AMNA NAWAZ: Well, coal mining has long been known as an arduous and dangerous job.

In addition to working in very tough conditions often miles below the earth's surface, coal miners are also susceptible to the respiratory disease known as black lung.

It currently afflicts one in five veteran miners, and the most severe form of the disease is at the highest it's been in decades.

As William Brangham reports, after years of inaction, new federal rules are aiming to protect those workers.

WOMAN: Catch your breath.

WILLIAM BRANGHAM: These men all worked for years mining coal deep underground, and now they are struggling to extend their lives a few more years.

They all have what is called complicated black lung disease, the most severe, incurable version.

It brutally scars the lungs, making it hard to breathe.

Exercises like this aim to strengthen their remaining lung capacity.

BILLY HALL, Former Coal Miner: A lot of people take breathing for granted.

But when it comes down you can't breathe, that's something else.

And you wanted to do things, and your mind said you could, but then your body says you can't.

WILLIAM BRANGHAM: Sixty-seven-year-old Billy Hall was diagnosed with the disease at 48 after starting work in the mines just out of high school.

Last February, he was lucky.

He got a double lung transplant.

How did you reconcile doing a job you loved when you knew it was also starting to harm your health?

BILLY HALL: Got to feed your family, is one way of putting it, you know?

WILLIAM BRANGHAM: In the last decade, hundreds of coal miners suffering with black lung have come to the New Beginnings Pulmonary Rehab Clinic in Southwest Virginia.

Many of these men are no longer alive.

Scientists have long known that breathing cold dust can cause black lung, a disease that got its name as miners' lungs turned the color of coal.

But an even more dangerous culprit has been identified, silica dust, which comes out of the rocks that get demolished during mining.

That dust is 20 times more toxic to the lungs.

This stretch of Southwestern Virginia is where federal investigators found the largest cluster of complicated black lung cases ever officially recorded.

It's affecting younger miners and sickening them much more quickly, men like Denver Hoskins.

DENVER HOSKINS, Former Coal Miner: My great-grandfather, my grandfather, my dad and me also, we was coal miners.

And, I mean, it -- once you become one, you -- it's hard to get away from.

WILLIAM BRANGHAM: At 17, this Kentucky native had a tryout with the Cincinnati Reds and plan to go to college.

But when his dad got sick with black lung, he stayed home and, like everyone else in his family, went to work in the mines.

At 43, Hoskins was diagnosed with the worst kind of black lung at the Stone Mountain Black Lung Clinic across the border in Virginia.

He now can't work and sometimes requires oxygen to breathe.

Can you put that into words what that is like to live with what you're living with in your chest?

DENVER HOSKINS: Whenever you having that -- having trouble, I guess the best -- the best illustration is cap your hand over your mouth or put a bag over your head, seal it off, see if you can breathe.

I can't get air out, but I can't get enough air in.

WILLIAM BRANGHAM: The reason young miners like Hoskins are getting silica into their lungs is because modern coal mining is using high-powered machinery to cut through increasing amounts of rock to get at this limited amount of coal that is left.

That cutting releases silica dust into the air and they breathe it into their lungs.

DR. BRANDON CRUM, Radiologist: And when you tell the younger monitors they have complicated black lung, you don't get much of a reaction at all.

They're kind of just stunned.

WILLIAM BRANGHAM: Every day, fourth-generation coal miner turned radiologist Brandon Crum reads lung X-rays with the telltale cloudy masses that signal black lung disease.

Since 2015, his clinic in the town of Coal Run, Kentucky, has diagnosed nearly 700 miners with complicated black lung.

DR. BRANDON CRUM: If they never go to work again, they continue to get worse, because, once that dust is in the lungs, it's there for the rest of their life.

And it is a chronic inflammatory process in their lungs with some of them that leads to this significant progression and shortness of breath.

WILLIAM BRANGHAM: Still, Crum says, the initial cause of this severe disease is preventable.

DR. BRANDON CRUM: We don't have to have this disease.

