The Bay Area Air Quality Management District issued the second Spare the Air Alert of 2013 for Friday.
The order does not ban burning wood, but residents are encouraged to walk, bike, carpool or take public transit.
High temperatures and low winds, paired with motor vehicle exhaust, are expected to cause unhealthy ozone accumulation in the Bay Area.
Ozone pollution is particularly harmful for young children, seniors and those with respiratory and heart conditions.
Outdoor exercise should be done in the early morning when ozone concentrations are lower.
To find out when a Spare the Air Alert is in effect, people can register at www.sparetheair.org, call 1-800-HELP-AIR, download the Spare the Air app, or register with Spare the Air on Facebook or Twitter.