A collection of studies has found that more coal miners in central Appalachia have suffered the advanced stages of the deadly disease black lung than previous government research has found, and more miners working in the region today have earlier stages of the disease.

NPR, which first reported on the issue in 2016, that Kirsten Almberg, an occupational health researcher at the University of Illinois, Chicago and co-author of a black lung study discussed their findings at a San Diego conference of the American Thoracic Society.

For their study, Almberg and Robert Cohen looked at claims filed with the federal government since 1970. It was the first time this method has been used and they found that more than 4,600 cases of severe black lung, also known as progressive massive fibrosis or PMF. More than half the cases they found occurred in the last 16 years, NPR reported.

They also found sharp increases in disease year after year in central Appalachian coal mining states, including 30 percent in West Virginia and 16 percent each in Kentucky and Virginia.

Previous research has been limited to a federal black lung testing and surveillance program to track the occurrence of simple and advanced black lung, conducted for decades by the National Institute for Occupational Safety and Health in every coal region of the country.

The problem with that method is that is only for working miners and it is voluntary. NIOSH says only about 40 percent of working miners get the X-rays that can diagnose the disease. That means these official records have vastly undercounted the problem. From 2011 to 2016, NIOSH counted just 99 cases of PMF nationwide. NPR, meanwhile, conducted its own survey of black lung clinics and legal providers and has found more than 2,000 cases in the same time frame.

“It’s not that we’re discovering a new disease. We’e seeing a resurgence of a disease that should have been eradicated,” said Cohen. “You know, it's something we should not be seeing ... at all and we're seeing thousands of cases still in the 21st century.”

"So it is certainly not a blip. It's not just a small spike. It's kind of a relentless and increasing progression of disease.”

Two other studies involve the damage black lung disease does to the lungs. One was also discussed at the San Diego conference; it found that lung function continues to get worse in miners with simple black lung, even after they stop working and they're no longer exposed to the coal mine dust that causes the disease. That suggests the disease could eventually develop into PMF.

The other study was released two weeks ago and documents a growing number of lung transplants for miners with the most severe symptoms of PMF