Late in the day Wednesday, a new fire erupted several miles to the west above the city of La Canada Flintridge, but it also was burning upslope and away from homes. At least 10 acres had burned by late afternoon, U.S. Forest Service spokesman Bruce Quintelier said.
"It's pretty bad, the smoke," said Natacha Cuvelier, a 20-year-old student at the University of Southern California in Los Angeles. "Once I stepped out of the door, I could smell it."
The air was considered unhealthy in many areas, and regional officials urged people to avoid strenuous activities, indoor and out.
Schools were advised to suspend physical education and sports, said Dr. Jonathan Fielding, Los Angeles County's director of public health.
The smoke lingered for lack of a sea breeze.
"Sometimes when we have these fires that will last for several days, the smoke can kind of build and get sloshed back and forth, so to speak," said Sam Atwood, spokesman for the South Coast Air Quality Management District. "With each successive day, the extent of the smoke will get bigger and bigger."
The first fire, which erupted Tuesday near Morris Dam in San Gabriel Canyon, was 10 percent contained as temperatures headed up to triple digits, said forest dispatcher Chris Rush.
"The fire is pretty much doing whatever it wants to do. It's burning freely," said Capt. Jim Wilkins, a Forest Service spokesman.
Eight air tankers and nine helicopters dropped water and retardant as hundreds of firefighters struggled to contain flames that leapfrogged among steep, rocky ridges where the brush had not burned for at least 25 years and was bone-dry.
No injuries were reported, and no homes were in immediate danger, authorities said. However, a mandatory evacuation was ordered Wednesday afternoon for Camp Williams, a recreational vehicle campsite and resort on a fork of the San Gabriel River.
Other campgrounds remained closed, a day after picnickers and campers were forced to flee and 18 Boy Scouts had to be flown out of the area.
Investigators believe the fire was caused by a person, but it was unclear whether it was accidental or deliberate, Rush said. Mountain highs will run up to 105 degrees through Friday, said meteorologist David Sweet of the National Weather Service.
The weather service issued a fire danger warning for mountain areas stretching from Los Angeles County northwest through Santa Barbara and Ventura counties, but the forecast did not call for any strong Santa Ana winds that typically stoke the worst Southern California wildfires.
Forecasters said a strong high pressure system was expected to build over the Southwest through Friday as onshore flow of moist air from the Pacific weakened. That was expected to result in continued warming and drying. Single-digit humidity levels were already being observed in mountain areas.
For weeks this summer, low clouds and fog hung close to the Southern California coast, and the moist marine layer often pushed well inland overnight.
The Angeles National Forest sprawls across about 1,000 square miles of chaparral, pine and fir in towering mountains and rugged canyons north and northeast of Los Angeles.
It is a major recreation area for the metropolitan region, drawing millions of hikers, mountain bikers, picnickers and campers.