Earlier studies by Nelson et al. (1943) with seleniferous corn and wheat in the diet, by Schroeder and Mitchener (1971a) with sodium selenite or sodium selenate in drinking water, and by Volgarev and Tscherkes (1967) with sodium selenate in the diet, reported that selenium was carcinogenic in animals. However, subsequent studies revealed that the earlier studies were flawed and that there was no association between selenium intake and the incidence of cancer in humans and animals (USDHHS 1994). Also, in animals, selenium supplementation has been shown to significantly inhibit tumours induced by chemicals, viruses, or ultraviolet light (USDHHS 1994).
The only selenium compound that has been shown to be carcinogenic is selenium sulphide administered orally to mice and rats. Hepatocellular carcinomas and adenomas (in rats and mice), and alveolar/bronchiolar carcinomas and adenomas (in mice) were reported in the animals following chronic oral exposure to selenium sulphide (NTP 1980). Selenium sulphide is a pharmaceutical compound used in antidandruff shampoos. However, it is not absorbed through the skin and the use of shampoos containing this compound should be safe, unless one has cuts or sores on the scalp or hands. Chemically, selenium sulphide is very different from the organic and inorganic selenium compounds in foods and in the environment (USDHHS 1994).
According to the USEPA, selenium carcinogenicity in humans and rats is not classifiable. The evidence for selenium sulphide, however, is sufficient to classify it as a group B2 (probable human carcinogen) chemical (IRIS 1993).