Health advisories urging residents to stay indoors to limit exposure to the smokey air were issued Saturday from Bakersfield north to Redding, a distance of nearly 450 miles.
Air pollution readings in the region are two to 10 times the federal standard for clean air, said Dimitri Stanich, spokesman for the California Air Resources Board.
Some areas are experiencing the worst air quality on record, with the smoke hanging down to the ground like a fog.
Air quality agencies are especially concerned about small-particle pollution. The tiniest particles can penetrate past the body's immune defenses, traveling deep into the lungs and the bloodstream.
"When you have it on the scale we are seeing now, it is very dangerous to the general public health," Stanich said. "This is a very serious problem."
Changing weather brought smoke-clearing breezes and brief relief to some areas Saturday, but it could also bring lightning storms similar to the ones that ignited fires across Northern California a week ago.
Thunderstorms could strike anywhere in the northern Sierra Nevada or the northern Central Valley on Saturday night, said National Weather Service forecaster Johnnie Powell in Sacramento.
The thunderstorms could also bring a small amount of much-needed rain, he said. The front was expected to pass by Sunday, setting up a second week of abysmal air quality.
The renewed threat of dry lightning and stiffer breezes that could stir the wildfires led fire officials to declare a "red flag warning" - meaning the most extreme fire danger - until 5 a.m. Monday for Northern California.
On Saturday, President Bush issued an emergency declaration for California and ordered federal agencies to assist in firefighting efforts in many areas. Gov. Arnold Schwarzenegger had made the request on Friday.
More than 17,000 firefighters, 1,500 fire engines and bulldozers, and more than 80 helicopters and aircraft were fighting more than 1,000 fires Saturday, said state emergency services spokesman Kelly Huston.
"The summer has just begun, and fire conditions will only get tougher," Ruben Grijalva, director of the California Department of Forestry and Fire Protection, warned in a weekly radio address on behalf of the governor.
The fires have destroyed 47 structures and injured 85 people and continue to threaten nearly 10,000 homes, businesses and buildings, according to his department.
A lightning-sparked wildfire in the Big Sur region of the Los Padres National Forest has burned 42 square miles and destroyed 16 homes. The blaze, which was only 3 percent contained, has forced the closure of a 12-mile stretch of coastal Highway 1 and driven away visitors at the peak of the tourist season.
Farther south in the forest, a wildfire that started three weeks ago has scorched 92 square miles of remote wilderness. It was 80 percent contained Saturday.
Stanich, of the Air Resources Board, advised people to stay inside and keep activity to a minimum. Children, the elderly and people with heart and lung problems are particularly vulnerable, but pollution levels are high enough to affect healthy adults.
Health officials have reported an increase in people complaining of eye and throat irritation and coughing. The poor air can also trigger asthma attacks and bronchitis.
Some veterinary offices said pet owners were bringing in dogs and cats with symptoms ranging from weepy eyes and irritated skin to difficulty breathing or unusual lethargy. Vets were advising that pets remain inside until the smoke clears.
Smoky air canceled this weekend's 100-mile Western States Endurance Run for the first time in its 31-year history. The decision disappointed 370 runners who had traveled from as far away as Africa for the annual race from Squaw Valley at Lake Tahoe to Auburn in the Sierra foothills.
In Sonoma County, the limited visibility kept the Energizer Bunny and dozens of other colorful hot air balloons from lifting off during Saturday's Hot Air Balloon Classic in Windsor.
Cities also closed public pools, canceled softball games and called off July Fourth fireworks displays. Schwarzenegger urged residents not to buy fireworks this year and said local governments should consider an outright ban, though he would not impose one statewide.
In central New Mexico, a blaze caused by lightning that forced the evacuation of 400 people was 35 percent contained. Thunderstorms were forecast, and firefighters welcomed the possibility of rain but feared that winds could change the fire's direction.
In Arizona, an effort to stop a blaze in the Phoenix suburb of Laveen failed Saturday, forcing the evacuation of residents of 16 homes near the brush-choked Gila River bed. The fire has almost doubled in size in a day and consumed nearly 8 square miles, officials said.