Far from the fires, the deadly risks of smoke are intensifying.

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. Gould, 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.