Dose response relationships of exposure and lung function of salt miners – comparative analysis of a longitudinal study with a linear and a mixed model

A longitudinal study in two salt mines in Germany suggested effects of the mixed exposure of salt dust, diesel exhaust and nitrogen oxides on lung function of the miners. With regard to the discussion of occupational exposure limit values an additional analysis of the considerable data should analyse effects of the single components of exposure separate as far as possible. 70% of the first cohort (568 miners, mean age 45.0 years) was re-examined after a 5 year period. Mean individual lifetime exposure concentrations were 10.1 mg/m3 of inhalable and 1.46 mg/m3 of respirable salt dust, 0.12 mg/m3 of diesel exhaust (EC), 0.78 ppm of NO2 and 1.59 ppm of NO. Dose response relationships were calculated by multiple linear model and linear mixed model analysis. Analyses indicated clear dose response relationships in both models and confirmed the results of the previous study. Effects pointed out in the direction of chronic obstructive lung disease. With the linear regression a loss of FEV1 of 11.5 mL/yr could predicted in relation to an av. exposure in the 5-yr-periode. In the mixed model some exposure effects could be modelled as random effects. There was a mean effect of -74.2 (SD ) 40.5) ml per mg/m3 respirable dust on FEV1. A stronger effect of more than -100 mL per mg/m3 on FEV1 was found when FEV1 was lower at the start of the study. As in the first analysis it was impossible to determine the effect of a single exposure component separately in these models because of the high correlation of the individual concentrations (>0.8) as well as to contribute to recommendations for occupational exposure limit values. However, results give information about the situation of employees, exposed in the ranges of concentrations of individual substances described here. With the mixed model a widened understanding of exposure effects is enabled. With the view to occupational health and safety technology and medical preventive measures should be continued in the mines.