DALLAS — Rising prevalence of the most deadly form of black lung disease among U.S. miners appears largely due to shifting mining methods resulting in increased inhalation of crystalline silica, according to research presented here at the American Thoracic Society’s annual meeting.
A review of mortality data from 1991 through 1996 from NIOSH’s National Coal Workers’ Autopsy Study (NCWAS) showed little evidence of a transition to silica predominant disease. Apparently, more recent shifts in exposures are driving the resurgence in the rapidly progressing form of black lung known as progressive massive fibrosis (PMF).
Another analysis of PMF cases recorded in the NCWAS data before and after 1990 suggested a historical shift in the disease, with silicotic PMF accounting for a greater percentage of cases in the latter period.
And a third analysis showed a proportional increase in mortality from non-malignant respiratory diseases among younger miners (<65 years) in more recent birth cohorts compared to miners born earlier.
The three studies examining the changing pathology and demographics of black lung disease were all led by Robert Cohen, MD, of the University of Illinois at Chicago’s Black Lung Center of Excellence.
Cohen told MedPage Today that the increase in the silicotic form of black lung underscores the need for tougher federal standards regulating respiratory exposures associated with modern mining practices.
“Existing regulations don’t address this very well,” he said.
He added that animal and toxicology studies show that dust from modern mining which contains very small particle silica from rock is up to 20 times more toxic than the dust derived from coal alone.
Silica is also a recognized carcinogen, whereas the evidence linking coal dust to lung cancer remains weak.
Radiographic and pathologic evidence suggests a strong link between the increase in pulmonary massive fibrosis and increased exposure to respirable crystalline silica.
In an effort to further explore this link, occupational pathologists reviewed lung tissue under light microscopy from 325 miners who had been classified as having PMF with sufficient lung material available for review.
Silicotic PMF was defined as fused silicotic nodules which made up more than 75% of the area of the lesion. Mixed type PMF was defined as having less than 75% silicotic nodules and coal-type PMF was defined as having 25% or less of silicotic nodules.
The miners included in the analysis were born from 1885 through 1942, and the pathologists identified 141 coal-type PMF cases, 107 mixed type and 77 silicotic-type cases of PMF, with no statistical difference in the type of PMF cases over time.
No significant differences in the distribution of these cases by PMF type were seen over time, suggesting that “the shift in mining exposures driving the resurgence in rapidly progressive pneumoconiosis and PMF likely occurred more recently,” the researches wrote in a poster.
In another analysis of NCWAS data, the researchers documented a significant increase in the proportion of silicotic PMF occurring after 1990 – 40% vs 24% before 1990 (P=0.002).
An examination of mortality trends among coal miners filing for federal black lung benefits from 1970 through 2016 also suggested a more recent shift in exposure type.
In that study, proportional mortality from pneumoconiosis and other non-malignant respiratory diseases was most pronounced among the miners in the cohort born after 1940.
“This increase is pronounced among younger miners and may reflect increased mortality from progressive massive fibrosis, which is occurring more frequently and in younger miners,” the researchers wrote.
Proportional mortality from lung cancer was also highest among older miners in the most recent birth cohort examined (those born after 1940), possibly reflecting exposure to workplace carcinogens such as respirable silica, the researchers noted.