SACRAMENTO, Calif. -- 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.

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 high readings of 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 15,000 firefighters, 1,000 fire engines and more than 80 helicopters and aircraft were fighting more than 1,000 fires Saturday, said Ruben Grijalva, director of the California Department of Forestry and Fire Protection.

"The summer has just begun, and fire conditions will only get tougher," Grijalva warned in a weekly radio address on behalf of the governor.

Areas hardest hit include Butte County, where 31 fires have burned 19 square miles and threatened 1,200 homes; Mendocino County, where 121 fires have burned 45 square miles and threatened 900 homes; and Shasta and Trinity counties, where about 160 fires have burned 58 square miles and threatened 230 homes.

South of the tourist town of Big Sur in the Los Padres National Forest, a wildfire that started three weeks ago had burned 92 square miles and destroyed 16 structures including two homes. 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.

They said surgical masks, wet cloths and bandanas are not enough to filter the smoke. Only N95- and P100-rated masks filter out the smallest and most dangerous particles.

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.

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Associated Press writers Terence Chea and Amanda Fehd contributed to this report.