California ? 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.

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.