Limit Value Assessment for Respirable Coal Mine Dust in Germany

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Abstract
Occupational limit values for airborne noncarcinogenic substances are defined as threshold limit values in Germany. This concept presumes a “hockey stick” relationship between mean shift concentrations and health effects. A population-based threshold limit value (in Germany called the MAK value) is defined as the maximum value on the exposure scale below which a constant baseline risk is observed. This concept was originally developed for toxic substances and is modified to handle the case of fibrogenic dust exposure. The incidence of profusion category 1/1, International Labor Organization (ILO) 1980 of coal workers' pneumoconiosis (CWP) is used as the outcome measure to evaluate the health effects of respirable coal mine dust. The basic concept is modified firstly to analyze the dependence of disease incidence on the mean long-term concentration, allowing for linear and nonlinear effects of time since first exposure, and secondly to handle incomplete clearance in terms of a time-dependent threshold. Moreover, an adaptation factor is introduced to transform long-term threshold value estimates into shift limit proposals. This concept is applied to German CWP studies.(11,14) Long-term thresholds are estimated by switching linear regression modeling. Exposure variability of shift-based static gravimetric dust measurements is analyzed in comparison to indirectly determined long-term personal average respirable dust exposure intensity to estimate the adaptation factor. By this procedure, an epidemiologically based range of 1.5 to 6 mg/m3 is derived for the long-term limit proposal for the respirable coal mine dust concentration at the underground work sites of the German coal mining industry. A corresponding range of 5 to 20 mg/m3 is calculated for the 8-hour shift limit proposal.