Adverse health effects of asbestos: solving mysteries regarding asbestos carcinogenicity based on follow-up survey of a Chinese factory

Our plain-English summary of Adverse health effects of asbestos: solving mysteries regarding asbestos carcinogenicity based on follow-up survey of a Chinese factory, by E. Yano (Environmental Health and Preventive Medicine, 2018).

The study

• Author: E. Yano
• Journal: Environmental Health and Preventive Medicine
• Published: 2018;23(1):35
• Type of article: Review
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The present review summarizes the results of several follow-up studies assessing an asbestos product manufacturing plant in Chongqing, China, and discusses three controversial issues related to the carcinogenicity of asbestos. The first issue is the amphibole hypothesis, which asserts that the carcinogenicity of asbestos is limited to amphiboles, such as crocidolite, but not serpentines, such as chrysotile. However, considering the possible multiple component of asbestos carcinogenicity in the presence of tobacco smoke or other carcinogens, chrysotile cannot be regarded as non-carcinogenic. Additionally, in a practical sense, it is not possible to assume "pure" chrysotile due to its ubiquitous contamination with tremolite, which is a type of amphibole. Thus, as the International Agency for Research on Cancer (IARC) assessed, all forms of asbestos including chrysotile should be regarded carcinogenic to humans (Group 1). The second issue is the chrysotile/tremolite paradox, which is a phenomenon involving predominant levels of tremolite in the lung tissues of individuals who worked in locations with negligible levels of tremolite due to the exclusive use of chrysotile. Four possible mechanisms to explain this paradox have been proposed but this phenomenon does not support the claim that amphibole is inert. The final issue discussed is the textile mystery, i.e., the higher incidence of cancer in asbestos textile plants compared to asbestos mines where the same asbestos was produced and the exposure levels were comparable. This phenomenon was first reported in North America followed by UK and then in the present observations from China. Previously, levels of fiber exposure were calculated using a universal converting coefficient to estimate the mass concentration versus fiber concentration. However, parallel measurements of fiber and mass concentrations in the workplace and exposed air indicated that there are wide variations in the fiber/mass ratio, which unjustifies the universal conversion. It is possible that contamination by airborne non-fibrous particles in mines with mass fiber conversion led to the overestimation of fiber concentrations and resulted in the textile mystery. Although the use and manufacturing of asbestos has been banned in Japan, more than 10 million tons of asbestos had been imported and the majority remains in existing buildings. Thus, efforts to control asbestos exposure should be continued.

In plain English

This single-author review summarized the author's group's follow-up studies of an asbestos products factory in Chongqing, China. The factory's manager said only chrysotile was used but could not rule out contamination with amphibole. In a 25-year follow-up of 515 male workers, the review reported adjusted figures of 1.5 (95% CI 1.0–2.3) for relative mortality and 8.1 (95% CI 1.8–36.1) for lung cancer risk, comparing high- and low-exposure workers. In a 1972–2008 follow-up of 577 asbestos workers and 435 control workers, the asbestos workers' adjusted hazard ratio for lung cancer death was 3.31 (95% CI 1.60–6.87). The author interpreted a case-control study within the factory as showing a multiplicative interaction between smoking and asbestos for lung cancer. The factory had two confirmed mesothelioma cases, one pleural and one peritoneal. Textile workers had higher lung cancer risk than Sichuan mining workers at low exposure levels. The author concluded that all forms of asbestos, including chrysotile, should be regarded as carcinogenic to humans. The author suggested overestimated fiber exposure in mines may explain the higher textile risk, but said this could not be confirmed.

