Pulmonary effects and biopersistence of deposited silicon carbide whisker after 1-year inhalation in rats

Our plain-English summary of Pulmonary effects and biopersistence of deposited silicon carbide whisker after 1-year inhalation in rats, by I. Akiyama, A. Ogami, T. Oyabu, et al (Inhalation Toxicology, 2007).

The study

• Authors: I. Akiyama, A. Ogami, T. Oyabu, H. Yamato, Y. Morimoto, I. Tanaka
• Journal: Inhalation Toxicology
• Published: 2007;19(2):141-7
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Silicon carbide whisker (SiCW) is a durable asbestos substitute. To clarify the biopersistence, the changes of geometrical configurations of deposited whiskers, and potential carcinogenicity and fibrogenicity, 42 Wistar male rats were exposed to SiCW for 6 h/day, 5 days/wk for 1 yr by inhalation. The mass median aerodynamic diameter (geometric standard deviation, GSD), the geometric mean fiber diameter (GSD), and the geometric mean fiber length (GSD) were 2.4 microm (2.4), 0.5 microm (1.5) and 2.8 microm (2.3), respectively. The daily average exposure concentrations were 2.6 +/- 0.4 mg/m3 (98 +/- 19 fibers/ml) and the rats were sacrificed at 6 days and 3, 6, and 12 mo after the exposure. The SiCW amount deposited in each rat lung at 6 days after the exposure, determined by an x-ray diffraction method, was 5.3 +/- 1.4 mg. The biological half-time (BHT) was 16 mo calculated from the deposited SiCW at 6 days and 3, 6, and 12 mo. The BHT was more prolonged than normal physiological clearance. The diameter of SiCW in the lung at each sacrificed time during 12 mo of clearance after the inhalation did not change. However, as for the length of SiCW in the lung, longer SiCW tended to be retained in the lung as the clearance time increased, especially after 6 mo. The histopathological examination revealed bronchoalveolar hyperplasia (BAH) in 2 rats at 1 yr after the exposure and severe fibrotic changes around aggregated SiCW.

In plain English

This animal experiment studied silicon carbide whisker (SiCW), described by the authors as a durable asbestos substitute. Of 84 male Wistar rats, 42 breathed SiCW for 6 hours a day, 5 days a week, for 1 year, at an average of 2.6 mg/m3 (98 fibers/ml). The other 42 breathed clean air. Rats were killed 6 days and 3, 6 and 12 months after exposure ended. Survival did not differ significantly between the groups, but exposed rats had significantly heavier lungs than controls at 6 days, 6 months and 12 months. The lungs of exposed rats examined six days after exposure held 5.3 ± 1.4 mg of SiCW. From amounts measured over the follow-up period, the authors estimated a biological half-time of almost 16 months. Fiber diameter in the lungs did not change significantly, while the share of long whiskers (over 20 μm) rose over time, especially after 6 months. At 12 months, marked fibrotic changes surrounded some fiber clusters, and bronchoalveolar hyperplasia was seen in two exposed rats. The authors suggested these effects came from both the hazardous effects of SiCW and an excess burden of fiber in the lung.

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: 1-year inhalation exposure study in rats with a clean-air control group, followed by sacrifice at intervals after exposure ended
• Who was studied: 84 male Wistar rats, 4 weeks old at the start: 42 exposed to SiCW and 42 controls exposed to clean air. Groups were sacrificed at 6 days and 3, 6 and 12 months after the 1-year exposure. Table 3 lists 10, 5, 5 and 11 exposed rats and 10, 5, 5 and 13 control rats at those times. The study was conducted at the University of Occupational and Environmental Health, Japan.
• Exposure: Silicon carbide whisker (TWS-100, Tokai Carbon Co., 98% SiC). Rats inhaled it 6 h/day, 5 days/wk for 1 yr at a daily average of 2.6 ± 0.4 mg/m3 (98 ± 19 fibers/ml). Mass median aerodynamic diameter was 2.4 μm, geometric mean fiber diameter 0.5 μm and geometric mean fiber length 2.8 μm. The authors described this concentration as close to the occupational exposure limit.
• Main results:
  • Survival rates did not differ significantly between exposed and control rats.
  • Exposed rats had significantly heavier lungs than controls at 6 days, 6 months and 12 months after exposure (p less than .01). For example, at 6 days exposed lungs weighed 2.13 ± 0.20 g versus 1.88 ± 0.13 g in controls. Body weight and other organ weights did not differ significantly.
  • In the exposed rats examined 6 days after the 1-year exposure, the SiCW lung burden was 5.3 ± 1.4 mg.
  • The deposited SiCW decreased exponentially over the observation period. The biological half-time, calculated from these measurements with a one-compartment model, was almost 16 months.
  • The diameter of SiCW recovered from the lungs did not change significantly over the observation period.
  • The fraction of long whiskers (over 20 μm) in the lungs increased, and the fraction of short whiskers decreased, with increasing clearance time, especially after 6 months. Geometric mean length was 1.55 μm at 6 days and 2.62 μm at 12 months.
  • At 6 days after exposure, small fiber clusters were spread through the lungs, infiltration of inflammatory cells around them was not remarkable, and there was slight thickening of part of the pleura due to fiber deposition.
  • At 12 months after exposure, fibrotic changes were remarkable around some fiber clusters, with inflammatory cell infiltration.
  • Bronchoalveolar hyperplasia was observed in two rats in the exposed group, with fiber aggregations scattered within it. The figure caption places this at 12 months.
• The authors' conclusion: The authors concluded that 1 year of SiCW inhalation near the occupational exposure limit produced increased lung weight, higher biopersistence and pathological changes. They suggested these were caused by the hazardous effects of SiCW together with an excess burden of fiber in the lung. They concluded that insoluble fibers whose aerodynamic diameter leads to high deposition should be controlled, taking deposition amounts as well as hazardous effects into account.

Topics

Medical terms in this study, explained in our Lexicon: biopersistence, Fiber Length, X-Ray Diffraction (XRD).

Related studies

• Expression of Clara cell secretory protein in the lungs of rats exposed to silicon carbide whisker in vivo — Y. Morimoto, L. Ding, et al. (2003)
• Histopathological changes in rat lung following intratracheal instillation of silicon carbide whiskers and potassium octatitanate whiskers — A. Ogami, Y. Morimoto, et al. (2007)
• Pulmonary response, in vivo, to silicon carbide whiskers — G.L. Vaughan, S.A. Trently, et al. (1993)
• The carcinogenicity of WHO fibers of silicon carbide: SiC whiskers compared to cleavage fragments of granular SiC — K. Rödelsperger, B. Brückel (2006)
• Gene expression of surfactant protein-A and thyroid transcription factor-1 in lungs of rats exposed to silicon-carbide whisker in vivo — Y. Morimoto, L. Ding, et al. (2003)
• In vitro activity of silicon carbide whiskers in comparison to other industrial fibers using four cell culture systems — N.F. Johnson, M.D. Hoover, et al. (1992)

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