For Wolfe and other researchers at the National Institute of Occupational Safety and Health (NIOSH), the resurgence of this old-time killer is not just a moral outrage. It's also an epidemiological mystery. After decades of steady decline, why has black lung disease made a comeback? Why is it progressing faster and striking younger miners - those who have spent less time on the job, and who never worked in the bad old days before the federal law took effect? And why is the spike limited to a few geographic hot spots?

The NIOSH researchers in Morgantown, West Virginia, have a couple of theories. Neither is yet proven, but they are determined to solve the deadly mystery.

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Dr. Edward L. "Lee" Petsonk was a respiratory disease specialist, but not a coal mining expert, when NIOSH put him in charge of its Morgantown-based black lung program about 10 years ago. The program offers each underground coal miner a free chest X-ray every five years. That's how NIOSH tracks rates of the disease.

Although coal production is booming, the number of underground miners has dropped to fewer than 45,000 nationwide. For a variety of reasons, most miners don't take advantage of the X-rays. So when Petsonk took over the black lung program, he decided it was feasible to orient himself by looking at every single new X-ray that came in.

"After a couple of years, something changed," he recalls. "I began to see the type of disease that was only in the textbooks -- this massive fibrosis, where the lung is basically destroyed. It's nothing but black scar tissue. I was incredulous. And it was young people. It wasn't the older miners. I thought, something is wrong here. We decided we'd better do some research."

Dressed in the khaki uniform of the U.S. Public Health Service, Petsonk sat down for a two-hour interview in Morgantown in December 2008, just days before he retired. He was joined by Wolfe, a public health analyst who sipped coffee from a blue-and-yellow West Virginia mug, and Michael Attfield, their sweater-clad boss.

Lean and balding, Petsonk teases Wolfe about taking black lung disease personally. But he shows every bit as much passion about solving the mystery of the lethal illness.

As soon as he stumbled upon the surprisingly advanced cases, "I said, 'The clock is running,'" he recounts. "'These miners out there are getting sick and dying, and we know about it.'"

The team presented its information to NIOSH's then-director, John Howard. He agreed it was urgent and came up with money for a mobile X-ray unit. Headed by Wolfe, a crew would take the van out into the field, encouraging miners to come for black lung screening. The purpose was twofold: to alert miners who had developed disease, and to gather data so NIOSH could fill in the current black lung picture.

In September 2007, Petsonk reported that the disease rate had more than doubled among miners who worked 25 years or more underground, from about 4 percent in 1997 to 9 percent in 2006. The rate among miners with 20 to 24 years' experience jumped even more, from 2.5 percent to 6 percent. While those are still small percentages, the trend is going in the wrong direction.

"The statistics are important, and they help us pinpoint and evaluate the problem," Petsonk says. But statistics don't tell the whole story. The federal Mine Safety and Health Act, passed in 1969 and fully effective since 1973, was specifically designed to eliminate the most advanced black lung cases altogether. Even without the statistics, "what we know is these cases of young people getting sick. And that's wrong. That's a failure of the Act."

"The real tragedy," he continues, "is that these are hardworking people who are doing a service for their companies and our society, and what they get for it is a really -"

"Raw deal," Wolfe interjects.

"Well, more than a raw deal," Petsonk replies. "If you see the suffering of a person struggling to breathe, every minute of every day, this is like a diabolical torture."

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So what have the NIOSH researchers learned that might explain the increased torture?

The short answer is: The miners are breathing too much dust. That, pure and simple, is what causes the inflammation and scarring that characterize black lung.

Under the mine act of 1969, dust in coal mines must total no more than 2 milligrams per cubic meter of air. That's too high, the researchers agree: Since 1995, NIOSH has recommended cutting the limit in half, to 1 milligram per cubic meter. But NIOSH, which is part of the Centers for Disease Control, has no regulatory power. That falls to a separate agency, the labor department's Mine Safety and Health Administration, which has declined to set a stricter standard.

But the short answer doesn't solve the mystery, since miners have been breathing dust for as long as they've been digging coal out of the earth. Something has changed to make the disease more common and more aggressive in the hot spots of southern West Virginia, eastern Kentucky and western Virginia. In the western U.S. coal mines, by contrast, black lung rates continue to fall.

Petsonk figures that, whatever changed, it probably began in the 1980s, since black lung takes years to develop. He, Wolfe and Attfield offer two main explanations.

First, miners are working longer hours. The 2-milligram dust limit "was set for an eight-hour shift [and] a 40-hour week," Petsonk says. "Most miners now say they're working 60-hour weeks, and often 12-hour or 16-hour shifts."

That packs a double whammy, he explains. "If you work 50 percent more, not only do you get 50 percent more dust in, but you have a lot less time to cough it out. The effect on the lungs is greater than would be considered just from the increase of work hours."

Indeed, statistics on the MSHA website show that the average underground coal miner worked just over 2,000 hours in 1998, a peak production year. That marked a 32 percent jump from 1978. (Work hours continued to rise through 2007, to more than 2,100 per miner.)

During that same 20-year period of 1978 to 1998, productivity more than quintupled, to 9,545 short tons per miner. "They are working hard, fast, and generating lots of dust," Petsonk observes. What's more, "they're using very aggressive equipment" that also may produce more dust than older mining techniques.

Wolfe and Attfield are currently trying to track down data that would let them look for a possible correlation between longer work hours and black lung rates.

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A second theory isn't about coal at all. It's about rock.

In the old Appalachian areas that are black-lung hot spots, "all the easy coal has been mined," Wolfe notes. Much of the remaining black fuel lies in what the industry calls "thin seams," 28 inches or less.

Coal companies used to leave the thin seams alone, because mining Them brought too much rock along with the coal. But Petsonk says modern techniques make it easier to wash out the rock. And rising coal prices make that effort pay off. "The coal industry tells me, 'Yeah, we take up to 40 percent rock.'"

So what?

Well, the rock contains silica, which is 20 times as toxic as coal dust. If miners are breathing even a slightly higher percentage of silica dust than in the past, they could be seeing significantly more silicosis. That lung ailment is medically distinct from black lung -- but both diseases produce "small, rounded cavities in about the same area of lung," Petsonk says.

A pathology lab can easily tell the diseases apart. But "on the X-ray, they don't look that different. In any individual miner, it's really hard to look at the X-ray and say, 'that's silicosis' or 'that's black lung.'" So Petsonk suspects that some of what is being diagnosed as an increase in black lung is actually an increase in silicosis.

The NIOSH team has prepared some research on silica levels and black lung rates, which it will present at a conference in May. Petsonk says he can't reveal the results yet, but implies that they should answer some of the questions about the resurgence of black lung disease. Meanwhile the researchers -- like the miners themselves - will keep digging.

Carole Bass was a 2008 fellow of the Alicia Patterson Foundation, whose financial support paid for the reporting and writing of this article.

Click on the play arrows here and at the top of this story to watch a two-part video the NIOSH researchers prepared about their research.