As the Kincade Fire burned through some 80,000 acres in Northern California, Ismael Barcenas, felt his lungs burning and “knots in [his] throat.” Barcenas, a farmhand at a vineyard in Santa Rosa, has asthma but kept showing up to work and choking through the smoke. After a few days, Barcenas left the county for cleaner air and checked into a hotel.

With strong gusts of wind blowing east, smoke from the Kincade Fire spread all the way to Sacramento last week, about 60 miles from where Barcenas works. There, Michal Borton, a student at Cosumnes River College, found it difficult to breathe as a well-ventilated chemistry lab let smoke in. Borton ended up leaving in the middle of the class.

Several hundred miles south near Long Beach in Southern California, Demetria Maldonado called in sick from her job as an aide for students with special needs. Smoke from the Getty and Castlewood fires had her coughing all day.

Monster fires in California have killed at least three people so far and burned tens of thousands of acres over the last couple of weeks. At least five fires are burning in the state; the Kincade Fire — which began two weeks ago — is still just 88 percent contained. The blazes have closed schools and businesses, forced hundreds of thousands of people to evacuate, and left behind charred rubble where entire communities once stood.

The effects have also been felt by people out of the path of the fires. Smoke from the Kincade Fire hung over the Bay Area for days, resulting in school closures and a “Spare the Air” alert — a call to avoid driving in order to reduce pollution. In Oakland, Fresno, Visalia and other cities, local public health officials have reported “unhealthy” and “very unhealthy” levels of air pollution and asked residents to stay indoors as much as possible.

Of primary concern is particulate matter, specifically PM2.5 — fine particles of soot and dust that are about 30 times smaller in diameter than a strand of human hair. They can burrow their way deep into the lungs, causing asthma and cancer. As wildfires burn through towns, spurred on by a warmer and drier climate, that soot and dust also picks up toxic chemicals from burning buildings.

“Things like lead or other toxins can attach on to that particular matter,” said Mary Prunicki, a pollution biologist at Stanford University. “When that’s inhaled, these other heavy metals or toxic pollutants hitchhike on the PM2.5.”

Researchers expect that particulate matter from wildfires will rise dramatically in the Western U.S. as the planet warms. One study estimates that between 2046 and 2051, wildfire-related PM2.5 levels will likely increase by 160 percent on average if temperatures continue to rise. Northern California, the Pacific Northwest, and forests in the northern Rocky Mountains will experience the worst of it, the researchers concluded.

Hotter and longer fires, especially those burning through towns with plastic and chemical materials, may also mean more toxic particulate matter, Prunicki said. “It may make things combust that otherwise wouldn’t, and when that’s put into the air, it can attach on to the particulate matter.”

Barcenas, the farmworker, has been working at the same vineyard for over two decades but said that the fires this year had him reaching for his inhaler more often than when blazes swept Northern California in 2017 and 2018. Leaving the county means missing work and less money for his family. “To me the worst thing about this fire is I’ve been without work for six days, and now four more days,” he said in Spanish. “In the last fire, I was out only for one day.” He fears that if the fires continue like this, year after year, it could shutter farms in the region and put him out of work.

Barcenas and other asthma sufferers who’ve struggled to breathe the last couple of weeks may discover new health problems months from now. Researchers have found that wildfire smoke can trigger cardiovascular and respiratory illnesses months after the initial exposure, sometimes leading to premature deaths.

One study found that smoke from the Camp and Woolsey fires in California last year contributed to the premature deaths of as many as 1,400 people. That’s excluding the 88 people who died during the fires. A separate analysis last year by Reveal, a nonprofit news organization, concluded that in the months after the 2017 Tubbs Fire in Northern California that left at least 22 dead and burned about 37,000 acres, emergency rooms saw a 20 percent increase in visits by patients for cardiovascular diseases.

That spike in health problems is felt most acutely by the young, elderly, and people of color — in part a function of where they live. When researchers looked at Medicare hospital admission data between 2004 and 2009 for the Western United States, they found that more than 70 percent of black patients were exposed to more than one smoke wave, compared to just 56 percent of white patients. Overall, black residents in the West had a higher risk of hospital admissions as a result of respiratory illnesses.

A lot about wildfire smoke and public health remains unknown. Though many people wear masks as a protective measure, research on their effectiveness is scant, Prunicki said. It’s a question that she and a colleague are hoping to tackle along with looking into whether air purifiers can help people avoid breathing difficulties and other illnesses.

“There’s very little data when you try to guide people on who should be putting on masks,” said Prunicki. “That just makes it hard to make recommendations on what people should do because there’s not research to back it up.”

People also have different levels of comfort with masks. While Maldonado, the special education aide in Southern California, said that using a mask and putting a scarf across her mouth helped her breathe better, Borton, the college student, said that he found wearing a mask suffocating. He relied instead on two daily medications for asthma. “If I wear a mask, then I’m mostly just breathing in the air that I breathed out,” he said. “I just have to suffer through it.”

Jorge Rodriguez assisted with translations for this story.