Behavior of new type of rock wool (HT wool) in lungs after exposure by nasal inhalation in rats

Our plain-English summary of Behavior of new type of rock wool (HT wool) in lungs after exposure by nasal inhalation in rats, by Y. Kudo, K. Shibata, T. Miki, et al (Environmental Health and Preventive Medicine, 2005).

The study

• Authors: Y. Kudo, K. Shibata, T. Miki, M. Ishibashi, K. Hosoi, T. Sato, N. Kohyama, Y. Aizawa
• Journal: Environmental Health and Preventive Medicine
• Published: 2005;10(5):239-48
• Cited by: 9 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: Previous types of rock wool has been recently replaced with high-temperature wool (HT wool). HT wool is characterized by a chemical composition with a higher concentration of Al(2)O(3) and a lower concentration of SiO(2), lower biopersistence, and a higher melting point than previous types of rock wool. To evaluate the safety of HT wool, an asbestos substitute, we examined the biopersistence of HT wool in the lungs, based on changes in fiber count according to the length and fiber size (length and width), by performing a nose-only inhalation exposure study in rats.

Methods: Male Fischer 344 rats were exposed to fibers at the target exposure concentration of 30 mg/m(3) continuously for 3 hours daily for 5 consecutive days. Rats were sacrificed shortly after exposure, and 1, 2, and 4 weeks after exposure, and their lung tissues were incinerated at a low temperature. Then, fiber counts and sizes in the lungs were analyzed using a phase contrast microscope.

Results: The fiber count in the lungs 4 weeks after exposure significantly decreased from the baseline value (shortly after exposure). The half-life of fibers calculated from the approximation curve was 34 days for all fibers and 11 days for fibers longer than 20 μm.

Conclusions: Both the length and width significantly decreased 4 weeks after exposure, probably because fibers were ingested by alveolar macrophages, discharged to outside of the body by mucociliary movement, or lysed by body fluid. In future studies, it is necessary to examine the long-term persistence of fibers in the lungs.

In plain English

This was a rat experiment on HT wool (high-temperature wool), a newer type of rock wool used as an asbestos substitute. No asbestos was tested. In Japan, 24 male Fischer 344 rats were assigned to breathe HT wool through the nose for 3 hours a day for 5 days. One rat died on day 3 and was excluded, leaving 5 rats in the shortly-after group and 6 in each other group. The target was 30 mg/m3; the measured chamber average was 11.0 mg/m3 and 31.2 fibers/cm3. Groups were killed shortly after exposure and 1, 2 and 4 weeks later, and their lung fibers were counted. At 4 weeks, the total lung fiber count was 54.8% of the shortly-after level, a statistically significant decrease. The count of fibers longer than 20 μm was 15.0% of that same category's shortly-after count. The calculated half-life was 34 days for all fibers and 11 days for fibers longer than 20 μm. Average fiber length fell from 7.93 to 6.37 μm, and eroded or folded fibers were seen. The authors concluded that HT wool fibers longer than 20 μm appeared relatively safe because of their shorter half-life, and that longer-term studies are needed.

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 exposure study in rats, with lung fiber burden measured at four time points after exposure
• Who was studied: 24 male Fischer 344 rats, 6 to 10 weeks old, in four groups of six. One rat died on day 3 and was excluded, leaving 5 rats in the shortly-after group and 6 in each of the other groups. The study was done at Kitasato University School of Medicine, Japan; dates not given.
• Exposure: HT wool, a high-temperature rock wool (man-made vitreous fiber) with 43% SiO2 and 18% Al2O3, supplied by the Rock Wool Association of Japan. It was inhaled through the nose for 3 hours a day for 5 consecutive days, at a target mass concentration of 30 mg/m3.
• Main results:
  • Measured exposure chamber concentrations (arithmetic mean, standard deviation) were 31.2 (21.6) fibers/cm3 and 11.0 (7.5) mg/m3.
  • Total fiber count in the lungs fell from a geometric mean of 9.036 x10^5 per lung shortly after exposure to 4.955 x10^5 at 4 weeks (54.8% of the shortly-after value). This was significantly lower than the shortly-after, 1-week and 2-week groups (p less than 0.05).
  • Fibers longer than 20 μm fell from 0.628 to 0.094 x10^5 per lung at 4 weeks, which was 15.0% of that category's shortly-after count. This was significantly lower than the shortly-after and 1-week groups (p less than 0.05).
  • At 4 weeks, the total count and the counts of fibers 5 to 20 μm long, longer than 20 μm, and WHO fibers were significantly lower than in the shortly-after group (p less than 0.05).
  • Calculated half-life in the lungs was 34 days for all fibers and 105 days for fibers 5 μm or shorter.
  • Calculated half-life was 29 days for fibers longer than 5 μm up to 20 μm, 11 days for fibers longer than 20 μm, and 27 days for WHO fibers.
  • Geometric mean fiber length in the lungs fell from 7.93 μm shortly after exposure to 6.37 μm at 4 weeks (p less than 0.05).
  • Geometric mean fiber width in the lungs fell from 1.30 μm shortly after exposure to 1.16 μm at 4 weeks (p less than 0.05).
  • Partly eroded or transversely folded fibers were seen in the rats' lungs.
• The authors' conclusion: The authors concluded that their results suggest HT wool fibers longer than 20 μm are relatively safe because of their shorter half-life. They said this short-term study suggested HT wool is a relatively safe substance.
• Limitations the authors state:
  • The authors stated that the long-term persistence and pathological (disease) effects of the fibers in the lungs still need to be investigated; this study followed rats for only 4 weeks.

Topics

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

Related studies

• Biopersistence of rock wool in lungs after short-term inhalation in rats — Y. Kudo, Y. Aizawa (2008)
• Comparative cytotoxicity study of rock wool and chrysotile by cell magnetometric evaluation — Y. Kudo, M. Watanabe, et al. (2003)
• Effects of rock wool on the lungs evaluated by magnetometry and biopersistence test — Y. Kudo, M. Kotani, et al. (2009)
• Cytotoxicity study of rock wool by cell magnetometric evaluation — Y. Kudo, M. Kotani, et al. (2009)
• Studies on the chronic toxicity (inhalation) of four types of refractory ceramic fiber in male Fischer 344 rats — R.W. Mast, E.E. McConnell, et al. (1995)
• Alveolar type II cell responses to chronic inhalation of chrysotile asbestos in rats — K.E. Pinkerton, S.L. Young, et al. (1990)

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