It’s July and the Green fire is tearing through Northern California. An elite federal firefighting crew called the La Grande Hotshots has been sent to help. The 24-person crew has been working for days on the front lines, where invisible toxins hide in the thick haze.

More than 1,000 firefighters are on the fire. Several crews, including the La Grande Hotshots, are trying to contain the flames by building a trench of bare earth that will stretch from a road to a river bank. They’re doing this at night, in hopes that the cooler air will tamp down the smoke.

The crew knows that they’re risking their health.

One longtime member died last year after being diagnosed at 40 with brain cancer. A former crew leader is being treated for both leukemia and lymphoma diagnosed in his 40s. Another colleague was recently told that he has the lungs of a lifelong chainsmoker.

Wildfire fighters nationwide are getting sick and dying at young ages, The New York Times has reported. The federal government acknowledges that the job is linked to lung disease, heart damage and more than a dozen kinds of cancer.

But the U.S. Forest Service, which employs thousands of firefighters, has for decades ignored recommendations from its own scientists to monitor the conditions at the fire line and limit shifts when the air becomes unsafe.

To find out how harmful the air gets on an average-size wildfire, Times reporters brought sensors to the Green fire this summer. We tracked levels of some of the most lethal particles in the air, called PM2.5, which are so tiny that they can enter the bloodstream and cause lasting damage.

Readings above 225.5 micrograms per cubic meter are considered hazardous. On the fire line, levels regularly exceeded 500.

Around 6 p.m., the La Grande Hotshots started their shift and set off toward the fire line.

Capt. Nick Schramm, a crew leader, assumed the air was reasonably safe. He has done this work for nearly two decades, and like most firefighters, he often has coughing fits after long shifts. But he believes that exposure to hazardous air is unavoidable.

“That’s just the harsh truth,” he said later.

As climate change makes fire seasons worse, several states have tried to shield outdoor workers from wildfire smoke, which can contain poisons like arsenic, benzene and lead. California now requires employers to monitor air quality during fires, and to provide breaks and masks when the air turns unhealthy.

But these rules don’t apply on the wildfires themselves, because state agencies and private companies successfully argued that those constraints would get in the way of fighting fires.

Until recently, federal firefighters weren’t even allowed to wear masks on the job. Masks are now provided, but they are still banned during the most arduous work, closest to the fire. The Forest Service says face coverings could cause heatstroke, though wildland firefighters in other countries regularly use masks without this problem.

Firefighters say that during their shifts they worry more about immediate dangers — falling trees, burns, sharp tools — than about smoke exposure. As the La Grande crew hiked down the steep terrain, Lily Barnes, a squad leader, concentrated on keeping her footing.

Back home in the off-season, she sometimes wonders what the smoke is doing to her body, she said in an interview. “Maybe I’ll realize one day I shouldn’t have been doing this work.”

The handbook issued to Forest Service crews has 10 words of guidance for smoke exposure on the fire line: “If needed, rotate resources in and out of smoky areas.” The agency declined to comment for this story, but in the past has told The Times that while exposure cannot be completely eliminated, rotating crews helps limit risk.

In practice, according to interviews with hundreds of firefighters, workers feel as though they are sent into smoke and then forgotten. Over months of reporting, Times journalists never saw a boss pull a crew back because of exposure.

Even experienced supervisors can’t tell exactly how unhealthy the air is just by looking.

Chuy Elguezabal, the La Grande superintendent, says he pulls his crews out of smoke when it becomes impossible for them to work — when they cannot see or breathe, or they are overcome by headaches and coughing fits.

On the Green fire, he said, the smoke seemed like more of an inconvenience, like the 105-degree daytime heat or the poison oak that had given many of the firefighters weeping sores.

Since the 1990s, Forest Service researchers have suggested giving crews wearable air sensors, but the agency hasn’t done it. Other dangerous workplaces, like coal mines, have long been required to monitor airborne hazards.

On the Green fire, The Times used a device that weighs as much as a deck of cards and costs about $200.

Last year, firefighters wore the same devices during a small federal research project to measure their exposure. For hours, those readings stayed at 1,000 — as high as the monitors go — according to Zach Kiehl, a consultant who worked on the project.

Mr. Kiehl said that ideally, crews would be issued monitors to know when to put on masks or pull back from a smoky area. “You can pay now and prevent future cases, or pay out later when a person is losing a husband or a father,” he said.

The firefighters believe that the decision to work at night has paid off: The smoke occasionally got thick, but didn’t seem bad compared with other fires they have worked. They think the exposure was fleeting.

In fact, the monitors show, the air was never safe.