More than 20 percent of longtime underground coal miners in Central Appalachia, which includes Eastern Kentucky, have black lung, the highest level recorded in a quarter of a century, according to a study published this week.
The study by researchers at the National Institute for Occupational Safety and Health is the latest to document a troubling increase in black lung, an incurable disease caused by inhaling dust churned up during mining.
Black lung impairs breathing and often leads to premature death.
The study found that 16.1 percent of miners with at least 25 years of experience had simple black lung, the initial phase of the disease, and 4.5 percent had progressive massive fibrosis, the most severe form.
"We can think of no other industry or workplace in the United States in which this would be considered acceptable," researchers said of the level of occupational injury.
Black lung is progressive, so simple black lung can develop into progressive massive fibrosis.
The fact that the disease can take years to develop also means more cases are likely to show up in coming years.
Black lung has caused or contributed to the deaths of more than 76,000 miners since the late 1960s, according to federal figures.
The findings in the report were for miners who had received X-rays between 1970 and 2017.
Not all miners do, but the results likely reflect the prevalence of black lung among all miners, the study said.
Other reports have documented hundreds of cases of black lung among retired miners.
The study released this week said there has been an increase in black lung nationwide since 2000, but the most significant spike is concentrated in Central Appalachia, made up of Eastern Kentucky, West Virginia and Virginia.
The national prevalence of black lung among long-time miners -- those on the job for 25 years or more -- exceeds 10 percent, and more than 5 percent among miners with 20 to 24 years' experience, the study said.
The prevalence of black lung among long-time underground coal miners once topped 30 percent, but declined significantly under rules adopted in 1969 to protect miners' health, including a limit on their exposure to breathable dust.
By the late 1990s, the prevalence among long-time miners was down to about 5 percent, while the prevalence of progressive massive fibrosis dropped from 3.5 percent in the mid-1970s to 0.5 percent, the study said.
However, the disease rebounded beginning about 2000, especially in Central Appalachia.
Researchers have cited a number of potential factors in the resurgence, including miners working longer shifts and therefore having longer exposure to dust; more mining of thinner coal seams, which requires cutting through more rock and so churns out more silica; inadequate dust-control rules; and failure by coal companies to comply with the rules.
The U.S. Mine Safety and Health Administration adopted rules beginning in 2014 aimed at providing better protection for miners, including lower limits on respirable dust and tougher dust-sampling requirements.
The agency recently put out a call for information for a study of the rules, which went into full effect in 2016.
Some mine-safety advocates have been concerned that President Donald Trump's administration would ease the dust rule -- as it has with many Obama-era environmental standards -- but MSHA chief David G. Zatezalo said in a news release that the goal of the study is to see if the new dust rule is working as intended.
"MSHA has no intention of rolling back the protections afforded to coal miners under the final dust rule," Zatezalo said.
MSHA said it could take at least a decade to fully assess the health effects of the rule because of the time it takes for black lung to develop after miners are exposed.
The NIOSH researchers said the increase in black lung indicates the rules remain essential to protect miners.
"Enhancement and diligent enforcement of the 2014 standards remains critical for reversing these trends," the researchers said.
The study was published online Thursday in the journal of the American Public Health Association.