Components of Coal Mine Dust Exposure and the Occurrence of Prestages of Pneumoconiosis

In the last 20 years the exposure of German coal miners to respirable dust has been lowered and controlled according to enacted dust standards. The incidence of coal workers' pneumoconiosis (CWP) dropped substantially, but the question remains whether the CWP risk has been eliminated. A historical cohort study was performed enrolling all hard coal miners at two typical collieries of the Ruhr area (Heinrich Robert, high rank coal: n = 564; Walsum, low rank coal: n = 693) and at all six collieries of the Saar area (n = 4521, special low rank coal) who began work underground without earlier exposure between 1974 and 1979 (Ruhr) or 1980 and 1986 (Saar). Thus all three typical stratigraphic horizons of German coal seams were considered. Complete individual data on shifts worked underground, exposures to respirable coal mine dust and quartz dust, and CWP development during time underground were compiled for each miner. Historical exposures were based mainly on 43,842 static gravimetric measurements. CWP outcome was ascertained by a new evaluation of 47,781 chest radiographs according to International Labour Office (ILO) 1980, read independently by at least two physicians. The mean concentration of respirable coal mine dust exposure varied from 1.6 mg/m3 (Saar) and 2.3 mg/m3 (Walsum) up to 2.9 mg/m3 (Heinrich Robert). Quartz dust exposure was inversely related: 0.21,0.10, and 0.05 mg/m3 for the Saar, Walsum, and Heinrich Robert mines, respectively. A total of 59 cases of profusion category 0/1 (ILO 1980) were determined (Saar: 45 cases; Walsum: 4 cases; Heinrich Robert: 10 cases). No higher categories were observed. Risk assessment by Cox modeling with time-dependent covariables shows an association between early radiographic signs (0/1, ILO 1980) and dust exposure, especially quartz dust exposure modified by stratigraphic horizon. The significant differences between subcohorts persisted. Based on this study, a long-term (15-year) threshold limit value for respirable coal mine dust was estimated to be not less than 2.5 mg/m3.