Toxicity of silicon carbide whiskers

Abstract

There is growing interest in the use of ceramic materials, such as A1203, Si3N 4 and SiC in the form of whiskers for the reinforcement of ceramics and metals [1 3]. Epidemiological studies have established that exposure to certain fibrous minerals of natural occurrence is associated with increased incidence of mesothelioma a malignant tumour of the pleural or peritoneal cavity. Studies in experimental animals have shown that carcinogenicity (or at least the ability to induce mesothelioma) is restricted to long (> 10 #m), thin ( < 1 ~tm diameter) fibres and is relatively independent of chemical nature, providing the fibres are durable in the body (e.g. [4, 5]). Many whisker products consist of, or contain, fibres in the sub-micrometre diameter range and some have been shown to produce tumours in experimental animals [6]. We have prepared sub-micrometre diameter SiC whiskers by the carbo-thermic reduction of SiO2 and subjected this material to an in vitro cell culture test in which the response correlates well with the results of injection or implantation studies in animals [7]. A comparison has been made with crocidolite (blue) asbestos, which is well established as a causative agent for mesothetioma in human populations. The cellular response to the SiC whiskers is of the same order as crocidolite suggesting that in the absence of any additional information, such whiskers should be considered as potentially carcinogenic with appropriate handling precautions applied. A whisker-containing product of the carbo-thermic reaction (Fig. 1) contained a proportion of nonfibrous materials. The diameter range of the fibrous component is shown in Fig. 2 and no attempt to separate fibrous from non-fibrous material was made. The presence of free silica can lead to confusing results in the in vitro cell tests used and to avoid this the material was extracted with hot aqueous NaOH and washed in distilled water prior to use in the test. X-ray examination showed the SiC preparation to consist of highly crystalline cubic/?-SIC together with a minor amount of hexagonal c~-SiC and traces of mullite and corundum (A1203) from contamination by the crucible. The cell culture test has been previously described [8] and its specificity in terms of fibre size has been established [9]. Survival of V79-4 Chinese hamster lung cells was determined by cloning efficiency from a single-cell suspension. Cell suspension (20ml) was added to the appropriate quantity of test fibre in I ml of physiological saline in a sterile McCartney bottle. Aliquots of this suspension were seeded into 60-mm diameter plastic Petri dishes and the surviving cells allowed to grow and form colonies for 5 to 6 days. Colonies were then washed with physiological saline