Toxicity of methylmercury chloride in rats. III. Long-term toxicity study.

Four groups, each of 25 male and 25 female weanling rats, were given dietary levels of 0, 0.1, 0.5 and 2.5 ppm MeHgCl for 2 years. Observations were made on behaviour, growth, food intake, haematology, serum enzymes, urinalysis, microsomal liver enzymes, organ weights and histology with special reference to the nervous system, histochemistry of the kidneys and cerebellum and on tissue Hg concentrations. Significant findings included a slight growth reduction in females at 2.5 ppm, increased relative kidney weight at 2.5 ppm and histochemical changes in kidney enzymes at 2.5 ppm. No effect was seen on the nature or incidence of pathological lesions or tumours at any level. From the results obtained in the short-term, reproduction and long-term studies, the no-toxic effect level for rats appears to be between 0.1 and 0.5 ppm MeHgCl in the diet. Exposure of the Dutch population does not appear to present a health hazard at the moment because the mean intake of total Hg is still far below the intake deemed to be safe.