Dose-response Relationship between a Particular Form of Cryptocrystalline Silica and the Incidence of Silicosis

Objectives It has been known for centuries that crystalline silica induces silicosis. However, there are different entities of crystalline silica. Siliceous earth is a worldwide unique blend of amorphous and cryptocrystalline silica and lamellar kaolinite. This retrospective cohort study is the first that investigated the silicosis risk of siliceous earth. Methods The cohort consists of employees exposed to siliceous earth in one company in Bavaria, Germany. Their job activities were collected from personnel files. Workers were included, if they had worked at least one year from the date of recruitment to the end of the study in 2008. The disease data were collected from occupational disease records of the Raw Materials and Chemical Industry Employers’ Liability Insurance Association (BG RCI). Two pneumologists reviewed the radiographic findings independently on the basis of reports of radiological findings according to ILO 2000. Workers with an ILO profusion ≥ 1/1 (p, q, r) were considered to have silicosis. Using the existing 709 measurement records of the years 1961–2008 exposure to silica was assessed for each subject with a job-exposure matrix. To determine the risk of silicosis Poisson regression was performed and to determine the dose-response relationship cubic splines were applied. Results In total 675 persons were enumerated, including 84 women. Among women, there were no silicosis but 55 among men. The evaluation of the exposed men showed a silicosis risk at a cumulative exposure of more than 6 mg/mxyears (RR = 25; 95% CI 5.3–111.5) and especially in underground mining (RR = 12; 95% CI 4.9–28.6). In addition, a dose-response relationship with increasing cumulative exposure was found. Conclusions We found a dose-response relationship between silicosis and cumulative exposure to siliceous earth. The silicosis risk of siliceous earth is according to these results not higher than that described in the literature for crystalline or cryptocrystalline silica.