Silica and asbestos exposure at work and the risk of bladder cancer in Canadian men: a population-based case-control study

Our plain-English summary of Silica and asbestos exposure at work and the risk of bladder cancer in Canadian men: a population-based case-control study, by L. Latifovic, P.J. Villeneuve, M.É. Parent, et al (BMC Cancer, 2020).

The study

• Authors: L. Latifovic, P.J. Villeneuve, M.É. Parent, L. Kachuri, Canadian Cancer Registries Epidemiology Group, S.A. Harris
• Journal: BMC Cancer
• Published: 2020;20(1):171
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Silica and asbestos are recognized lung carcinogens. However, their role in carcinogenesis at other organs is less clear. Clearance of inhaled silica particles and asbestos fibers from the lungs may lead to translocation to sites such as the bladder where they may initiate carcinogenesis. We used data from a Canadian population-based case-control study to evaluate the associations between these workplace exposures and bladder cancer.

Methods: Data from a population-based case-control study were used to characterize associations between workplace exposure to silica and asbestos and bladder cancer among men. Bladder cancer cases (N = 658) and age-frequency matched controls (N = 1360) were recruited within the National Enhanced Cancer Surveillance System from eight Canadian provinces (1994-97). Exposure concentration, frequency and reliability for silica and asbestos were assigned to each job, based on lifetime occupational histories, using a combination of job-exposure profiles and expert review. Exposure was modeled as ever/never, highest attained concentration, duration (years), highest attained frequency (% worktime) and cumulative exposure. Odds ratios (OR) and their 95% confidence intervals (CI) were estimated using adjusted logistic regression.

Results: A modest (approximately 20%) increase in bladder cancer risk was found for ever having been exposed to silica, highest attained concentration and frequency of exposure but this increase was not statistically significant. Relative to unexposed, the odds of bladder cancer were 1.41 (95%CI: 1.01-1.98) times higher among men exposed to silica at work for ≥27 years. For asbestos, relative to unexposed, an increased risk of bladder cancer was observed for those first exposed ≥20 years ago (OR:2.04, 95%CI:1.25-3.34), those with a frequency of exposure of 5-30% of worktime (OR:1.45, 95%CI:1.06-1.98), and for those with < 10 years of exposure at low concentrations (OR:1.75, 95%CI:1.10-2.77) and the lower tertile of cumulative exposure (OR:1.69, 95%CI:1.07-2.65). However, no clear exposure-response relationships emerged.

Conclusions: Our results indicate a slight increase in risk of bladder cancer with exposure to silica and asbestos, suggesting that the effects of these agents are broader than currently recognized. The findings from this study inform evidence-based action to enhance cancer prevention efforts, particularly for workers in industries with regular exposure.

In plain English

This population-based case-control study used data from Canada's National Enhanced Cancer Surveillance System. It compared 658 men aged 40 or older with bladder cancer and 1360 men without cancer, from seven provinces, who answered questionnaires in 1994–97. Industrial hygienists rated each man's lifetime jobs for exposure to crystalline silica and asbestos. Asbestos fiber types were not specified. In fully adjusted models, ever exposure to asbestos gave an odds ratio of 1.32 (95% CI 0.98–1.77), which was not statistically significant. Odds were higher for men exposed at least 20 years before (2.04; 1.25–3.34), exposed during 5–30% of work time (1.45; 1.06–1.98), exposed under 10 years at low concentrations (1.75; 1.10–2.77), and in the lowest third of cumulative exposure (1.69; 1.07–2.65). They were not significantly higher in the middle or highest thirds, and no clear exposure-response pattern emerged. For silica, 27 or more years of exposure gave 1.41 (1.01–1.98). Exposure to both agents gave 1.67 (1.06–2.62). The authors concluded that their results indicate a slight increase in bladder cancer risk with both exposures. They noted possible bias and unmeasured confounding.

