SAN FRANCISCO (AP) — Smoke from the wildfires north of San Francisco plunged air quality levels in the Bay Area to the same unhealthy level as China's notoriously polluted capital, sending people to emergency rooms and forcing schools to close and people to wear masks when they step outside.

The region has endured days of choking smoke since the fires began Sunday night and claimed at least 31 lives and destroyed thousands of homes and businesses. The smoke cast a dull haze over San Francisco's scenic skyline, and poor visibility has led to numerous flight delays and cancellations at the city's airport.

Air quality in most of the region Thursday and Friday was as bad as smog-choked Beijing, according to the Bay Area Air Quality Management District.

"We have unprecedented levels of smoke and particles in the air that we normally don't see," said Ralph Borrmann, a spokesman for the district.

He called it the worst air quality ever recorded in many parts of the Bay Area.

Since Wednesday, about 15 percent of flights in and out of San Francisco International Airport experienced delays because air traffic controllers have had to separate aircrafts at a greater distance, airport spokesman Doug Yakel said Friday.

Officials warned that very fine smoke particles, thinner than a human hair, can get lodged in the lungs and into the bloodstream, causing irreparable damage to the body. In Solano County, hospitals there received more than 250 people who complained of toxic air inhalation, county health officer Bela Matyas said Thursday.

At an Ace Hardware store in San Francisco's financial district, phones were ringing nonstop with customers looking to buy breathing masks. But they were sold out, as were most stores in the area.

With winds expected to keep blowing in smoke from the fires to populated areas this weekend, many schools decided to close Friday. Organizers canceled weekend events, including an Oktoberfest in Walnut Creek and a fitness festival and half marathon in San Francisco.

Sports teams are monitoring the air quality as they prepare to host games.

The Oakland Raiders are still planning to host the Los Angeles Chargers in Oakland on Sunday, but the team said it will keep monitoring and will update fans if there are changes. The NFL had considered moving the game if it becomes necessary.

Officials at the University of California, Berkeley and Stanford University planned to proceed with games Friday and Saturday, respectively. Stanford encouraged fans to bring donations for victims of the fires to the game.