The Nordic Expert Group for Criteria Documentation of Health Risks from Chemicals. 150. Silicon carbide

Abstract

Silicon carbide (SiC) is a rare naturally occurring mineral, but manufactured industrially at a large scale. SiC may exist in non-fibrous (angular) and fibrous forms (polycrystalline fibres and single crystal whiskers). It may also appear as cleavage fragments, platelets and in amorphous form, and in a large range of particles sizes. Angular SiC is produced mainly for use as abrasives and cutting devices, but also as a refractory product, in ceramic applications, in heating elements, electronic devices, and in composites and metallurgy. Polycrystalline SiC fibres are formed unintentionally during the production of angular SiC. SiC whiskers are manufactured for specific use such as strengthening of composite materials and in electronic components. SiC nanoparticles are increasingly being produced. Nearly all epidemiological studies on SiC have been performed in the SiC production industry where workers have been exposed to a mixture of angular SiC, SiC fibres, quartz, cristobalite and other dusts and gases. Adverse health outcomes in the respiratory system are lung fibrosis, obstructive lung diseases and lung cancer. Due to the mixed and complex exposures, the human data cannot be used for assessing exposure-response relationships for angular SiC and polycrystalline SiC fibres. There are no human data on the health effects of SiC whiskers. Angular SiC: Some animal and in vitro studies indicate a slight inflammatory activity, but most studies conclude that the toxicity is low. There are no experimental data to identify dose-response relationships. The physico-chemical properties indicate that angular SiC should be considered to confer similar toxicity as other poorly soluble, low toxicity (PSLT) dusts. Data from other PSLT dusts indicate that the effects may be related to particle surface area rather than to mass. Fibrous SiC: SiC whiskers induce inflammation and fibrosis in the lungs of rats and mice after intratracheal instillation and inhalation. In life-long studies with SiC whiskers, rats develop local mesotheliomas after inhalation and intrapleural and intraperitoneal injection, in accordance with the fibre paradigm stating that long thin durable fibres may be fibrogenic and tumourigenic. The inflammatory, fibrogenic and carcinogenic effects observed in experimental animals exposed to SiC whiskers are comparable to those of amphibole asbestos fibres. For the setting of occupational exposure limits (OELs), considering the similarities in morphology, biopersistence and effects in experimental animals, SiC whiskers and polycrystalline SiC fibres should be considered to be equally potent in humans as amphibole asbestos fibres.