Refractory ceramic fiber (RCF) toxicity and epidemiology: a review

Our plain-English summary of Refractory ceramic fiber (RCF) toxicity and epidemiology: a review, by M.J. Utell, L.D. Maxim (Inhalation Toxicology, 2010).

The study

• Authors: M.J. Utell, L.D. Maxim
• Journal: Inhalation Toxicology
• Published: 2010;22(6):500-21
• Type of article: Review
• Cited by: 20 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This paper provides a review of the relevant literature on refractory ceramic fibers (RCFs), summarizing relevant data and information on the manufacture, processing, applications, potential occupational exposure, toxicology, epidemiology, risk analysis, and risk management. RCFs are amorphous fibers used for high-temperature insulation applications. RCFs are less durable/biopersistent than amphibole asbestos, but more durable/biopersistent than many other synthetic vitreous fibers (SVFs). Moreover, as produced/used, some RCFs are respirable. Toxicology studies with rodents using various exposure methods have shown that RCFs can cause fibrosis, lung cancer, and mesothelioma. Interpretation of these animal studies is difficult for various reasons (e.g., overload in chronic inhalation bioassays). Epidemiological studies of occupationally exposed cohorts in Europe and the United States have demonstrated measurable effects (e.g., mild respiratory symptoms and pleural plaques) but no disease (i.e., no interstitial fibrosis, no excess lung cancer, and no mesothelioma) to date. The RCF industry, working cooperatively with various government agencies in the United States, has developed a comprehensive product stewardship program (PSP) to identify and control risks associated with occupational exposure. One provision of the PSP is the adoption of a voluntary recommended exposure guideline (REG) of 0.5 fibers/milliliter (f/ml). Selected on the basis of prudence and demonstrated feasibility, compliance with the REG should reduce risks to levels between 0.073/1000 and 1.2/1000, based on extrapolations from chronic animal inhalation studies.

In plain English

This article was a review of published literature on refractory ceramic fibers (RCFs). These are amorphous fibers made by melting alumina and silica and used for high-temperature insulation. It covered manufacture, workplace exposure, animal toxicology, studies of workers in the United States and Europe, and risk estimates. The authors reported that animal inhalation studies found high concentrations of RCF can cause fibrosis and tumors. They said these studies were hard to interpret because of lung "overload." Among the worker studies reviewed, a US study found pleural changes in 27 workers (2.7%), with an odds ratio of 6.0 (95% CI 1.4 to 31.0) at the highest cumulative exposure level. A US mortality study found no increase in all deaths or cancer deaths. It did find a statistically significant excess of cancers of the urinary organs (SMR 334.8, 95% CI 111.6 to 805.4), whose biological significance the authors called uncertain. The authors concluded that RCF exposure irritates the skin and causes some respiratory symptoms and pleural plaques. They said studies had so far failed to show progressive lung function loss, interstitial fibrosis, excess lung cancer or mesothelioma. They noted the mortality study was small, its cohort young, and its power limited.

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 literature on refractory ceramic fibers (an update of an earlier review), covering manufacture, exposure, toxicology, epidemiology, risk analysis and risk management
• Who was studied: Reviewed rodent studies and worker studies. The worker studies included a US cross-sectional study begun in 1987 of 1030 eligible RCF industry employees (753 current, 277 former) and companion European studies.
• Exposure: Refractory ceramic fibers (RCFs): amorphous synthetic vitreous fibers made by melting alumina and silica, used for high-temperature insulation. Some comparisons were made with crocidolite, amosite and chrysotile asbestos.
• Main results:
  • In the animal inhalation studies reviewed, exposure to high concentrations of RCF caused fibrosis and tumors.
  • In a reviewed US study (LeMasters et al., 1998), 29.6% of male production workers and 11.3% of male nonproduction workers reported one or more respiratory symptoms. The adjusted odds ratio was 2.9 (95% CI 1.4 to 6.2).
  • The same US study found a statistically significant FVC decline of 165 ml and 156 ml per 10 years of RCF production work in male current and past smokers. The review authors characterized this as statistically but not clinically significant.
  • A later longitudinal analysis of 361 male production workers (Lockey et al., 1998) found no excessive decline in lung function over 7 years of follow-up.
  • In the US cohort, pleural changes were seen in 27 workers (2.7%).
  • In the US cohort (Lockey et al., 2002), the odds ratio for pleural changes at the highest cumulative exposure level (more than 135 fiber-months per cubic centimeter) was 6.0 (95% CI 1.4 to 31.0).
  • The US mortality study of male workers at two RCF plants (LeMasters et al., 2003) found a statistically significant association with cancers of the urinary organs: SMR 334.8 (95% CI 111.6 to 805.4).
  • In the same mortality study, a proportional hazards analysis did not show elevated total risk with cumulative RCF exposure: risk ratio 0.99 (95% CI 0.85 to 1.16).
  • A reviewed analysis (Walker et al., 2002) found the cohort's lung cancer mortality was statistically incompatible with RCF being as potent as amphibole asbestos. It could not exclude RCF being as potent as chrysotile asbestos.
  • Industry monitoring data showed weighted average workplace fiber concentrations across job categories of about 0.2 to 0.3 f/ml, with most concentrations at or below 1 f/ml.
  • The NIOSH estimate quoted in the review was that keeping exposures below the 0.5 fiber recommended limit would reduce lung cancer risk to between 0.073 and 1.2 per 1,000. This estimate was based on extrapolations from animal-based risk models.
• The authors' conclusion: The authors concluded that RCF exposure is irritating to the skin and causes selected respiratory symptoms and pleural plaques. They added that epidemiological studies had thus far failed to show progressive decrements in lung function, interstitial fibrosis, excess lung cancer or mesothelioma. They said the biological significance of the urinary cancer finding is uncertain.
• Limitations the authors state:
  • The US mortality study's cohort was relatively young and small.
  • The mortality study had only 40% power to detect a 2-fold increase in lung cancer.
  • Interpretation of the main animal inhalation experiments is problematic because they exhibited lung overload.
  • Estimated risks for exposed workers vary by a factor of nearly 1000 depending on the models and assumptions used.

Topics

Medical terms in this study, explained in our Lexicon: Epidemiology, Occupational Exposure, Amphibole Asbestos, Vitreous, Fibrosis, Lung Cancer, Mesothelioma, Pleural Plaques.

Related studies

• Refractory ceramic fibers: Fiber characteristics, potential health effects and clinical observations — M.J. Utell, L.D. Maxim (2018)
• Review of refractory ceramic fiber (RCF) toxicity, epidemiology and occupational exposure — L.D. Maxim, M.J. Utell (2018)
• Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers — H. Greim, M.J. Utell, et al. (2014)
• Are airborne refractory ceramic fibers similar to asbestos in their carcinogenicity? — A.M. Walker, L.D. Maxim, et al. (2012)
• Risk analysis for mortality from respiratory tumors in a cohort of refractory ceramic fiber workers — A.M. Walker, L.D. Maxim, et al. (2002)
• Refractory ceramic fiber exposure and pleural plaques — J. Lockey, G. Lemasters, et al. (1996)

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