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Re: Quantitative risk assessment of silicosis death for quartz exposed German workers

Dear Editor

In a recent article of Occupational and Environmental Medicine [1], Mannetje and co-workers presented quantitative evidence of an exposure- response relationship between occupational exposure to crystalline silica and silicosis mortality in a carefully designed pooled analysis. This paper impressively demonstrated 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 longevity of silica exposed workers. The quantitative evidence (exposure-response relationship) presented in this paper provided a sophisticated basis for quantitative assessment of the 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. However, some methodological limitations related to this estimation may, on the other hand, lead to an extreme overestimation of these lifetime risks. One important limitation may be the lack of consideration of latency period in the assessment. The authors reported in their paper 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 a quantitative risk assessment of silicosis deaths for German workers (life table of German population is used in the assessment) exposed to crystalline silica at exposure levels of 0.1 and 0.05 mg/m³ for 45 years, we get a lifetime risk of silicosis death of 1.6 and 0.7 per 1000, respectively. These values are about 9 times lower than the estimates given by Mannetje et al. Even though, we believe that these values are more likely be overestimated, since the assumption of a 28-year latency is the most conservative assumption. Furthermore, this latency was estimated form 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 the quantitative risk assessment (maximal cumulative exposure of 4.5 mg/m³-year). Therefore, longer latency period should be considered in a more realistic assessment. If latency period is not considered in our assessment, we will get nearly the same results as those given by Mannetje et al. (see Table 1).

Reference

(1) Mannetje A, Steenland K, Attfield M, et al. Exposure-response analysis and risk assessment for silica and silicosis mortality in a pooled analysis of six cohorts. Occup Environ Med 2002; 59: 723-728.

Table 1. Estimated lifetime risk of silicosis death for German workers exposed to crystalline silica for 45 years

*Life table of German population in the year of 1995 was used in the assessment