Air-quality alerts have been called 12 times since Nov. 1 in the Bay Area, which has exceeded federal health standards for fine particle pollution seven times, the San Francisco Chronicle reported Friday.
"The worst of those seven days was Christmas Day," said Lisa Fasano, a spokeswoman for the Bay Area Air Quality Management District.
She blamed smoke from traditional holiday fires in residential fireplaces for most of the pollution.
Air quality has been poor the entire winter season, she said, and the situation isn't expected to change any time soon.
"We're calling the alerts because we're having stagnant cold air, and it's trapping particle pollution here in the Bay Area and making it very unhealthy to breathe," Fasano said.
"There are a number of families and a number of people who have asthma and can't be around wood smoke. I am one of them."
A particularly persistent high-pressure system has been pushing storms away from Northern California, meteorologists said, so car exhaust and smoke from factories and chimneys has remained in place for people to breathe.