We appreciate the recent contribution of Ulmet al 1 to the controversial question as to whether occupational exposure to crystalline silica, a classified carcinogen, increases the risk of lung cancer irrespective of silicosis. There are, however, some serious conceptual and methodological questions regarding the design, conduct, and analysis of the study which may affect the conclusions.

If silicosis is a surrogate for the internal dose or on the causal pathway between silica exposure and lung cancer, this question might only be addressed with detailed individual exposure information and classification of the silicosis grade in a population of silicotic and non-silicotic subjects.

The authors presented the pooled data from two matched case-control studies among non-silicotic subjects in two German industries. The study populations were not clearly defined with respect to the region. Cohorts for the recruitment of cases and controls as suggested by the authors cannot be reconstructed. Given the lack of a nationwide cancer registry in Germany, complete case ascertainment is questionable. In particular, the selection of potential cases from the workforce due to health problems poses a threat to internal validity.

Recruitment of controls and exposure levels varied between the two industries. Selection of more highly exposed or silicotic control subjects, in particular in the stone and quarrying industry, cannot be ruled out. We think that more detailed information on the data and results from the two industries, which have been …