The closures came as president Donald Trump was set to visit the western state Saturday to survey the damage and meet victims of the giant Camp Fire that has devoured an area roughly the size of Chicago since it broke out last week, killing at least 63 people.
In an interview with Fox News ahead of his visit, Trump doubled-down on his earlier claim that mismanagement of Califonia's forests was to blame for the fires. But he acknowledged that climate change may have contributed "a little bit" to the wildfires.
"You need forest management. It has to be," Trump told Fox. "I'm not saying that in a negative way, a positive -- I'm just saying the facts."
More than 600 people have been reported missing since the inferno erupted 8 November, laying waste to the town of Paradise at the northern foothills of the Sierra Nevada mountains, and destroying around 10,000 homes.
Some 180 miles (290 kilometres) to the southwest, San Francisco on Friday ordered all public schools shut and its iconic cable cars returned to their stations as the Air Quality Index soared to 271, comparable to Dhaka, Bangladesh and worse than Kolkata, India.
"San Francisco's air quality has moved from red or 'unhealthy' to purple or 'very unhealthy' due to local wildfires and weather patterns," the SFMTA transport authority said on its website.
"The Department of Public Health highly recommends that everyone stay indoors and avoid exposure to the outside air."
Mayor London Breed announced that public buses would be free for the day in order to ensure people have access to enclosed transportation.
A thick blanket of haze enveloped the region and the famous Golden Gate Bridge was shrouded in thick smog.
"It's bad," said local resident Melvin Karsenti. "You have this constant haze over the city. The air feels thicker. I've never seen that many people wear (face) masks."
People drop off freshly baked cookies and cupcakes to Camp Fire evacuees who are living in a Walmart parking on 16 November 2018 in Chico, California. Photo: AFP
Missing toll jumps
On Thursday, authorities raised the number of missing people from 300 to 631 as investigators went back and reviewed emergency calls and reports of missing made when the Camp Fire erupted on 8 November.
Much of the rescue work is now focused on Paradise, a community that is home to many retirees who found themselves unable to get out in time.
Hundreds of rescuers, backed by sniffer dogs, were going from house to house, as authorities called on relatives of the missing to provide DNA samples to help with body identification.
"I'm still going to keep on looking and hope for the best," Jhonathan Clark, who was hunting for his brother, sister-in-law and nephew, told AFP.
"My dad is starting to lose hope a little bit," he added.
The remains of seven additional victims discovered by rescuers Thursday brought to 63 the number of people who have died in the Camp Fire, which remained only 45 per cent contained Friday afternoon.
Three other people have died in southern California in another blaze dubbed the Woolsey Fire, which engulfed parts of Malibu, destroying the homes of several celebrities.
That inferno, which is about two-thirds the size of the Camp Fire, was close to 70 per cent contained by Friday, as authorities predicted they would have it under control by Monday.
Many of the victims of the Camp Fire have been housed in temporary shelters and are facing homelessness as they try to rebuild their lives.
Adding to their misery, an outbreak of the highly contagious norovirus has been reported at several shelters.
Public health officials said 41 people had been sick with vomiting and diarrhea as of Wednesday evening and 25 had to be hospitalised.
"The number of sick people is increasing everyday," the Butte County Health Department said in a statement.
While the cause of the Camp Fire is still under investigation, a lawsuit has been filed against the local power company, PG&E, by fire victims claiming negligence.