Hundreds of lightning-sparked wildfires have turned the air of Northern California into an unhealthy stew of smoke and ash, forcing the cancellation of athletic events and other outdoor activities.

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.

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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.

On Saturday, President George W. 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 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.

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.

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.

Near Las Vegas, a small plane crashed in a forested area, killing four people and touching off a wildfire that is threatening about 40 homes, said Ian Gregor, spokesman for the Federal Aviation Administration. Authorities recommended that residents evacuate.