Risk factors associated with progressive massive fibrosis among U.S. coal miners – a case-control study

Rationale: Progressive Massive Fibrosis (PMF) is the most disabling form of pneumoconiosis. This study aims to investigate the risk factors associated with PMF in coal miners. Methods: PMF cases were determined by International Labour Office (ILO) classification of radiographs of pneumoconiosis based on final readings of last x-ray taken in the NIOSH Coal Workers' X-Ray Surveillance Program (CWXSP) during 1970 to 2000. A 1 to 1 matched case-control approach was used (controls having no PMF), yielding 314 pairs for assessment of PMF risk factors, including coal dust and quartz exposures, coal rank, and x-ray appearances. Results: Cumulative coal dust and quartz exposures were higher in cases (102.7 vs. 82.9 mg/m3-yrs, p<0.0001; and 2.48 versus 1.54 mg/m3-yrs, p = 0.0002, respectively). Cases were more likely from mines with higher coal rank than controls (21.4% vs. 13.9%, p=0.0023). The small opacity profusion on average was 4 minor categories higher in PMF cases (equivalent to category 2/2 versus 1/0, p< 0.0001). Among cases and controls with small opacities on chest radiograph, the small opacity “r” shape was present in 44% of cases versus 15% of controls (p<0.0001). The logistic regression for matched case-control analysis resulted that the odds of having PMF was 3.1, 1.6 and 2.4 times higher among miners in the fourth quartile of coal dust and quartz exposures, and in higher coal rank of coal mines, respectively. Conclusion: Coal dust and quartz exposures, higher coal rank, higher small opacity profusion and “r” shaped opacities were significantly related to PMF.