Silica, asbestos, man-made mineral fibers, and cancer

Our plain-English summary of Silica, asbestos, man-made mineral fibers, and cancer, by K. Steenland, L. Stayner (Cancer Causes & Control, 1997).

The study

• Authors: K. Steenland, L. Stayner
• Journal: Cancer Causes & Control
• Published: 1997;8(3):491-503
• Type of article: Review
• Cited by: 68 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Approximately three million workers in the United States are estimated to be exposed to silica, man-made mineral fibers, and asbestos. The lung is the primary target organ of concern. Each of these substances is composed predominantly of silicon and oxygen; asbestos and silica are crystalline, and asbestos and man-made mineral fibers are fibers. Man-made mineral fibers and asbestos are used as insulating agents, with the former having generally replaced the latter in recent years. Silica is used in foundries, pottery, and brick making, and is encountered by miners. A meta-analysis of 16 of the largest studies with well-documented silica exposure and low probability of confounding by other occupational exposures, indicates a relative risk (RR) of 1.3 (95 percent confidence interval [CI] = 1.2-1.4). Lung cancer risks are highest and most consistent for silicotics, who have received the highest doses (RR = 2.3, CI = 2.2-2.4, across 19 studies). The data for mineral fibers continue to support the International Association for Research on Cancer's 1988 judgment that mineral fibers are a possible human carcinogen (Group 2B). Recent epidemiologic studies provide little evidence for lung carcinogenicity for either glass wool or rock/slag wool. Ceramic fibers, a much less common exposure than glass wool and rock/slag wool, are of concern because of positive animal studies, but there are insufficient human data. Regarding asbestos, its carcinogenicity for the lung and mesothelium is well established. With regard to the controversy over chrysotile and mesothelioma, the data suggest chrysotile does cause mesothelioma, although it may be less potent than amphibole asbestos.

In plain English

This article reviewed published evidence on lung cancer and mesothelioma from crystalline silica, man-made mineral fibers and asbestos. Pooling 16 larger studies of silica-exposed workers, the authors found a combined lung cancer relative risk of 1.3 (95% confidence interval 1.2-1.4). For 19 studies of workers with silicosis, the summary relative risk was 2.3 (confidence interval 2.2-2.6 in the text, 2.2-2.4 in the abstract). Two large mineral fiber cohorts showed small lung cancer excesses, larger for rock/slag wool than glass wool. The authors said smoking could still explain most if not all of these excesses, and that the data still support IARC's 1988 rating of them as possibly carcinogenic. They called asbestos carcinogenicity for the lung and mesothelium well established. The authors wrote that asbestos lung cancer risks appeared lowest in cement and friction products and highest in mining and textiles, with no clear fiber-type pattern, though their table also lists insulation cohorts with lung cancer mortality ratios of 4.1 and 5.0. They concluded the data suggest chrysotile does cause mesothelioma, though it may be less potent than amphibole asbestos.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Review of published animal and human (epidemiologic) evidence, with summary relative risks calculated for some agents using inverse-variance weighting and a random effects model
• Who was studied: Published studies of exposed workers, limited to lung cancer and mesothelioma. These were tabulated as 19 cohort and case-control studies of silicotics, 16 studies of silica-exposed workers, cohort and nested case-control studies of man-made mineral fiber workers, and the larger mining and industrial cohorts exposed to asbestos.
• Exposure: Crystalline silica (foundries, mining, quarrying, granite, pottery, brick making); man-made mineral fibers (glass wool, glass filament, rock/slag wool, refractory ceramic fibers); and asbestos, including chrysotile, crocidolite, amosite, tremolite and anthophyllite, in mining, cement, friction products, textiles and insulation
• Main results:
  • Across the 16 larger studies of silica-exposed workers without known confounding lung carcinogen exposures (Table 2), the combined lung cancer relative risk was 1.3 (CI = 1.2-1.4).
  • Across 19 cohort and case-control studies of workers with silicosis (silicotics), the summary lung cancer relative risk was 2.3. The article body gives a 95 percent CI of 2.2-2.6, while the article's abstract gives 2.2-2.4.
  • In the two large cohort studies of rock/slag wool and glass wool workers, there was no lung cancer excess for glass filament, a slight excess for glass wool, and a more pronounced excess for rock/slag wool. In the US cohort, the lung cancer SMR was 1.12 (1.00-1.24) for glass wool and 1.36 (1.06-1.71) for rock/slag wool. In the European cohort, it was 1.12 (0.95-1.31) for glass wool and 1.39 (1.18-1.62) for rock/slag wool.
  • For the man-made mineral fiber cohorts, the authors noted that the relative risks were small, so confounding by smoking could still explain most if not all of the excesses. The four nested case-control studies were generally negative.
  • The one cohort study of refractory ceramic fiber workers did not show a lung cancer excess. Because of its size and the young age of the cohort, the authors viewed it as inconclusive rather than negative.
  • In most but not all of the asbestos cohort studies, there was an excess of lung cancer mortality and evidence of an exposure-response relationship.
  • The authors wrote that, among the asbestos cohorts, lung cancer risks appeared lowest in the cement and friction products industries and highest in mining and textiles. They saw no clear patterns of difference in risk by asbestos fiber type.
  • Table 4 also lists insulation worker cohorts with lung cancer SMRs of 4.1 (3.9-5.0) (Selikoff et al) and 5.0 (4.1-6.0) (Seidman et al, amosite). These are higher than the listed mining and textile cohorts.
  • In the asbestos studies that controlled for tobacco consumption, the lung cancer excesses did not appear to be explained by differences in smoking habits.
• The authors' conclusion: The authors concluded that the weight of the evidence supports silica being a human lung carcinogen. They also concluded that the data continue to support IARC's 1988 judgment that man-made mineral fibers are possibly carcinogenic in humans. They described asbestos carcinogenicity for the lung and mesothelium as well established, and concluded that the data suggest chrysotile does cause mesothelioma, although it may be less potent than amphibole asbestos.
• Limitations the authors state:
  • Most of the relative risks pooled showed substantial heterogeneity.
  • There were insufficient data to characterize with confidence how smoking and exposure interact.
  • Most silica studies did not report exposure levels.
  • The authors did not examine in detail the quality of historical exposure estimates in the mineral fiber case-control studies.
  • Human data on whether asbestos lung cancer requires preceding fibrosis were too sparse for firm conclusions.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile, Lung cancer.

Medical terms in this study, explained in our Lexicon: Mineral Fiber, Crystalline, Meta-Analysis, Confounding, Occupational Exposure, Relative Risk, confidence interval, Lung Cancer.

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• Mortality among a cohort of US man-made mineral fiber workers: 1985 follow-up — G.M. Marsh, P.E. Enterline, et al. (1990)
• Review of animal/in vitro data on biological effects of man-made fibers — S.A. Ellouk, M.C. Jaurand (1994)
• In vitro genotoxicity of asbestos substitutes induced by coupled stimulation of dissolved high-valence ions and oxide radicals — T. Huo, F. Dong, et al. (2018)
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