At the end of the 20th century, black lung, a common term for several respiratory diseases caused by coal mine dust exposure, hit an all-time low. Experts reported just 31 cases of the worst form of the disease. But reports of black lung have surged. Now, epidemiologists from the National Institute for Occupational Safety and Health (NIOSH) have found 416 cases of complicated black lung (progressive massive fibrosis​) in just three clinics from 2013 to 2017, report Howard Berkes and Adelina Lancianese for NPR.

Since 1970, the Coal Workers' Health Surveillance Program, administered by NIOSH, has monitored the health of coal miners offering free chest radiographs and examinations. In 2014, the program reported an increase in black lung disease or coal workers' pneumoconiosis. Last year, the director of the three clinics (not associated with the surveillance program) asked researchers for help in determining the scope of the problem. The results of that request were detailed this week in a letter published in the Journal of the American Medical Association.

"This is the largest cluster of progressive massive fibrosis ever reported in the scientific literature," Scott Laney, a NIOSH epidemiologist involved in the research tells NPR.

Black lung is caused by the inhalation of coal mine dust. As the miners cut into coal seams, microscopic particles of dust are kicked into the air and can slip into miner's lungs, where they become trapped. These particles include not just coal, but also silica—a potent lung irritant, as Maya Wei-Haas reported for Smithsonian.com last year. The particles provoke the body's immune response, but because the invaders are mineral—rather than bacterial or viral—the response spirals. Cells die, and inflammation damages tissues. Overtime, the lungs gradually degrade, depriving victims of air. Only a lung transplant can replace the dying tissue, but miners need to be healthy enough to qualify for a transplant. And even then, transplants only extend patient's lives by only a few more years.

Since the survey portion of the NIOSH study concluded a year ago, reports for black lung have continued. The three clinics, who serve populations in Virginia, Kentucky and West Virginia, have since diagnosed an additional 154 new miners.

These clinics are just one window into the problem. In 2016, NPR conducted its own survey of 11 black lung clinics in Virginia, West Virginia, Pennsylvania and Ohio and found 962 cases, Berkes reported in December 2016. Since then, the ongoing investigation has counted 1,000 more, he writes.

The new study also confirms previous observations that miners are getting diagnosed with the disease earlier than in the past. Miners with black lung are now younger and have been mining for far less time than those decades before. More studies are now underway to figure out just how many miners have the disease and to better understand what may be causing it, Berkes and Lancianese write for NPR.

The NPR investigation and other experts point to longer work shifts, the mining of thinner seams that produce dust with more silica and to retirements and layoffs that may be sending miners who previously put off check-ups into the clinics.

In 2012, Rachel Nuwer wrote about the resurgence as well as the trend for younger miners to receive a diagnoses for Smithsonian.com. Recognizing this problem isn't new, but figuring out why it is happening and what to do is taking time.

New regulations intended to protect workers from dangerous coal dust were fully implemented in 2016, but as Benny Becker reports for The Ohio Valley Resource, President Trump has ordered a review of those rules as part of his push to rollback regulations.

Meanwhile, Berkes and Lancianese report that 50,000 coal miners are still working.