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

But as wildfires intensify and become more common in a warming world, smoke from those fires is becoming a new and deadly source of pollution, health experts say. According to some estimates, wildfire smoke — which contains a mix of dangerous air pollutants such as particulate matter, nitrogen dioxide, ozone and lead — is already causing as many as 675,000 premature deaths year all over the world, as well as a number of respiratory, heart and other diseases.

Research shows that the smoke from the fire is starting to spread they are eroding world progress in cleaning pollution from exhaust pipes and chimneys, because climate change encourages fires.

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

Those health concerns came to the fore this week as wildfires raged through the Los Angeles area. Residents started returning to their neighborhoodmany strewn with smoldering ash and rubble, to survey the damage. Air pollution levels remained high in many parts of the cityincluding the northwest coast of Los Angeles, where the air quality index rose to “hazardous” levels.

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

That means the current death toll, “while tragic, is probably a significant underestimate,” he said. People with underlying health problems, as well as the elderly and children, are particularly vulnerable.

The rapid spread of this week’s fires in densely populated neighborhoods, where homes, furniture, cars, electronics and materials like paint and plastic burned, made the smoke more dangerous, said Dr. Lisa Patel, San Francisco Bay Area Pediatrician and CEO Medical Society Climate and Health Consortium.

A recent study found that even for homes that are spared destruction, smoke and ash blown inside can stick to carpets, sofas and drywall, creating health hazards which can last for months. “We breathe in this toxic mixture of volatile organic compounds and polycyclic aromatic hydrocarbons and hexavalent chromium,” said Dr. Patel. “It’s all harmful.”

Meanwhile, increasingly intense and frequent fires are changing 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 heathlands at the University of Colorado Boulder. “Throughout the year, we have fires that repeatedly affect the same population.”

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

A 2022 United Nations report concluded that the risk of devastating forest fires worldwide will increase in the coming decades. Warming and drying caused by climate change, along with development in fire-prone places, is expected to intensify “global forest fire crisis“, the report states. And the frequency and intensity of extreme forest fires more than doubled in the last two decades. In the United States, the average acreage burned a year has increased since the 1990s.

Now, pollution from wildfires is reversing decades of improvements in air quality brought about by cleaner cars and electricity generation. As of at least 2016, in nearly three-quarters of states in the mainland US, smoke from wildfires has undermined about 25 percent of progress in reducing concentrations of a type of particulate matter called PM 2.5, nature study Found in 2023.

In California, the effect of wildfire smoke on air quality offsets the public health gain caused by a reduction in air pollution from cars and factories, state health officials found. (By releasing carbon dioxide and other planet-warming gases into the atmosphere, wildfires themselves contribute greatly to climate change: Wildfires Ravaging Canada’s Boreal Forests in 2023 produced more greenhouse gases but the burning of fossil fuels in all but three countries.)

“It’s not a pretty picture,” said Dr. Gold from UC San Diego, who participated in the Nature study. If emissions of the planet-warming gases continue at current levels, “we have some work suggesting that wildfire smoke deaths in the U.S. could increase by 50 percent,” he said.

One bright side is that the Santa Ana winds that have fanned the flames so fiercely in recent days have blown some of the smoke toward the ocean. This contrasts with the smoke from the fires in Canada in 2023 who wandered off to New York and other US states hundreds of miles away, causing spikes 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 an air quality advisory due to smoke from Canadian wildfires. “We are seeing new and worsening threats in places that are not used to them,” said Dr. Patel, pediatrician.

The new normal brings changes in health care, said Dr. Patel. Multiple health systems are sending air quality alerts to vulnerable patients. At the small community hospital where she works, “every child who 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 check the air quality and say they should ask for an air purifier,” added Dr. Patel. He also warns that children should not participate in cleaning up after a fire.

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

For example, exhaust particles “are so small that when we inhale them, they go deep 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.”

This means that air pollution affects our whole body, he said. “It affects people who have diabetes, it has effects on the liver and kidneys, it has effects on the brain, it has effects on pregnancy,” he said. What is still not clear is whether pollution from wildfires has the same effects. “But it probably is,” he said.

Experts have a number of tips for people living in smoky areas. Heed air quality warnings and follow evacuation orders. Stay indoors as much as possible and use air purifiers. When you go out, wear N95 masks. Do not do strenuous exercises in bad weather. Keep children, the elderly and other vulnerable groups away from the worst of the smoke.

Ultimately, dealing with climate change and reducing all types 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 didn’t start cleaning the exhaust from our cars?” he said. “I try to think, the glass is half full, but it breaks my heart and worries me.”