Another forecast of unhealthy air prompted the air district to issue the health advisory for the eighth time this season.
"We have serious air quality conditions impacting the Bay Area right now," said Air District Executive Officer Jack Broadbent in a prepared statement. "The current combination of smoke and smog is making things particularly difficult for residents as they plan their outdoor activity."
Officials are warning residents that vigorous outdoor exercise should only be done in the early morning hours when ozone concentrations are lower, and are reminding residents that ozone pollution is especially harmful for young children, seniors and people with respiratory and heart conditions.
There have not been four consecutive Spare the Air days since 2006, said air district spokeswoman Lisa Fasano.
"It's not unusual to have a string of them when we have high-heat days," Fasano said. "What's unusual for us is the particulate matter combined with fog."
Bay Area transit agencies will not offer free transportation Thursday.