SAN FRANCISCO, June 24 (UPI) -- Smoke and haze from the hundreds of northern California wildfires is so thick that San Francisco pollution officials declared an air-quality alert.

The smoke is heaviest in Napa and Solano counties north of the Bay Area due to a major fire near Fairfield, Calif., but the hazy skies and smell of burning wood has become so ubiquitous that Northern Californians are getting accustomed to it, the San Francisco Chronicle reported Tuesday.

"It smells like you're right near a campfire," Benicia, Calif., waitress Riley Krull, 18, told the newspaper.

"We're advising people to use common sense," Kristine Roselius, a spokeswoman for the Bay Area Air Quality Management District, said, warning people to avoid exercise in smoky conditions and to keep children out of the haze.

State fire officials said more than 4,300 firefighters had joined local and federal personnel to battle more than 800 fires from near the Oregon border to the Sierra foothills to coastal Monterey County forests, the Contra Costa (Calif.) Times reported.

More than 112,000 acres had burned across Northern California, with 5,600 homes threatened.