Evaluation of the drinking water selenium standard: an update.

Recent studies suggesting a diabetogenic activity of selenium (Se) have raised concern about the human health effects of this metalloid of considerable nutritional and toxicological interest. Uncertainties exist about the adequate environmental levels of this metalloid, in particular, regarding the safe upper standard in drinking water, considering the very few epidemiologic studies carried out on this topic.We summarize results of our epidemiologic studies on long-term effects of consumption of municipal tapwater with naturally-occurring unusually high levels of Se (7-9 µg/l), within a non-seleniferous Italian area. Speciation analyses demonstrated that in that tapwater Se was almost entirely present in its hexavalent inorganic form, selenate. After checking for potential confounders, long-term consumers of the high-Se drinking water experienced no substantial change in overall mortality and in prevalence at birth of congenital anomalies, whilst incidence of amyotrophic lateral sclerosis and of some site-specific cancers exceeded that expected on the basis of reference rates. No beneficial effect on prostate cancer risk emerged, consistently with results of a recently published large trial carried out in the US.Our results are corroborated by laboratory studies, which demonstrated that inorganic (tetravalent) Se is toxic at concentrations as low as 0,8 µg/l, also confirming that toxicity of inorganic Se species far exceeds that of the organic forms.Overall, these findings indicate that current WHO drinking water upper Se standard, 10 µg/l, might be inadequate to protect human health, suggesting the need for additional epidemiologic studies on health effects of exposure to specific Se species through drinking water.