours when ozone concentrations are lower, district officials said.
This is the district's second Spare the Air alert for smog in 2013.