Cases of severe black lung disease, known as progressive massive fibrosis, have surged among coal miners in Central Appalachia to levels not seen in four decades.

In a letter published in the American Journal of Respiratory and Critical Medicine.

In 2012, the prevalence of severe black lung in miners in West Virginia, Virginia and Kentucky reached 3.2 percent, up from a low of 0.4 percent in 1998, according to findings published in the American Journal of Respiratory and Critical Care Medicine. In 1974, the level was 3.3 percent for miners in those states, The Wall Street Journal reported. 

More powerful machines that grind coal into finer particles could be to blame, safety experts say. They also suspect that mining the region's thinner coal seams is churning up more rock and hazardous silica dust.

The study analyzed results from a long-term surveillance program in which miners periodically undergo chest X-rays.

The prevalence reflects the proportion of underground miners with experience of 25 years or more who had X-ray evidence of severe black lung while still working. It excludes less experienced workers and those who became disabled. Prior published reports on the prevalence of black lung included data through 2009.

"We had a general sense that especially in Central Appalachia we were seeing a comeback, but all of us were very surprised by these latest numbers," said David Blackley, a Niosh researcher who published the data.

Davitt McAteer, who was head of the Mine Safety and Health Administration during the Clinton administration, called the new data alarming.

"What we've done by improving the productivity side of coal mining in the last 30 or 40 years has brought with it terrible consequences for the health side," he said.

The current head of MSHA, Joe Main, declined to comment on the new data before reviewing it. "I'm really concerned that this disease is far from over in this country," he said.

Bruce Watzman, a senior vice president of the National Mining Association, said the industry has been aware of increased black lung in the region and companies were taking steps to address the issue.

At the same time, he said the new industry wide dust rules fail to take into account that other regions don't have the same problems. The association filed a lawsuit in federal court in Atlanta in June seeking to block the rules. MSHA is expected to file a reply in October.

"We don't deny the fact that there is a problem in geographic pockets, but MSHA proposed a standard that applies to the entirety of the industry," he said. "That was one of our main objections."

Known medically as coal workers' pneumoconiosis, black lung develops from long-term exposure to coal dust and can lead to shortness of breath, disability and death.

Since Aug. 1, MSHA has collected 2,556 dust samples, while companies have gathered 744 samples. Nine resulted in violations. Main said the results indicate that operators can comply with the rules.

Watzman disputed that the new dust rules have had an effect on conditions in mines. "I don't think we should make any conclusions on such a small population of samples," he said.

Other experts said it would take years to slow the prevalence of black lung in Central Appalachia because the long latency period of the disease means other miners already have likely been exposed to unhealthy levels of dust but haven't been diagnosed.