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 summarizing the author's research group's own earlier follow-up studies (cohort and nested case-control studies, fiber measurements) of one Chinese asbestos factory, and discussing three controversial issues: the amphibole hypothesis, the chrysotile/tremolite paradox, and the textile mystery
• Who was studied: Workers at an asbestos products factory in Chongqing, China (operating since 1939; about 1200 employees at one point). The studies reviewed included a cohort of 515 male workers followed for 25 years; 577 asbestos workers and 435 control workers followed from 1972 to 2008; 41 male lung cancer cases among 1139 workers, each matched by age with five controls; and a chrysotile textile cohort and a Sichuan chrysotile mining cohort observed for 26 years
• Exposure: Chrysotile asbestos from Sichuan mines, handled in raw material, textile, asbestos cement, rubber and friction material workshops. The factory manager said only chrysotile was used but could not totally rule out amphibole contamination. Tremolite contamination in mine samples was less than 0.001%. Tremolite was overrepresented among airborne fibers, although the review estimated it at less than 1.5% at its highest concentration. Exposure was grouped by workshop into high, medium and low levels, or by estimated cumulative fiber exposure into four levels (Q1 to Q4)
• Main results:
  • 25-year follow-up of 515 male workers, adjusted for age and smoking: the review reported 1.5 (95% CI 1.0–2.3) for adjusted relative mortality and 8.1 (95% CI 1.8–36.1) for lung cancer risk, in a comparison of high- and low-exposure workers
  • 1972–2008 follow-up: 259 deaths among the 577 asbestos workers, including 53 from lung cancer. The control cohort had nine lung cancer cases
  • Asbestos workers compared with control workers, adjusted for age and smoking: hazard ratios of 2.05 (95% CI 1.56–2.68) for all-cause mortality and 1.89 (95% CI 1.25–2.87) for all cancers
  • Asbestos workers compared with control workers: hazard ratios of 3.31 (95% CI 1.60–6.87) for lung cancer and 3.23 (95% CI 1.68–6.22) for respiratory diseases
  • Nested case-control study (41 male lung cancer cases): adjusted odds ratio for lung cancer of 3.66 (95% CI 1.61–8.29) for the high-exposure group and 1.25 (95% CI 0.47–3.31) for the medium-exposure group. The odds ratio for smoking was 3.33 (95% CI 1.10–10.08)
  • Compared with low-exposure non-smokers, the odds ratio was 10.39 (95% CI 1.34–82.45) for high-exposure smokers and 5.23 (95% CI 0.50–54.58) for high-exposure non-smokers. The author interpreted this as a multiplicative interaction between smoking and asbestos for lung cancer
  • The factory had two confirmed cases of malignant mesothelioma (one pleural, one peritoneal) and one suspected pleural case not confirmed by histopathology
  • Textile versus mining cohorts, adjusted for age and smoking: at low exposure levels (Q1 and Q2), textile workers had a higher risk of lung cancer death than mining workers. At higher levels, both groups had similarly high risks (hazard ratios > 8 at Q3 and > 11 at Q4, as printed)
  • Parallel measurements of dust mass and fiber counts in the factory showed that the relationship between dust and fiber concentrations varied by workshop. Fiber levels were low relative to dust mass in the asbestos cement and rubber workshops
• The authors' conclusion: The author concluded that the amphibole hypothesis (that amphiboles cause cancer but chrysotile does not) cannot be accepted. The reasons given were possible multiple mechanisms of asbestos carcinogenicity and the unavoidable contamination of chrysotile with tremolite. As IARC assessed, all forms of asbestos including chrysotile should be regarded as carcinogenic to humans (Group 1). The author suggested that the textile mystery may result from overestimated fiber exposure in mines, where a single dust-to-fiber conversion factor was used. The author also stated that efforts to control asbestos should continue.
• Limitations the authors state:
  • In the textile-versus-mine comparison, the fiber-monitoring team could not get permission to visit the mine, so fiber exposure was estimated from dust measurements rather than standard fiber measurements
  • The explanation that mine fiber concentrations were overestimated could not be confirmed without exact fiber measurements, so it remains speculation
  • The factory's routine monitoring measured dust concentration, not fiber concentration

Topics

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

Medical terms in this study, explained in our Lexicon: amphibole hypothesis, Amphibole Group, Crocidolite, Serpentine Group, Chrysotile, Carcinogenic, Tremolite, IARC.

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