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In a recent article of Occupational and Environmental Medicine,1 Mannetje et al presented quantitative evidence of an exposure-response relation between occupational exposure to crystalline silica and silicosis mortality in a carefully designed pooled analysis. This paper impressively showed that simple silicosis, one of the oldest occupational diseases, is still a relevant occupational health problem nowadays which may have a negative effect on the life expectancy of silica exposed workers. The quantitative evidence (exposure-response relation) presented in this paper provided a sophisticated basis for quantitative assessment of the absolute risks of silicosis deaths for workers exposed to different level of crystalline silica.
In this manuscript, the authors exemplarily quantified the risk of silicosis deaths for workers exposed to crystalline silica at exposure levels of 0.1 and 0.05 mg/m3 for 45 years. The estimated lifetime risks of silicosis death were 13 and 6 per 1000, respectively. The authors concluded that, due to exposure misclassification and possible under-report of silicosis deaths, the lifetime risks of silicosis deaths may be underestimated.
Based on the estimated exposure-response relation provided by Mannetje et al,1 we recalculated the lifetime risk of silicosis deaths for quartz exposed workers by using the life table of the German population in the year 1995. The results of our calculation are presented in table 1. If latency of silicosis death is not considered in our calculation, we get nearly the same results as those given by Mannetje et al (11.4 and 5.3 per 1000 at exposure levels of 0.1 and 0.05 mg/m3 for 45 years, respectively). However, Mannetje et al reported in their manuscript that workers who died of silicosis had a median duration of exposure of 28 years and only 9% of silicosis death occurred within one year after leaving the job. The median latency of silicosis death may, therefore, account for at least 28 years. If we consider this latency period (28 years) in our calculation, we get a lifetime risk of silicosis death of 1.6 and 0.7 per 1000, respectively. These values are about seven times lower than our previous estimation without considering the latency period. We believe that these values are more likely be overestimated, since the assumption of a 28 year latency is the most conservative assumption (period after leaving jobs was not accounted into the 28 year latency period). Furthermore, this latency was estimated from workers exposed to a much higher level of quartz (median cumulative exposure of 7.15 mg/m3-year) than the maximal possible exposure level in our calculation (maximal cumulative exposure of 4.5 mg/m3-year). Therefore, longer latency period should be considered in a more realistic assessment of lifetime risk of silicosis deaths. We wonder whether Mannetje et al have considered latency period in their calculation of lifetime risk of silicosis deaths.