Lung cancer: another good reason to control silica

In this issue, Steenland and colleagues present strong evidence for the pulmonary carcinogenicity of silica dust exposure [1]. In a new analysis of previously published studies, the authors pooled and reanalyzed the data from ten large cohort studies, covering more than 1,000 lung cancer cases. The results demonstrate a clear relationship between cumulative exposure to silica occurring in a wide range of different industries, and lung cancer mortality. Quantitative exposure data, so often the weak link in occupational cancer studies, were either available or developed specifically for this reanalysis. The authors argue persuasively that smoking is unlikely to have been an important confounder of these results. This work adds important confirmatory evidence to the International Agency for Research on Cancer (IARC) determination in 1997 that silica is a human carcinogen (Group 1) [2]. A definitive link between silica and lung cancer is a bittersweet achievement for science, because silica has been known for centuries to cause disabling lung disease [3]. Effective silicosis prevention could have eliminated the large exposed populations used in the studies reanalyzed by Steenland and colleagues. Now that there is yet another reason to control human exposure to silica dust, we can only hope that cancer's "cachet" as a terrifying illness will spur industries and governments to eliminate a hazard that has taken far too many lives already. Often in the past, a carcinogenicity determination has led to a dramatic shift in perspective about the safety of the substance in question. For example, the realization that 4-aminobiphenyl caused bladder cancer led to drastic reductions in exposures, and to its substitution by safer chemicals [4]. Vinyl chloride too was handled very differently once its carcinogenicity was identified [5]. But the failure to eliminate silicosis as an occupational lung disease should alert the public health community that preventing silica's lung cancers may be much more difficult. In the United States, exposure to high levels of silica, and new cases of silicosis still occur with shocking frequency [6]. A recent study identified all known cases of silicosis in the state of Michigan for the period 19871995 [7]. The 577 cases were almost certainly a considerable underestimate the authors estimated that they identified approximately one third of all those occurring in th state in that period. In only a minority of cases were workplace monitoring data available, but among these, 43% of the workplaces in which a case had occurred were found to be out of compliance with the permissible exposure limits. Linch and colleagues used U.S. Occupational Safety and Health Administration (OSHA) compliance data to estimate the numbers of workers in the U.S. exposed to silica in 1993 [8]. Approximately 100,000 workers were likely to have been exposed above the legal limit of 0.1 mg/m3. Another revealing failure: 25-30% of air samples collected during OSHA inspections for potential silica standard violations between 1996 and 1998 were above the legal limit of 0.1 mg/m3. The average fine for this infraction was about $800 [9]. Steenland and colleagues conclude their pooled analy is for silica and lung cancer by suggesting that: "...existing permissible exposure limits for silica need to be lowered" [1]. We agree, but argue that lowering the allowable air concentration of silica is not enough. We have seen that the current U.S. standard is not adequately enforced, and so relying only on an exposure standard seems ill-advised. Due to low fines and the low likelihood of inspection, it has been shown that in the U.S., risk of being found out of compliance with occupational health standards represents only a minor incentive to employers to control workplace hazards [10-12]. The best way to control a hazard is to eliminate it [13], and this is especially true for silica, given the failure to reduce exposures. Devices which attempt to reduce exposure to a chemical by capturing it after it has been Dr. Rosenberg was supported in part by a Special Emphasis Research Career Award from the National Institute for Occupational Safety and Health.