Silicon carbide (SiC) dust and other dusts for comparison were injected intratracheally at a high dose (50 mg) into rats and the response of the lungs and the lymph nodes was studied after an appropriate experimental period. The indices studied were: histological changes in the lung and lymph nodes, organ weights, the formation of collagenous fibres, and the appearance of quartz typical areas. According to several epidemiological investigations and previous experimental animal studies, SiC produces silicogenic (fibrogenic) effects. No changes in the tissues studied in terms of damaging fibrogenic effects could be found after eight months (first series) and three and 12 months (second series). In particular, the histological findings and the absence of quartz typical areas as well as the quantitative determination of collagen fibres show that SiC had no harmful effects on tissues. Based on these results, the extent to which other exposures during the production of SiC can be responsible for the established radiological alterations is discussed. Without doubt the following may be confounders: SiC fibres, crystalline SiO2 (quartz, cristobalite, tridymite), and possibly gaslike emissions (SO2). From the hygienic medical point of view the workplaces during SiC manufacture should be examined carefully. The substance SiC dust as such can be considered as inert from the experimental results based on qualitative and extremely sensitive procedures. A revision of the present threshold value for SiC in ther German MAK list is called for.
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