Synthetic vitreous fibers: a review of toxicology research and its impact on hazard classification

Our plain-English summary of Synthetic vitreous fibers: a review of toxicology research and its impact on hazard classification, by T.W. Hesterberg, G.A. Hart (Critical Reviews in Toxicology, 2001).

The study

• Authors: T.W. Hesterberg, G.A. Hart
• Journal: Critical Reviews in Toxicology
• Published: 2001;31(1):1-53
• Type of article: Review
• Cited by: 71 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Because the inhalation of asbestos, a naturally occurring, inorganic fibrous material, is associated with lung fibrosis and thoracic cancers, concerns have been raised about the possible health effects of synthetic vitreous fibers (SVFs). SVFs include a very broad variety of inorganic fibrous materials with an amorphous molecular structure. Traditionally, SVFs have been divided into three subcategories based on composition: fiberglass, mineral wool (rock, stone, and slag wools), and refractory ceramic fiber. For more than 50 years, the toxicologic potential of SVFs has been researched extensively using human epidemiology and a variety of laboratory studies. Here we review the research and its impact on hazard classification and regulation of SVFs. Large, ongoing epidemiology studies of SVF manufacturing workers have provided very little evidence of harmful effects in humans. Several decades of research using rodents exposed by inhalation have confirmed that SVF pulmonary effects are determined by the "Three D's", fiber dose (lung), dimension, and durability. Lung dose over time is determined by fiber deposition and biopersistence in the lung. Deposition is inversely related to fiber diameter. Biopersistence is directly related to fiber length and inversely related to fiber dissolution and fragmentation rates. Inhaled short fibers are cleared from the lung relatively quickly by mobile phagocytic cells, but long fibers persist until they dissolve or fragment. In contrast to asbestos, most of the SVFs tested in rodent inhalation studies cleared rapidly from the lung (were nonbiopersistent) and were innocuous. However, several relativley biopersistent SVFs induced chronic inflammation, lung scarring (fibrosis), and thoracic neoplasms. Thus, biopersistence of fibers is now generally recognized as a key determinant of the toxicologic potential of SVFs. In vitro dissolution of fibers in simulated extracellular fluid correlates fairly well with fiber biopersistence in the lung and pulmonary toxicity, but several exceptions suggest that biopersistence involves more than dissolution rate. Research demonstrating the relationship between biopersistence and SVF toxicity has provided a scientific basis for hazard classification and regulation of SVFs. For a nonhazardous classification, legislation recently passed by the European Union requires a respirable insulation wool to have a low lung-biopersistence or be noncarcinogenic in laboratory rats. U.S. fiberglass and mineral wool industries and the Occupational Health and Safety Administration (OSHA) have formed a voluntary Health and Safety Partnership Program (HSPP) that include: a voluntary permissible exposure level (PEL) in the workplace of 1 fiber/cc, a respiratory protection program for specified tasks, continued workplace air monitoring, and, where possible, the development of fiber formulations that do not persist in the lung. RCF manufacturers have implemented a Product Stewardship Program that includes: a recommended exposure guideline of 0.5 fibers/cc; a 5-year workplace air monitoring program; and research into the development of high-temperature-resistant, biosoluble fibers.

In plain English

This article reviewed research on synthetic vitreous fibers (SVFs: fiberglass, mineral wools, refractory ceramic fiber or RCF) and its effect on hazard classification. It covered worker studies, rodent inhalation studies (many industry-sponsored) and laboratory dissolution tests. The authors reported that large worker studies gave very little evidence of harm. It cited a study finding pleural plaques in 21% of U.S. RCF workers with at least 20 years in production. In rats, two insulation fiberglasses produced lung tumors only at background levels, a finding the review notes was disputed. After chronic inhalation of high RCF1 concentrations, 41% of hamsters developed thoracic mesotheliomas. Durable JM 475 fiberglass produced one mesothelioma among 83 hamsters (1.2%). Chrysotile and crocidolite asbestos, used as positive controls, produced tumors in rats. The authors said they were unsuitable for direct comparison with SVFs. Long fibers had weighted rat-lung clearance half-times of 817 days for crocidolite and 418 for amosite, versus 6 to 67 days for SVFs. The authors concluded that biopersistence is a key determinant of SVF toxicity. They noted that rats and hamsters responded differently, leaving unclear which species, if either, represents humans.

