The Carcinogenicity of WHO Fibers of Silicon Carbide: SiC Whiskers Compared to Cleavage Fragments of Granular SiC

Abstract

A steep dose-response relationship for tumors was reported after application of 0.05 to 25 mg of whiskers of SiC in 1991 in a chronic intraperitoneal injection study in rats. In contrast, doses of 250 and 1000 mg of granular SiC did not cause tumors. The whiskers contained 107,000,000 fibers longer than 5 mum, per milligram while the granular sample was assumed to be free of such fibers. However, it has since been realized that even granular SiC may contain fibrous fragments that fulfill the definition of WHO fibers. It was the objective of our study to reanalyze the quantity of WHO fibers that were administered with the granular SiC. In addition, we examined whether the potency per WHO fiber was different for the SiC fragments and the SiC whiskers. Samples of the original granular and fibrous SiC were suspended in water and filtered. One half of each filter was analyzed by scanning electron microscopy (SEM, magnification x 2500), and transmission electron microscopy (TEM, x 10,000) was also performed for the SiC whiskers. The concentration of WHO fibers was 58,000 fibers/mg for the granular sample compared to 48,000,000 (SEM) and 42,000,000 (TEM) fibers/mg for the whiskers. The aspect ratio of the WHO fibers exceeded 10/1 for only 3.3% of the fragments but in each analysis for 96% of the whiskers. In addition, 0% of the fragments compared to 44% and 30% of the whiskers were more than 10 mum long. In total, 15 and 58 x 106 WHO fibers were injected with the granular SiC even though only 0.8% and 0% tumors were recorded. However, 20.1% and 43.3% tumors would have been expected if the carcinogenic potency were the same for the fragments and for the whiskers. We therefore conclude that carcinogenic potency is a function of the shapes of the WHO fibers and is much lower for SiC fragments than for whiskers. Hence carcinogenicity mainly is restricted to a subgroup of WHO fibers longer than about 10 and thinner than about 1 mum. This conclusion is in line with the optimal exposure index, which recently has been proposed by the U.S. Environmental Protection Agency (EPA) for asbestos.

Toxicological investigations on the respirable fraction of silicon carbide grain products by thein vitrovector model

“…Thus, a tumor number of 20.1% and 43.3% would be expected on a number CF to number SiC whisker comparison, respectively. Since the two SiC grain exposures together containing SiC-CF induced practically no tumor, the authors conclude that carcinogenicity of fibers is mainly restricted to a subgroup of WHO fibers longer than about 10 and thinner than about 1 mm, length/diameter ratio !10 (Rodelsperger & Bruckel, 2006). This conclusion is in accordance with the optimal exposure index proposed by the Environmental Protection Agency (EPA) for asbestos (EPA, 2003).…”

“…Also our inference of the biological significance of CFs is supported by the results of a re-evaluation (Rodelsperger & Bruckel, 2006) of an earlier carcinogenicity study of mineral fibers with the intraperitoneal test conducted by Pott et al, (1991). A steep dose-response was reported after intraperitoneal injection of SiC whiskers in five doses between 0.05 and 25 mg from 12.5% to 97% tumors in the animals.…”

Malignant Mesothelioma: Mechanism of Carcinogenesis

“…Intrapleural [102] or intraperitoneal injection of silicon carbide whiskers [103] manufacturing and application of refractory ceramic fibers, the rodent carcinogenicity assays raise concern that long thin biopersistent mineral fibers may be carcinogenic [104]. Erionite fibers are very potent in induction of malignant mesotheliomas following intrapleural injection [105] or inhalation [106].…”

Preparation, dynamic rheological behavior, crystallization, and mechanical properties of inorganic whiskers reinforced polylactic acid/hydroxyapatite nanocomposites

“…So, the SMCW could be a good candidate to be used to reinforce PLA/HA composite. As a kind of whisker, SMCW may cause the potential health hazard due to its shape and size, e.g., silicose disease, etc . However, such a risk could be avoided by melt compounding of SMCWs with polymer and introduction of maleic anhydride grafted polymer (enhancing the combination of whisker with polymer matrix).…”