From all subjects, information on occupational history, duration of exposure, workshop within the company, former occupational exposures, and smoking habits were available. Informed written consent was obtained from each participant.
Spirometry was performed with a Jaeger-Masterlab (two companies, n = 606) and a Jaeger Flowscreen (n = 441) [Jaeger-Toennies; Wurzburg, Germany] according to American Thoracic Society criteria. It included the measurement of the FVC and FEV1. Relative values were calculated for the measured lung function parameters by using the reference values proposed by the European Community for Coal and Steel.
Descriptive data analysis for the parameters FEV1, FVC, and FEV1 in percent of FVC (FEV1%FVC) comprise the calculation of means and SDs of these parameters in exposure categories of cumulative dust and fiber exposure, respectively. The categorization of these variables into ordinal groups was based on a priori defined scheme of cut points. Moreover, number and percentage of FVC values < 80% predicted were calculated.
In a second step, the potentially confounding effects of factory, age, sex, body mass index, and smoking habits were incorporated into the analysis by employing multiple linear regression modeling. Separate regression models were set up for the lung function parameters FEV1, FVC, and FEV1%FVC, which were included untransformed as dependent variables into the model, because their empirical distribution was approximately normal. Parameter estimates from these models show the adjusted effects of the specific dust/fiber exposure category on the FEV1, FVC, and FEV1%FVC, respectively, relative to the control group. Statistical significance of these changes is assessed by the test within the regression model whether the corresponding model parameters equals 0.