We don't have to have men in their 30s and 40s going through transplants.

We don't have to have wives that are widowed.

This is not our grandfathers' black lung anymore.

This is a whole different aggressive form of black lung that we're seeing.

This is severe, severe lung disease.

WILLIAM BRANGHAM: Some U.S. industries where silica exposure occurs, like construction and fracking, are regulated by OSHA, the Occupational Safety and Health Administration.

That agency recognizes the threat and mandates exposure limits and safety precautions.

But coal mining is covered by a different agency and has been an exception.

That's now changing.

In June, the federal Mine Safety and Health Association, or MSHA, proposed a rule to cut the exposure limit for silica dust in half for a work shift.

CHRISTOPHER WILLIAMSON, Mine Safety and Health Association: This is my great-grandfather's hard hat that he wore when he was a coal miners.

WILLIAM BRANGHAM: Chris Williamson heads MSHA.

CHRISTOPHER WILLIAMSON: I have seen more miners packing around oxygen tanks than I would ever like to see.

And if these things are put in place, all this is entirely preventable.

So it goes back.

We have known for decades that silica cause -- causes workers to be sick.

WILLIAM BRANGHAM: Decades ago, the CDC recommended reducing silica dust in coal mines by half, but no action was taken.

In the 1990s, MSHA warned the industry about silica exposure, but no rules were adopted.

Separately, the National Mining Association, which is the industry's biggest lobbying group, successfully blocked broader regulatory measures.

In 2014, the Obama administration passed a rule limiting overall coal dust, but stopped short of regulating silica.

REBECCA SHELTON, Appalachian Citizens Law Center: Regulations on coal mining have always had pushback from the industry.

The industry is interested in running as much coal as possible, and regulations slow that down.

WILLIAM BRANGHAM: Rebecca Shelton is the director of policy at the Appalachian Citizens Law Center.

She says, when silica levels are high, the best way to protect workers is to increase the spraying of water to tamp down the dust and boosting ventilation inside the mines.

But the industry has argued that giving miners additional respirator masks would be enough.

In a statement to the "NewsHour," the National Mining Association said: "The new proposed rule specifically indicates that the use of respiratory protection equipment cannot be used as a method of compliance.

We believe this is a mistake."

REBECCA SHELTON: We have represented many miners that have been punished for wanting to enforce mine safety regulations.

The enforcement is going to be really challenging if you don't have more sampling.

WILLIAM BRANGHAM: Shelton says effective enforcement will be difficult if Congress doesn't increase funding and resources for MSHA.

She says the agency has seen a 50 percent reduction in enforcement staff over the last decade.

CHRISTOPHER WILLIAMSON: The ultimate goal here is to prevent miners from getting sick.

I can't predict a year, two years from now what resources Congress are going to -- is going to make available to us.

It's going to be a priority.

And we're going to do the best we can with what we have.

And that's outside of my control.

WILLIAM BRANGHAM: In the proposed rule, mine operators will self-report silica dust exposure.

It's a detail that worries safety advocates.

But Williamson says those records would be double-checked by MSHA.

CHRISTOPHER WILLIAMSON: It's in a mine operator's best interest to do sampling in the way that's there too, because they want to know -- they want to know what is the health hazards out there, right?

Like, what is potentially in that environment that may make one of their employees sick?

WILLIAM BRANGHAM: These new rules have come too late to protect Denver Hoskins and countless others.

Men like him worked for decades to mine the coal that helped power and build this country, but they have suffered terribly for it.

On average, a diagnosis like his cuts 12 years off a person's life.

DENVER HOSKINS: I hope and pray that the good man above gives me another 20, 30 years.

I would like to see my children grow, become a granddad and get to see them grown, if it's his will.

But if it ain't.

it ain't.

He's blessed me so far.

He truly has.

WILLIAM BRANGHAM: A final rule on silica dust exposure is expected next year, but would be subject to reversal under a different administration.

For the "PBS NewsHour," I'm William Brangham in Central Appalachia.