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.

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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.

Smoke and haze fill the morning air in San Francisco on October 13, 2017. (CBS)

Smoke and haze fill the morning air in San Francisco on October 13, 2017. (CBS)

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 the inaugural 2017 Virgin Sports Festival of Fitness in San Francisco which featured a half marathon.

Thursday afternoon, Another Planet Entertainment announced that two shows by rock group the National scheduled for this weekend at the Greek Theatre in Berkeley have been postponed.

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Sports teams are monitoring the air quality as they prepare to host games. Some members of the Oakland Raiders wore masks during workouts Thursday.

The NFL has been exploring options to move Sunday’s game between the Oakland Raiders and Los Angeles Chargers if it becomes necessary.

Oakland, about 45 miles south of the fires, has been blanketed by smoke.

The Cal football game against Washington State scheduled for Friday night at Memorial Stadium in Berkeley is currently still set to start at 8 p.m.

The Pac-12 said that the conference and schools decided Friday afternoon to play the game as scheduled. The conference said the safety of players and fans drove the decision. Cal will work with the Bay Area Air Quality Management District to monitor conditions leading up to game time.

A sustained air quality index of 200 or more is the NCAA threshold for delaying or canceling games. Early Friday evening, Berkeley had an index of 177.

Another football game at Stanford is scheduled for Saturday night with the Cardinal hosting the Oregon Ducks.

Early Friday evening, Stanford’s air quality index was at 115.

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