Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers

Our plain-English summary of Perspectives on refractory ceramic fiber (RCF) carcinogenicity: comparisons with other fibers, by H. Greim, M.J. Utell, L.D. Maxim, et al (Inhalation Toxicology, 2014).

The study

• Authors: H. Greim, M.J. Utell, L.D. Maxim, R. Niebo
• Journal: Inhalation Toxicology
• Published: 2014;26(13):789-810
• Type of article: Review
• Cited by: 12 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

In 2011, SCOEL classified RCF as a secondary genotoxic carcinogen and supported a practical threshold. Inflammation was considered the predominant manifestation of RCF toxicity. Intrapleural and intraperitoneal implantation induced mesotheliomas and sarcomas in laboratory animals. Chronic nose-only inhalation bioassays indicated that RCF exposure in rats increased the incidence of lung cancer and similar exposures resulted in mesothelioma in hamsters, but these studies may have been compromised by overload. Epidemiological studies in the US and Europe showed an association between exposure and prevalence of respiratory symptoms and pleural plaques, but no interstitial fibrosis, mesotheliomas, or increased numbers of lung tumors were observed. As the latency of asbestos induced mesotheliomas can be up to 50 years, the relationship between RCF exposure and respiratory malignances has not been fully determined. Nonetheless, it is possible to offer useful perspectives. RCF and rock wool have similar airborne fiber dimensions and biopersistence. Therefore, it is likely that these fibers have similar toxicology. Traditional rock wool has been the subject of numerous cohort and case control studies. For rock wool, IARC (2002) concluded that the epidemiological studies did not provide evidence of carcinogenicity. Based on analogies with rock wool (read across), it is reasonable to believe that increases in lung cancer or any mesotheliomas are unlikely to be found in the RCF-exposed cohort. RCF producers have developed a product stewardship program to measure and control fiber concentrations and to further understand the health status of their workers.

In plain English

This review compared refractory ceramic fiber (RCF) with traditional rock wool using animal, fiber and worker studies. The authors judged their measurements of 4031 airborne workplace RCF fibers (geometric mean diameter 0.91 micrometers, length about 17 micrometers) similar to airborne rock wool. Average rat lung half-times for fibers 20 micrometers or longer were about 50 days for RCF and 59 for rock wool, a difference the authors called neither statistically significant nor material. Both were far less biopersistent than amosite or crocidolite asbestos. In rats, inhaled RCF produced fibrosis and tumors, in a study possibly compromised by overload; rock wool produced fibrosis but no tumors. One single-dose, nose-only hamster study (30 mg/m3, 18 months) found pleural mesotheliomas in 41% of RCF-exposed animals; this model's relevance to humans has been questioned. RCF worker studies showed dose-related pleural plaque increases but no interstitial fibrosis, excess lung cancer or mesothelioma. The 942-worker mortality study had limited power and found an unexpected urinary organ cancer association. The authors wrote that these similarities suggest possible RCF risks have been overstated, and that the mortality study must continue.

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 article comparing published data on refractory ceramic fiber and traditional rock wool, plus the authors' summary of airborne RCF fiber size measurements
• Who was studied: Published animal, biopersistence and epidemiological studies of RCF and rock wool, and 4031 airborne respirable RCF fibers from workplace personal monitoring samples collected under the RCF product stewardship program
• Exposure: Refractory ceramic fiber (aluminosilicate wool), a synthetic vitreous fiber, compared with traditional rock wool (MMVF21), with amphibole asbestos (amosite, crocidolite, tremolite) used as a reference
• Main results:
  • Airborne respirable RCF fibers had a geometric mean diameter of 0.91 micrometers and a geometric mean length of about 17 micrometers
  • Although bulk fiber diameters differ, the authors found airborne RCF and rock wool fiber diameters quite similar
  • The biopersistence measure used was the weighted half-time (WT1/2) for fibers 20 micrometers or longer, measured in laboratory animals
  • Published RCF weighted half-times (WT1/2) ranged from 41 to 64 days, with an arithmetic mean of about 50 days
  • The authors judged the difference between the RCF (50 days) and traditional rock wool (59 days) weighted half-times not statistically significant and not material
  • Both RCF and rock wool were much less biopersistent than amphibole asbestos such as amosite
  • In chronic rat inhalation studies, RCF produced fibrosis and tumors, while rock wool (MMVF21) produced fibrosis but no tumors
  • In one chronic, single-dose, nose-only inhalation study, 41% of Syrian golden hamsters exposed to RCF at 30 mg/m3 for 18 months developed pleural mesotheliomas; no comparable hamster study of rock wool exists
  • Epidemiological studies of RCF workers showed a dose-related increase in pleural plaques but no interstitial fibrosis, no elevated lung cancer rates and no mesotheliomas
  • The RCF mortality study found an unexpected, statistically significant association with cancers of the urinary organs
  • In one rock wool factory study, pleural plaques were found in 1.59% of exposed workers (12 of 754) and 0.55% of referents (3 of 541). The difference was not statistically significant (p of 0.09).
  • IARC moved rock wool from Group 2B to Group 3
• The authors' conclusion: The authors wrote that the similarities between RCF and rock wool suggest that possible risks of occupational RCF exposure have been overstated. By analogy with rock wool, they judged increases in lung cancer or any mesotheliomas unlikely in the RCF-exposed cohort. They added that the RCF mortality study must continue to provide definitive evidence on mesothelioma.
• Limitations the authors state:
  • The RCF mortality study is limited by sample size (942 workers) and by limited time since first exposure (mean of 21 years)
  • The RCF mortality study has limited power to address mesothelioma
  • The RCF rat inhalation study may have been compromised by overload from a test substance not representative of workplace fiber
  • The relevance of the hamster model for assessing mesothelioma in humans has been questioned
  • Only one study of pleural plaques in rock wool workers was available
  • Definitive evidence on mesothelioma requires continued follow-up of the RCF mortality study

Topics

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

Medical terms in this study, explained in our Lexicon: Inflammation, Mesothelioma, Sarcoma, Lung Cancer, Pleural Plaques, Interstitial Fibrosis, Latency, Fiber Dimensions.

Related studies

• The importance of fiber biopersistence and lung dose in determining the chronic inhalation effects of X607, RCF1, and chrysotile asbestos in rats — T.W. Hesterberg, G.A. Hart, et al. (1998)
• Are airborne refractory ceramic fibers similar to asbestos in their carcinogenicity? — A.M. Walker, L.D. Maxim, et al. (2012)
• Pulmonary and pleural leukocytes from F344 rats produce elevated levels of fibronectin following inhalation of refractory ceramic fibers — T.R. Gelzleichter, J.B. Mangum, et al. (1996)
• A 30-year mortality and respiratory morbidity study of refractory ceramic fiber workers — G. LeMasters, J.E. Lockey, et al. (2017)
• Refractory ceramic fibers: Fiber characteristics, potential health effects and clinical observations — M.J. Utell, L.D. Maxim (2018)
• Refractory ceramic fiber (RCF) toxicity and epidemiology: a review — M.J. Utell, L.D. Maxim (2010)

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