Comparative investigations of the biodurability of mineral fibers in the rat lung

Our plain-English summary of Comparative investigations of the biodurability of mineral fibers in the rat lung, by H. Muhle, B. Bellmann, F. Pott (Environmental Health Perspectives, 1994).

The study

• Authors: H. Muhle, B. Bellmann, F. Pott
• Journal: Environmental Health Perspectives
• Published: 1994;102 Suppl 5(Suppl 5):163-8
• Cited by: 29 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

The biodurability of various glass fibers, rockwool, and ceramic fibers was examined in rat lungs and compared with natural mineral fibers. Experiments were based on studies that have shown that the biodurability of fibers is one of the essential factors of the carcinogenic potency of these materials. Sized fractions of fibers were instilled intratracheally into Wistar rats. The evenness of distribution of fibers in the lung was checked by scanning electron microscopy (SEM) or careful examination of the fiber suspension before treatment. After serial sacrifices up to 24 months after treatment, the fibers were analyzed by SEM following low temperature ashing of the lungs. Parameters measured included number of fibers, diameter, and length distribution at the various sacrifice dates, so that analyses could be made of the elimination kinetics of fibers from the lung in relation to fiber length (FL). Size selective plots of the fiber elimination correlated with fiber diameters enables the mechanism of the fiber elimination (dissolution, fiber breakage, physical clearance) to be interpreted. The half-time of fiber elimination from the lung ranges from about 10 days for wollastonite to more than 300 days for crocidolite. The biodurability of man-made vitreous fibers (MMVF) is between these values and is dependent on the chemical composition of the fibers and the diameter and length distribution. Results indicate that the in vivo durability of glass fibers is considerably longer than expected from extrapolation of published data on their in vitro dissolution rates.

In plain English

In this animal experiment, the authors measured how long various mineral fibers stayed in rat lungs. Female Wistar rats each received a single dose of 2 milligrams of fibers placed directly into the windpipe (intratracheal instillation). Groups of three to six rats were killed at intervals up to two years later. Their lungs were ashed, and the remaining fibers were counted and measured by electron microscopy. The paper gave detailed results for one glass fiber, X607. It also compared earlier results for other glass fibers, glasswool, rockwool, a ceramic fiber, crocidolite, chrysotile and wollastonite, recalculating some of these data sets. For X607 fibers longer than 5 micrometres and thinner than 3 micrometres, the estimated elimination half-time was 46 days (95% confidence limits 41 to 54 days). Across fiber types, elimination half-times ranged from about 10 days for wollastonite to more than 300 days for crocidolite, with man-made vitreous fibers in between. The authors concluded that glass fibers lasted considerably longer in the lung than laboratory dissolution tests would predict. They noted that low fiber counts at late time points limited some earlier results.

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: Animal experiment: single intratracheal instillation of sized fibers into rats with serial sacrifices, plus comparison with earlier instillation and inhalation studies, some of whose data were recalculated
• Who was studied: Female Wistar rats weighing 200 to 220 g; three to six rats per group sacrificed at each time point, up to 24 months after treatment; work at the Fraunhofer-Institut, Hannover, Germany
• Exposure: 2 mg of fibers in 0.3 ml of 0.9% salt solution, given once intratracheally. Glass fiber X607 (CaO 38.3%, SiO2 59.6%) was presented in detail. The paper also compared results for other glass fibers, glasswool, rockwool, ceramic fiber, UICC crocidolite, chrysotile (Zimbabwe; in inhalation data also Calidria and Canada chrysotile) and wollastonite. Some of these data sets were recalculated.
• Main results:
  • Across fiber types, the half-time of fiber elimination from the lung ranged from about 10 days for wollastonite to more than 300 days for crocidolite. Man-made vitreous fibers fell between these values, depending on chemical composition and on fiber diameter and length.
  • For glass fiber X607, the estimated elimination half-time calculated from the number of fibers longer than 5 micrometres and thinner than 3 micrometres was 46 days (95% confidence limits 41 to 54 days)
  • This 46-day half-time for X607 compared with 77 days reported in another (inhalation) study of the same fiber
  • For X607, the half-time for fibers longer than 20 micrometres was short. The authors took this to indicate that breakage and dissolution caused the elimination of these fibers.
  • For X607 particles shorter than 2.5 micrometres, the half-time calculated from particle mass was 68 days. The authors compared this with the alveolar clearance of insoluble particles (about 60 days).
  • Data from earlier studies of a series of fibers were recalculated in the same way as for X607
  • In the recalculated instillation data, glass microfiber 104/475 had a half-time of 1794 days for fibers longer than 5 micrometres, but with a very wide confidence range
  • In the instillation data for chrysotile (Zimbabwe), the half-time calculated from fiber mass was 5000 days (lower 95% confidence limit 772 days)
  • Half-times calculated from another inhalation experiment were in relatively good agreement with those from intratracheal instillation
• The authors' conclusion: The authors concluded that the in vivo durability of glass fibers is considerably longer than expected from published in vitro dissolution rates. They also concluded that in vivo biodurability studies provide more relevant data than in vitro dissolution tests.
• Limitations the authors state:
  • In the earlier experiments, few fibers remained after 1 and 2 years, which made the analysis difficult
  • For glass microfiber 104/475, the number of animals and fibers analyzed may have been too low for adequate statistical analysis
  • Even 2 mg of long fibers may form clumps in the lung after instillation
  • Summarizing elimination as a half-time loses considerable information
  • Earlier calculations for glass fiber 104/E had incorrectly left out data, and the recalculated results were markedly different

Topics

Medical terms in this study, explained in our Lexicon: Mineral Fiber, Carcinogenic, Scanning Electron Microscopy (SEM), Fiber Length, Wollastonite, Crocidolite, Vitreous.

Related studies

• Investigation on the durability of man-made vitreous fibers in rat lungs — B. Bellmann, H. Muhle, et al. (1994)
• Investigation of the biodurability of wollastonite and xonotlite — B. Bellmann, H. Muhle (1994)
• Significance of durability of mineral fibers for their toxicity and carcinogenic potency in the abdominal cavity of rats in comparison with the low sensitivity of inhalation studies — F. Pott, M. Roller, et al. (1994)
• Investigation and analysis of asbestos fibers and accompanying minerals in biological materials — L. Le Bouffant (1974)
• Solubility of asbestos and man-made mineral fibers in vitro and in vivo: its significance in lung disease — A. Morgan, A. Holmes (1986)
• Mortality among a cohort of US man-made mineral fiber workers: 1985 follow-up — G.M. Marsh, P.E. Enterline, et al. (1990)

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