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: Population-based case-control study analysed with multivariable unconditional logistic regression
• Who was studied: Men aged 40 or older who had worked at least 1 year: 658 histologically confirmed bladder cancer cases and 1360 cancer-free controls from 7 Canadian provinces (Ontario controls excluded), questionnaires mailed 1994–97; response rates were 66% of cases and 64% of controls contacted
• Exposure: Workplace exposure to crystalline silica and to asbestos (fiber type not specified). Industrial hygienists assigned it from lifetime job histories, rating concentration, frequency and reliability as none, low, medium or high. It was analysed as ever exposure, highest concentration, highest frequency, duration and cumulative exposure. Low asbestos exposure was typically assigned to welders, medium to furnace installers and repairmen, and high to asbestos miners.
• Main results:
  • Asbestos, ever exposed (120 of 658 cases, 151 of 1360 controls), fully adjusted model: odds ratio 1.32 (95% CI 0.98–1.77)
  • Asbestos, restricted to those exposed at least 20 years ago: odds ratio 2.04 (95% CI 1.25–3.34); for at least 40 years ago, 1.26 (95% CI 0.90–1.78)
  • Asbestos exposure during 5–30% of work time: odds ratio 1.45 (95% CI 1.06–1.98)
  • Asbestos exposure for under 10 years at low concentrations: odds ratio 1.75 (95% CI 1.10–2.77)
  • Lowest tertile of cumulative asbestos exposure: odds ratio 1.69 (95% CI 1.07–2.65). Middle tertile 1.22 (0.76–1.97) and highest tertile 1.13 (0.70–1.82) were not statistically significant.
  • No clear exposure-response relationships emerged
  • Silica, ever exposed (254 of 658 cases, 431 of 1360 controls), fully adjusted model: odds ratio 1.20 (95% CI 0.95–1.51), not statistically significant
  • Silica exposure for 27 years or more: odds ratio 1.41 (95% CI 1.01–1.98)
  • Ever exposure to both silica and asbestos (51 cases, 54 controls), compared with men exposed to neither: odds ratio 1.67 (95% CI 1.06–2.62)
• The authors' conclusion: The authors concluded that their results indicate a slight increase in bladder cancer risk with exposure to silica and asbestos, suggesting the effects of these agents are broader than currently recognized. They added that the silica results were more consistent with an exposure-response relationship.
• Limitations the authors state:
  • The semi-quantitative exposure estimates do not capture the full variability at work sites
  • Exposure measurement error, especially for lower-confidence estimates
  • Reporting errors and differential recall of job histories may have misclassified exposure
  • Proxy respondents may have introduced error in exposure and confounders
  • Modest response rates for cases and controls
  • Unmeasured and residual confounding, including by other occupational carcinogens not measured
  • The asbestos results may partly reflect susceptibility bias: highly exposed workers may develop asbestosis or other lung diseases and be removed from exposure

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Case-Control Study, Carcinogenesis, Translocation, Cumulative Exposure, Odds Ratio, confidence interval.

Related studies

• Lung and bladder cancer in a Norwegian municipality with iron and steel producing industry: population based case-control studies — T.K. Grimsrud, H. Langseth, et al. (1998)
• Occupational exposure to asbestos and lung cancer in men: evidence from a population-based case-control study in eight Canadian provinces — P.J. Villeneuve, M.É. Parent, et al. (2012)
• Workplace exposure to asbestos and the risk of kidney cancer in Canadian men — C.E. Peters, M.É. Parent, et al. (2018)
• Workplace exposure to diesel and gasoline engine exhausts and the risk of colorectal cancer in Canadian men — L. Kachuri, P.J. Villeneuve, et al. (2016)
• Risk of lung cancer associated with occupational exposure to mineral wools: updating knowledge from a french population-based case-control study, the ICARE study — F. Guida, S. Paget-Bailly, et al. (2013)
• Arsenic, tobacco smoke, and occupation: associations of multiple agents with lung and bladder cancer — C. Ferreccio, Y. Yuan, et al. (2013)

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