It kills more people each year than car crashes, war or drugs do. This invisible killer is the air pollution from sources like cars and trucks or factory smokestacks.

But as wildfires intensify and grow more frequent in a warming world, the smoke from these fires is emerging as a new and deadly pollution source, health experts say. By some estimates, wildfire smoke — which contains a mixture of hazardous air pollutants like particulate matter, nitrogen dioxide, ozone and lead — already causes as many as 675,000 premature deaths a year worldwide, as well as a range of respiratory, heart and other diseases.

Research shows that wildfire smoke is starting to erode the world’s progress in cleaning up pollution from tailpipes and smokestacks, as climate change supercharges fires.

“It’s heartbreaking, it really is,” said Dr. Afif El-Hasan, a pediatrician who specializes in asthma care at Kaiser Permanente in Southern California and a board director of the American Lung Association. Wildfires “are putting our homes in danger, but they’re also putting our health in danger,” Dr. El-Hasan said, “and it’s only going to get worse.”

Those health concerns were coming to the fore this week as wildfires ravaged the Los Angeles area. Residents began to return to their neighborhoods, many strewed with smoldering ash and rubble, to survey the damage. Air pollution levels remained high in many parts of the city, including in northwest coastal Los Angeles, where the air quality index climbed to “dangerous” levels.

Los Angeles, in particular, has seen air pollution at levels that could be raising daily mortality by between 5 to 15 percent, said Carlos F. Gold, an expert in the health effects of air pollution at the University of California, San Diego.

That means current death counts, “while tragic, are likely large underestimates,” he said. People with underlying health issues, as well as older people and children, are particularly vulnerable.

The rapid spread of this week’s fires into dense neighborhoods, where they burned homes, furniture, cars, electronics and materials like paint and plastic, made the smoke more dangerous, said Dr. Lisa Patel, a pediatrician in the San Francisco Bay Area and the executive director of the Medical Society Consortium on Climate and Health.

A recent study found that even for homes that are spared destruction, smoke and ash blown inside could adhere to rugs, sofas and drywall, creating health hazards that can linger for months. “We’re breathing in this toxic brew of volatile organic compounds and polycyclic aromatic hydrocarbons and hexavalent chromium,” Dr. Patel said. “All of it is noxious.”

Intensifying and more frequent fires, meanwhile, are upending experts’ understanding of smoke’s health effects. “Wildfire season is no longer a season,” said Colleen Reid, who researches the effects of air pollution from wildfires on heath at the University of Colorado Boulder. “We have fires all year round that affect the same population repeatedly.”

“The health impacts are not the same as if you were exposed once, and then not again for 10 years,” she said. “The effects of that is something that we still don’t really know.”

A United Nations report from 2022 concluded that the risk of devastating wildfires around the world would surge in coming decades. Heating and drying caused by climate change, along with development in places vulnerable to fire, was expected to intensify a “global wildfire crisis,” the report said. Both the frequency and intensity of extreme wildfires have more than doubled in the past two decades. In the United States, the average acreage burned a year has surged since the 1990s.

Now, pollution from wildfires is reversing what had been a decades-long improvement in air quality brought about by cleaner cars and power generation. Since at least 2016, in nearly three-quarters of states in the U.S. mainland, wildfire smoke has eroded about 25 percent of progress in reducing concentrations of a type of particulate matter called PM 2.5, a Nature study in 2023 found.

In California, wildfire smoke’s effect on air quality is offsetting public health gains brought about by a decline in air pollution from automobiles and factories, state health officials have found. (By releasing carbon dioxide and other planet-warming gases into the atmosphere, wildfires are themselves a big contributor to climate change: The wildfires that ravaged Canada’s boreal forests in 2023 produced more greenhouse gases than the burning of fossil fuels in all but three countries.)

”It’s not a pretty picture,” said Dr. Gold of U.C. San Diego, who took part in the Nature study. If planet-warming gas emissions continue at current levels, “we’ve got some work that suggests that mortality from wildfire smoke in the U.S. could go up by 50 percent,” he said.

One silver lining is that the Santa Ana winds that so ferociously fueled the flames in recent days have been blowing some of the smoke toward the ocean. That stands in contrast to the smoke from the 2023 Canadian wildfires that drifted to New York and other American states hundreds of miles away, causing spikes in emergency room visits for asthma.

At one point that year, more than a third of Americans, from the East Coast to the Midwest, were under air quality alerts from Canadian wildfire smoke. “We’re seeing new and worsening threats in places that are not used to them,” Dr. Patel, the pediatrician, said.

The new normal is bringing about changes to health care, Dr. Patel said. More health systems are sending out air quality alerts to vulnerable patients. In the small community hospital where she works, “every child that comes in with wheezing or asthma, I talk to them about how air pollution is getting worse because of wildfires and climate change,” she said.

“I teach them how to look up air quality, and say they should ask for an air purifier,” Dr. Patel added. She also cautions that children should not participate in cleanup after a wildfire.

Scientists are still trying to understand the full range of wildfire smoke’s health effects. One big question is how much of what researchers know about vehicle exhaust and other forms of air pollution apply to wildfire smoke, said Mark R. Miller, a researcher at the Center for Cardiovascular Science at the University of Edinburgh who led a recent global survey of climate change, air pollution and wildfires.

For example, exhaust particles “are so small that when we breathe them in, they go deep down into our lungs and are actually small enough that they can pass from our lungs into our blood,” he said. “And once they’re in our blood, they can be carried around our body and start to build up.”

That means air pollution affects our entire body, he said. “It has effects on people who have diabetes, has effects on the liver and the kidney, it has effects on the brain, on pregnancy,” he said. What’s still not clear is whether pollution from wildfires has all of those same effects. “But it’s likely,” he said.

Experts have a range of advice for people living in areas with smoke. Keep an eye on air quality alerts, and follow evacuation orders. Stay indoors as much as possible, and use air purifiers. When venturing outside, wear N95 masks. Don’t do strenuous exercise in bad air. Keep children, older people and other vulnerable groups away from the worst smoke.

Ultimately, tackling climate change and cutting back on all kinds of air pollution is the way to reduce the overall burden on health, said Dr. El-Hasan of the American Lung Association. “Can you imagine how much worse things would be if we hadn’t started cleaning up emissions from our cars?” he said. “I’m trying to think, glass half full, but it does break my heart and it does worry me.”