Exposure-Response Association between Cumulative Exposure to Respirable Crystalline Silica Dust and Lung Cancer

Baur et al. (2013) published an overview of epidemiological studies that estimated quantitative exposure-response relationships between long-term occupational exposure to respirable crystalline silica dust and lung cancer risk. I agree to the main conclusions: every quantitative review in this field will suffer from the heterogeneity of materials and methods across studies and in particular from a missing standardization of the exposure assessments. I'd like to give some additional comments addressing two specific sub-topics: the estimation of lung cancer excess risks and exposure thresholds; the interpretation of studies on hard coal miners. The results published by Lacasse et al. (2009) and Lakhal and Lacasse (2009) do not allow a sensible conclusion about thresholds for lung cancer risk after silica dust exposure. The studies by Steenland et al. (2001) or Sogl et al. (2012) indicated lung cancer excess risks only at average concentrations of at least 0.15 mg/m(3) or 0.25 mg/m(3) over 40 years of exposure. The cohort studies in hard coal mining (Attfield and Kuempel 2008, Miller and MacCalman 2010, Morfeld et al. 2007) showed no effect of coal mine dust exposure including the quartz dust component on lung cancer risk. The weak association with quartz dust exposure found in the British study was mainly driven by an anomalous situation at one colliery. The prima facie perplexing result of the German study (no association of lung cancer with dust exposures; but with pneumoconiosis on Xrays, and pneumoconiosis was related to dust exposure, in particular to the quartz dust component) is compatible with the following causal scheme: pneumoconiosis operates as a biomarker of lung cancer susceptibility, and not only as an exposure marker. Such a disposition to lung cancer was described for idiopathic fibroses also. Any evaluation of dust exposures (e.g., classification as carcinogenic, assessment of limits, introduction of occupational diseases) should focus on the causal agent, i.e., on the dust exposures and it should not focus on dummy variables, like inter mediate diseases. Sidestepping to dummy variables appears to be only acceptable if the exposures were not assessed or just very crudely determined.