The Camp Fire roaring through Northern California — the deadliest and most destructive wildfire in California’s history — has left a stagnant plume of smoke over cities more than 100 miles away, causing stores to sell out of respirator masks and sending some of the state’s most vulnerable to the hospital.

Wildfire smoke, thick with soot and other particles, sent the air quality plummeting in the Bay Area to the “very unhealthy” level — the second-lowest rating, just above “hazardous” — in the week since the Camp Fire started, said Bay Area Air Quality Management District spokesman Walter Wallace. By midday Wednesday, the air-quality advisory for the Bay Region had improved slightly to the “unhealthy” level.

Stagnant weather over San Francisco has held the smoke in place. Limp winds that recently blew through the city could not clear the smoke away. Worse, an inversion layer — a sheet of warm air that traps cold air beneath it — sits like a lid above the haze.

San Francisco General Hospital has not observed any significant increase in respiratory complaints, spokesman Brent Andrew said. But Deanna Watson, 23, said she had to take her 4-year-old son to a hospital after he had trouble breathing on Monday.

She had taken Gideon out to play over the weekend when she noticed the skies become a deeper gray as the smoke began to settle around her home in Orangevale, Calif. By the afternoon, Gideon noticed the change in the gray skies, and his health.

Gideon had been diagnosed with Roifman syndrome, which means he has a compromised immune system and underdeveloped lungs. But this is the first time a change in air quality caused Watson to rush to him to the doctor.

“I was surprised at how quickly the smoke settled here,” Watson said. “I wish I stayed inside more, but the mornings looked clearer.”

Gideon was discharged from a hospital in Roseville, Calif., on Wednesday.

“He keeps asking me why the skies are so gray, but he really doesn’t understand that the smoke caused him to feel this way,” Watson said.

The Camp Fire is one of several raging throughout California. At least 48 people have died since the fire started Nov. 8, and fires elsewhere claimed at least two more lives. Nearly 100 people were listed missing Wednesday on the website of the sheriff’s office in Butte County, where the fire started.

But far away from there, residents say they are feeling the effects.

“With the help of wind, wildfire smoke can float hundreds of miles when there is no rain,” said environmental epidemiologist Jia Coco Liu, who previously studied wildfires at the Johns Hopkins Bloomberg School of Public Health. Liu lives in Palo Alto, far from the fires in Northern California, but she can smell “heavy smoke.”

People older than 65 and children younger than 4 are particularly vulnerable to smoke, according to Ralph Delfino, an environmental epidemiologist at the University of California at Irvine.

“When you grow older, just like your ability to fight off infection goes down, the ability to mount an antioxidant defense decreases, too,” he said.

For the past week, 58-year-old Tori Eckman has felt dreadful, she said.

“I have a sore throat, stinging eyes, and my asthma has flared up,” she said. But her 10-year-old white Bichon, Charlie, needs to be walked, so every day she’s zipped up her orange jacket with its red and white “I voted” sticker, and walked laps around Lower Pacific Heights, a San Francisco neighborhood.

“This is my only outing for the day,” she said. She’s worried about how the smoke will affect Charlie but doesn’t have any alternatives.

Delfino and his colleagues found that, during the 2003 California wildfires, hospital admissions due to asthma increased by a third. As particulate matter in the air increased, so did the percentage of individuals who sought treatment for illnesses like bronchitis and pneumonia. Though smoke inhalation is often associated with respiratory problems, it can harm the cardiovascular system, too. Tiny particles in soot and smoke can irritate lungs and, if they are small enough, enter the bloodstream.

“The primary people at risk are people who already have heart or lung disease,” Delfino said.

Wallace urged people to avoid going outside if possible. “Stay indoors, limit the time outside, don’t risk it,” he said. “Hold off on that morning jog.” Windows should be shut and air conditioners set on recirculate.

Masks, respirators rated N95 and P100, can offer some protection, but only if they fit properly. Many people feel uncomfortable wearing these respirators because they need to sit so snugly against the face, which can be a problem for people with facial hair.

"I’ve lived here for 35 years, and last week was the worst air I’ve ever seen,” said Luis Cauz, 54, a delivery driver for Bi-Rite Food Service. Cauz’s approach is to ignore it; he doesn’t wear a mask or stay indoors, despite being asthmatic.

“I don’t want to wear one. I just feel good; I feel healthy,” he said. “I noticed the air is improving because I’m breathing better now than last week.”

Not everyone can afford to stay inside. Construction workers, mailmen, and utility service workers were working on Wednesday, many of them without masks.

Joseph Vuong, a U.S. postal worker, made his delivery rounds in Folsom, Calif., on Wednesday afternoon. He began delivering mail at 10 a.m. but noticed a drastic change in the air quality by the early afternoon.

“I felt fine being outside in the morning,” Vuong said. “But it’s like I can feel and taste the air now. It’s getting worse.” Vuong’s employer gave him a turquoise N95 mask, but he hasn’t worn it yet. It sits in his mail truck. “Maybe I should start,” he said.

San Francisco’s public health department has published a map of libraries and other public buildings with filtered air. Although the main library branch increased its hours on Friday, fewer than a dozen of the city’s many homeless people stayed late, the Atlantic reported.

Two years ago, Liu and her colleagues predicted that the health impacts of wildfire smoke will increase in the coming decades because of hotter and drier summers. Their model suggested “smoke waves,” multiple days during which certain pollutants exceed healthy levels, would increase in intensity and frequency. In western states in the late 2040s, the number of these smoke waves will increase by half, the epidemiologists calculated.

If their predictions are correct, 82 million individuals will experience more smoke in California, Oregon and the Great Plains.

“It’s very important to point out that this is one of the adverse health impacts of climate change. Unquestionably,” Delfino said. Drought, early springs, late rains and dried-out plants result in bigger, more intense fires, he said. Delfino paused speaking to cough. Smoke had spilled into Irvine, too.

Correction: An earlier version of this story referred to Lower Pacific Heights as a San Francisco suburb. It is a neighborhood.

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