LOS ANGELES — Last week, the U.S. Forest Service and Department of the Interior expanded the situations in which their firefighters are allowed to wear N95 masks.

Starting in September, the federal government began allowing firefighters to wear the masks, but not when they were working on the fire line, only at times such as in camp and sitting in vehicles. Now they’ll be allowed to wear them during some work battling wildfires, including patrolling for areas where the blaze has jumped past fire lines and putting out smoldering remains after a fire is contained.

Masks are still prohibited during firefighters’ most grueling tasks — digging lines to stop fires and directly attacking flames. And the masks they’re using, N95s, do not protect against all of the toxic substances in wildfire smoke.

Nonetheless, health experts applauded the move as a step in the right direction.

Here’s why it took so long to get here:

Research has linked wildfire smoke to a range of long-term health issues, including respiratory problems, cardiovascular diseases and cancers.

“The fire service knows that,” said Rachael Jones, professor and chair of environmental health sciences at UCLA’s Fielding School of Public Health. “They all have stories” of lung damage or cancer, either their own or their coworkers’.

But this wasn’t always common knowledge.

“The verbiage was that wildland smoke was benign,” one firefighter told Jones’ colleagues for a recent study on firefighters’ thoughts on mask use. “It was like sitting around a campfire.”

Even as scientists and fire officials came to terms with the very real long-term health risks, some firefighters still had concerns that masks could muffle communication, make it hard to breathe and interfere with other equipment. That slowed adoption.

Missing crucial orders because a voice is muffled, or struggling to pull out an emergency fire shelter because a mask is in the way, could be the difference between life and death.

“These are not trivial things when the fact is that they reflect life safety outcome,” Jones said.

Deciding on the right type of mask or respirator has slowed adoption too. There are no commercially available respirators that protect against all of the dangerous pollutants in wildfire smoke.

Scientists have not even fully determined which pollutants pose the greatest risks to firefighters, further complicating a choice.

N95s filter for solid particles in the air but not dangerous gases. Heavy smoke or sweat can cause them to clog.

Half-face respirators — often gray rubber with pink canisters — offer different filters for different gases, but none can filter all of the concerning gases in wildfire smoke. The masks and backup canisters are also much bulkier to carry around than N95s.

Respirators that can filter out all of the “literally hundreds” of concerning compounds in smoke “just simply don’t exist,” said Matt Rahn, research director for the Wildfire Conservancy, a nonprofit dedicated to protecting firefighters.

The result: “In our pursuit for perfection in finding the best respiratory devices for firefighters, we’ve basically fallen into a decision paralysis of doing nothing,” Rahn said. “It’s been that way for years.”

The federal government acknowledges the limitations of N95s in its educational material for firefighters. In a statement to The Times, the Forest Service said it will begin studying different respirators in a small pilot program to “determine if their use will be suitable for the wildland fire environment.”

The Forest Service said that N95s are already “readily available” to its firefighters and that it has more than 30,000 of them.

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