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: Narrative review of SVF toxicology research (epidemiology, rodent inhalation and biopersistence studies, in vitro dissolution) and of hazard classification and regulation
• Who was studied: Published studies of SVF manufacturing workers in Europe, the U.S. and Canada; rodent inhalation studies (mainly Fischer 344 rats and Syrian golden hamsters), many conducted 1988 to 1999 under fiber-industry sponsorship; and in vitro dissolution studies
• Exposure: Synthetic vitreous fibers: fiberglass, mineral wool (rock, stone and slag wools) and refractory ceramic fiber. Chrysotile, crocidolite and amosite asbestos were included in some rodent studies as positive controls.
• Main results:
  • The authors summarized worker epidemiology as giving very little evidence of harmful effects in humans
  • In a cited study of U.S. RCF workers (Lockey et al., 1996), pleural plaques were found in 21% of those who had worked at least 20 years in RCF production. The same study reported 3% among all RCF workers and 1% among nonproduction employees.
  • In the European mortality cohort through 1990, five mesotheliomas were identified by death certificate: one among more than 11,000 glass wool workers and four among more than 10,000 rock/slag wool workers. Two of the four came from a location where asbestos had been processed.
  • In rats exposed for 2 years to two building insulation fiberglasses (MMVF10 and MMVF11), lung tumor incidence was at background levels. The review notes that this finding was disputed.
  • In parallel rat studies, chrysotile asbestos produced a 20% tumor incidence (13 lung tumors and 1 mesothelioma) and crocidolite asbestos produced 14% (15 lung tumors and 1 mesothelioma). The authors stated that these two asbestos types were not appropriate for fiber-to-fiber comparison with SVFs.
  • After chronic inhalation of high concentrations of RCF1, 41% of hamsters but only 1.7% of rats developed thoracic mesotheliomas
  • In hamsters, JM 475 special application fiberglass produced minimal lung fibrosis and one mesothelioma among 83 animals at risk (1.2%)
  • In the same hamster study, amosite asbestos produced mesotheliomas in the low-, mid- and high-dose groups: 3 (3.6%), 22 (25.9%) and 17 (19.5%) animals, respectively
  • In rat biopersistence studies, fibers longer than 20 µm had weighted clearance half-times of 817 days for crocidolite and 418 days for amosite, compared with 6 to 67 days for the SVFs
  • In the authors' comparison, the fiber concentration in rat inhalation studies was 30-fold higher than the highest human exposure, which was blowing installation of unbound fiberglass
• The authors' conclusion: The authors concluded that how long fibers persist in the lung (biopersistence) is a key determinant of the toxic potential of SVFs, and that this provides a basis for hazard classification
• Limitations the authors state:
  • Rats and hamsters responded very differently to the same fiber, and it is unclear which species, if either, represents humans
  • Chrysotile and crocidolite asbestos differed from the SVFs in fiber dimensions and aerosol concentrations, so they could not be compared fiber-to-fiber with SVFs
  • Methods for measuring fiber dissolution in the laboratory have not yet been agreed or standardized
  • The review does not fully cover the in vitro dissolution literature

Topics

Medical terms in this study, explained in our Lexicon: Vitreous, Fibrosis, Epidemiology, Deposition, biopersistence, Fiber Length, Chronic Inflammation, Neoplasm.

Related studies

• Investigation on the durability of man-made vitreous fibers in rat lungs — B. Bellmann, H. Muhle, et al. (1994)
• Comparison of cytotoxicity of man-made vitreous fibres — K. Luoto, M. Holopainen, et al. (1997)
• Free radical activity of synthetic vitreous fibers: iron chelation inhibits hydroxyl radical generation by refractory ceramic fiber — D.M. Brown, C. Fisher, et al. (1998)
• Relationship between lung biopersistence and biological effects of man-made vitreous fibers after chronic inhalation in rats — T.W. Hesterberg, W.C. Miiller, et al. (1994)
• Differential in vitro cellular response induced by exposure to synthetic vitreous fibers (SVFs) and asbestos crocidolite fibers — G. Cardinali, D. Kovacs, et al. (2006)
• Interspecies comparisons of the toxicity of asbestos and synthetic vitreous fibers: a weight-of-the-evidence approach — L.D. Maxim, E.E. McConnell (2001)

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