SANTA ROSA, Calif. — For generations, firefighters fought mostly in desolate forests, where most of the dangers were fatigue and falling trees. But a confluence of modern factors — namely America’s rapid suburban expansion into the wilderness, combined with the growing ferocity of wildfires — is posing a host of new health threats to the men and women who fight these blazes.

While burning wood poses some threat to lungs, man-made products and the gases and particles they produce when burned are far more dangerous. In the last three years, California has seen a record number of devastating fires, and thousands of firefighters have been exposed to chemicals they had not previously encountered in such high volumes.

Unlike urban firefighters dealing with structural blazes, these wildfire responders do not wear heavy gear that filters air or provides clean air because the gear is unwieldy and too limited to allow the kind of multi-hour, high-exertion efforts demanded on the front lines of these large outdoor infernos.

Capt. Matt Alba, who has been with the San Francisco Fire Department for 18 years, spent 11 days working in Paradise, California, last year, in a smog so thick it burned his lungs. As he picked his way through the wreckage, he said, his crew began to fall sick: severe headaches, brutal coughs.

“I was just thinking about 9/11,” he said of the many firefighters who fell ill after the 2001 terrorist attacks. “I asked myself: Is history repeating itself here?”

Several studies have examined the health of firefighters who battle structural blazes. The largest, by the National Institute for Occupational Safety and Health, acknowledged that urban firefighters may be exposed to carcinogens, and found firefighters have higher rates of several types of cancers than the population as a whole.

On Thursday, California’s fight against fire continued. More than 7,000 firefighters were battling blazes up and down the state, including new wildfires in the heavily populated areas of Riverside and San Bernardino counties. The fires began just as winds eased in the north and firefighters wrangling the state’s largest active blaze, the Kincade fire, managed to contain more than half of its 76,800-acre footprint for the first time.

About 5,800 people remained under a mandatory evacuation order, a small fraction of the 180,000 who had been ordered to leave their homes Sunday.