Biopersistence of rock wool in lungs after short-term inhalation in rats

Our plain-English summary of Biopersistence of rock wool in lungs after short-term inhalation in rats, by Y. Kudo, Y. Aizawa (Inhalation Toxicology, 2008).

The study

• Authors: Y. Kudo, Y. Aizawa
• Journal: Inhalation Toxicology
• Published: 2008;20(2):139-47
• Cited by: 5 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

To evaluate the safety of rock wool (RW), an asbestos substitute, we examined the biopersistence of RW fibers in rat lungs based on the changes of fiber number and fiber size (length and diameter) by a nose-only inhalation exposure study. Twenty-four male Fischer 344 rats were exposed to RW fibers at a concentration of 30 mg/m(3) continuously for 3 h daily for 5 consecutive days. Six rats each were sacrificed shortly and at 1, 2, and 4 wk after exposure, and their lung tissues were ashed by a low-temperature plasma asher. Then the fiber numbers and fiber sizes in lungs were determined using a phase-contrast microscope and computed image analyzer. During the study period, the arithmetic mean (SD) values of fiber and weight concentrations were 78.5 (35.7) fibers/cm(3), and 29.9 (28.3) mg/m(3), respectively. The fiber number in lungs 4 wk after exposure significantly decreased from the baseline value (shortly after exposure) (p < .05). The half-life of fibers calculated from the approximate curve was 28 days for all fibers and 16 days for fibers with L > 20 microm, and the rate of decrease in fiber number was 46.3% at 4 wk after exposure (shortly-after group = 100%). Likewise, both length and diameter significantly decreased at 4 wk after exposure (p < .05), probably because fibers were phagosytosed and digested by alveolar macrophages, discharged to outside of the body by mucociliary movement, or dissolved by body fluid. It will be necessary in the future to further confirm the safety of RW fibers by assessing the biopersistence of fibers in the lungs and their pathological effects in our ongoing study performed in accordance with the guidelines established in the "Methods for Determination of Hazardous Properties for Human Health of Man Made Mineral Fibers" (EC protocol).

In plain English

This was an animal experiment on how long rock wool stays in rat lungs. Rock wool is a man-made vitreous fiber used as an asbestos substitute. Twenty-four male Fischer 344 rats breathed rock wool fibers through the nose for 3 hours a day for 5 consecutive days. Six rats were examined shortly after exposure and six each at 1, 2 and 4 weeks. Four weeks after exposure, the total number of fibers in the lungs was 46.3% of the number found shortly after exposure, a statistically significant drop. The calculated half-life was 28 days for all fibers and 16 days for fibers longer than 20 micrometers. Average fiber length in the lungs fell from 7.51 to 6.35 micrometers, and average diameter fell from 0.85 to 0.74 micrometers. The authors concluded that these fibers dissolved more easily and cleared faster than the rock wool used in an earlier study. They also said their results cannot be directly compared with earlier studies. They called for further work to confirm safety, including the fibers' effects on lung tissue, in an ongoing study.

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: short-term nose-only inhalation study measuring how long rock wool fibers persist in rat lungs (biopersistence)
• Who was studied: 24 male Fischer 344 rats, 6 to 10 weeks old, studied in two experiments of 12 rats each. Six rats were killed shortly after the last exposure and six each at 1, 2 and 4 weeks after. The study was done at Kitasato University School of Medicine, Japan. Study dates are not given.
• Exposure: Rock wool (a man-made vitreous fiber) made by NT Co. Ltd. and supplied by the Rock Wool Association, Japan. Its composition was 40% SiO2, 37% CaO, 14% Al2O3 and 6% MgO, plus minor components. The fibers were pressed to adjust their size. Rats breathed them through the nose for 3 hours a day for 5 consecutive days. The target concentration was 30 mg/m3 by mass and 50 plus or minus 10 fibers/cm3.
• Main results:
  • Measured airborne concentration in the exposure chamber, as given in the Results section: arithmetic mean 240.5 (SD 62.9) total fibers/cm3, including 73.0 (20.4) fibers/cm3 longer than 20 micrometers. The abstract instead gives 78.5 (35.7) fibers/cm3 and 29.9 (28.3) mg/m3.
  • In the group examined 4 weeks after exposure, the total number of fibers per lung was 46.3% of the number in the group examined shortly after exposure (the shortly-after group is set at 100%).
  • At 4 weeks, the total fiber number, fibers 5 to 20 micrometers long, fibers longer than 20 micrometers, and WHO fibers were all significantly lower than in the shortly-after group (p less than 0.05).
  • Calculated half-life of fibers in the rat lungs, by fiber length:
• all fibers: 28 days
• 5 micrometers or shorter: 47 days
• over 5 up to 20 micrometers: 29 days
• longer than 20 micrometers: 16 days
• WHO fibers: 27 days
  • The geometric mean length of fibers in the lungs fell from 7.51 micrometers shortly after exposure to 6.35 micrometers at 4 weeks. The decrease was significant at 2 and 4 weeks (p less than 0.05).
  • The geometric mean diameter of fibers in the lungs fell from 0.85 micrometers shortly after exposure to 0.74 micrometers at 4 weeks. The decrease was significant at 1, 2 and 4 weeks (p less than 0.05).
  • The authors estimated the dissolution rate of this rock wool from its chemical composition using Eastes's formula. The estimate was 2451 ng/cm3/h, compared with 25 ng/cm3/h for the rock wool in a 1996 study by Hesterberg et al.
• The authors' conclusion: The authors concluded that the rock wool fibers in this study dissolved more easily and cleared faster from rat lungs than the rock wool used in a 1996 study by Hesterberg et al.
• Limitations the authors state:
  • The authors said their results cannot be directly compared with earlier studies. The concentration of long fibers, the way half-life was calculated, and the exposure time all differed.
  • The authors noted that 30% of inhaled long fibers have been reported to clear within 24 hours. Because of how their starting measurement was timed, they said their results should be evaluated carefully.
  • The authors said the safety of rock wool fibers still needs further confirmation. They plan to assess biopersistence and pathological effects in an ongoing study that follows the EC protocol.

Topics

Medical terms in this study, explained in our Lexicon: biopersistence, Alveolar Macrophage, Mucociliary Clearance, Mineral Fiber.

Related studies

• Comparison of Calidria chrysotile asbestos to pure tremolite: inhalation biopersistence and histopathology following short-term exposure — D.M. Bernstein, J. Chevalier, et al. (2003)
• Evaluation of the deposition, translocation and pathological response of brake dust with and without added chrysotile in comparison to crocidolite asbestos following short-term inhalation: interim results — D.M. Bernstein, R. Rogers, et al. (2014)
• Comparison of Calidria chrysotile asbestos to pure tremolite: final results of the inhalation biopersistence and histopathology examination following short-term exposure — D.M. Bernstein, J. Chevalier, et al. (2005)
• 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)
• 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)
• Comparative cytotoxicity study of rock wool and chrysotile by cell magnetometric evaluation — Y. Kudo, M. Watanabe, et al. (2003)

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