Exercise capacity in coal workers' pneumoconiosis: an analysis using causal modelling.

Miners disabled from black lung disease (coal workers' pneumoconiosis, CWP) are entitled to disability benefits under United States federal and state laws. The determination of disability currently involves several scientific controversies. The Federal Department of Labor states that one second forced vital capacity (FEV1) is an important marker of disability from CWP. The Department of Labor also states that disability may occur in simple CWP, in the absence of progressive massive fibrosis (PMF). Both these contentions may reasonably be challenged. To investigate these issues, and to investigate the relation between exercise tolerance and several other variables, including age, weight, radiographic findings, and exposure to mining and smoking, we studied 690 miners. Simple correlation analysis was not helpful because many variables were correlated with each other. Linear regression analysis led to conclusions that were thought to be misleading. Causal modelling provided an analysis that appeared to be most explanatory. In the model tested years of coal mining did not affect FEV1; if this conclusion is substantiated by others FEV1 should be eliminated as an indicator of disability from CWP as it is not related to mining experience. On the other hand, the Department of Labor's position that disability may occur in simple CWP seems reasonable, as years of underground coal mining does affect forced vital capacity (FVC), which in turn impairs exercise capacity, even in the absence of PMF. FVC should be the major spirometric value used in determining disability from CWP because it alone is seen to decrease in relation to mining, and a decrease in FVC does affect exercise capacity. Thus this analysis addresses issues in determining black lung disability and shows the value of casual modelling.