The blanket of wildfire soot suffocating the San Francisco Bay Area got thicker Thursday, and air quality quickly deteriorated.

At 3 p.m., San Francisco had among the most polluted air in the region, with an air quality index reading of 235. That's in the "very unhealthy" range, meaning outdoor activity should be avoided.

This marks the the highest reading the city has seen since the Camp Fire started a week ago.

This afternoon, the index was 182 in San Jose, 186 in San Rafael and 217 in Oakland; these readings are in the "unhealthy" range. "Good" air quality falls in the 0 to 50 range.

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Residents have been choking on toxic, eye-stinging smoke for days, as the Camp Fire in Butte County continues to rage. The horrid conditions have shut down schools — San Francisco schools will be closed on Friday — cancelled sporting events and led many to purchase special masks to filter out the particulate matter. San Francisco Airport has seen delays due to low visibility in recent days.

National Weather Service meteorologist Drew Peterson says smoke from the fire is filling the Sacramento Valley, creating a deep reservoir of polluted air. The current weather pattern has a light wind gently pushing the smoke from the valley to the southwest toward the Delta. When the smoke hits this narrower opening in the valley, it fans out, spreading across the Bay Area. Here the air is stagnant, and little smoke is escaping through the Golden Gate's skinny opening.

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"More smoke is able to enter through the Central Valley than is able to exit through the Golden Gate," explains Peterson. "We're just getting this steady feed of smoke from the Central Valley and then once it's here, it doesn't have a way to get out."

He adds: "Within the last 36 hours, this is definitely the worst air quality we've seen. Very little of the smoke is leaving the state. The only smoke that has left the state is going through the Golden Gate Gap and through Point Reyes. Pretty much all the smoke generated by this fire is ending up here."

What's more, a temperature inversion is helping to trap the smoke.

"During the days, we're seeing 70s, but at night the temperatures are really cold in the 30s and 40s," explains Simrun Dhoot, a spokesperson with the Bay Area Air Quality Management District. "That's creating an inversion layer and that's what trapping the air pollution near the ground."

Livermore sits just south of the Delta and the smoke is pouring into this area, creating another reservoir of crummy air. At 8 a.m., Livermore registered the worst reading in the region with an air quality index creeping into the "very unhealthy range" at 205. By noon, things had improved slightly and the index dropped to 193. On Thursday morning, visibility in the East Bay town was one mile; on a day with clean air, it's more than 10 miles.

The current weather pattern is expected to hold through the weekend with the bad air quality continuing. The overall air quality will likely be worse on Friday than on Thursday. "Tomorrow is expected to be in the 'unhealthy' to 'very unhealthy' range," says Dhoot.

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Sunday night a weak system is expected to kick up more powerful winds that will mix up the atmosphere and clear out some of the smoke. Later in the week, rain is expected to clear up the air even more.

"I want people to know they're not the only ones suffering," says Peterson. "The air quality is worse in Sacramento and Butte County. As bad as this is, there are still people who have lost everything in this fire."