Coal workers' pneumoconiosis and progressive massive fibrosis are increasingly more prevalent among workers in small underground coal mines in the United States.

OBJECTIVE To determine whether the prevalence of coal workers' pneumoconiosis (CWP) or progressive massive fibrosis (PMF) among United States underground miners is associated with mine size. METHODS We examined chest radiographs from 1970 to 2009 of working miners who participated in the National Coal Workers Health Surveillance Program for the presence of small and large opacities consistent with pneumoconiosis, based upon the International Labour Organization classification system. RESULTS A total of 145 512 miners contributed 240 067 radiographs for analysis. From the 1990s to the 2000s, the prevalence of radiographic CWP increased among miners in mines of all sizes, while miners working in mines with fewer than 50 employees had a significantly higher prevalence of CWP compared to miners who worked in mines with 50 or more employees (p<0.0001). When adjusted for age and within-miner correlation, the difference in prevalence of CWP by mine size was significant for all decades. Since 1999, miners from small mines were five times more likely to have radiographic evidence of PMF (1.0% of miners) compared to miners from larger mines (0.2% of miners) with a prevalence ratio of 5.0 and 95% CI 3.3 to 7.5. CONCLUSION The prevalence of CWP among United States coal miners is increasing in mines of all sizes, while CWP and PMF are much more prevalent among workers from underground mines with fewer than 50 workers.