Water Quality Standards

Selenium levels in drinking-water vary greatly in different geographical areas but are usually much less than 0.01 mg/litre. Foodstuffs such as cereals, meat, and fish are the principal source of selenium in the general population. Levels in food vary greatly according to geographical area of production.

Selenium is an essential element for humans and forms an integral part of the enzyme glutathione peroxidase and probably other proteins as well. Most selenium compounds are water-soluble and are efficiently absorbed from the intestine. The toxicity of selenium compounds appears to be of the same order in both humans and laboratory animals. 

Except for selenium sulfide, which does not occur in drinking-water, experimental data do not indicate that selenium is carcinogenic. IARC has placed selenium and selenium compounds in Group 3. Selenium compounds have been shown to be genotoxic in in vitro systems with metabolic activation, but not in humans. This effect may be dose-dependent in vivo. There is no evidence of teratogenic effects in monkeys, but no data exist for humans.

Long-term toxicity in rats is characterized by depression of growth and liver pathology at selenium levels of 0.03 mg/kg of body weight per day given in food. 

In humans, the toxic effects of long-term selenium exposure are manifested in nails, hair and liver. Data from China indicate that clinical signs occur at a daily intake above 0.8 mg. Daily intakes of Venezuelan children with clinical signs were estimated to be about 0.7 mg, on the basis of their blood levels and the Chinese data on the relationship between blood level and intake. Effects on synthesis of a liver protein were also seen in a small group of patients with rheumatoid arthritis given selenium at a rate of 0.25 mg/day in addition to selenium from food. No clinical or biochemical signs of selenium toxicity were reported in a group of 142 persons with a mean daily intake of 0.24 mg (maximum 0.72 mg). 

On the basis of these data, the NOAEL in humans was estimated to be about 4 µg/kg of body weight per day. The recommended daily intake of selenium is about 1µg/kg of body weight for adults. An allocation of 10% of the NOAEL in humans to drinking-water gives a health-based guideline value of 0.01 mg/litre (rounded figure).