Objectives: To estimate a threshold value for the respirable quartz dust concentration and silicosis incidence (1/1, ILO 1980/2000) in the German porcelain worker cohort.
Methods: Forty incident cases occurred among 17,144 workers. Estimation of the relationship to cumulative respirable quartz dust exposures by using Cox regression with restricted cubic splines (7 knots or less) and fractional polynomials (degrees 5 or less). Threshold estimates were obtained by subtracting candidate values from the time-dependent concentration data (setting the result to zero if negative) and recalculating cumulative exposure.
Results: Best threshold estimate was 0.25 mg/m3 (95% confidence interval: 0.15 to 0.30 mg/m3).
Conclusions: A concentration threshold for silicosis risk is plausible. A threshold Cox model fitted the data significantly better than a nonthreshold model, summarized the cohort information without a loss in extracted information and more simply than restricted cubic splines and fractional polynomials.
From the Institute for Occupational Epidemiology and Risk Assessment of Evonik Industries AG (Dr Morfeld), Essen, Germany; Institute and Policlinic for Occupational Medicine, Environmental Medicine and Prevention Research (Dr Morfeld), University of Cologne, Germany; ENVIRON International Corporation (Dr Mundt), Amherst, Mass; Institute for Prevention and Occupational Medicine of the German Social Accident Insurance (Dr Taeger), Institute of the Ruhr-Universität Bochum (IPA), Germany; Verwaltungs-Berufsgenossenschaft (Dr Guldner and Mr Steinig), Würzburg, Germany; Institute of Occupational Medicine (Dr Miller), Edinburgh, United Kingdom.
Address correspondence to: PD Dr Peter Morfeld, Institute for Occupational Epidemiology and Risk Assessment of Evonik Industries, AG Rellinghauser Straße 1-11, 45128 Essen, Germany (firstname.lastname@example.org).
The study was supported by EUROSIL, representing the European industrial silica producers (http://www.eurosil.eu), the National Industrial Sand Association (North America) (http://www.sand.org/), and the Verwaltungs-Berufsgenossenschaft (www.vbg.de).
EUROSIL and the National Industrial Sand Association did not participate in any scientific aspects of this investigation.
The authors declare no conflicts of interest.