The selenium in drinking water is usually in the form of inorganic sodium selenate and sodium selenite, which are also easily absorbed in the digestive tract. The human body can change these inorganic selenium compounds into forms the body can use. Selenium compounds, including those used in some medicated dandruff shampoos, are not absorbed well through the skin. Selenium can build up in the human body, mostly in the liver and kidneys and, to a lesser extent, in the blood, lungs, heart, and testes. It also can build up in hair, depending on the length of time and amount of exposure.
Selenium leaves the body mainly in the urine, and less in feces and breath.
Selenium in the urine increases as the amount of selenium to which a person is exposed increases. Selenium compounds can be harmful, however, at daily levels that are only somewhat higher than needed. The seriousness of the effects of excess selenium depends on how much is eaten and how often. If amounts of selenium only somewhat higher than needed were eaten over long periods of time, several health effects could occur, including brittle hair, deformed nails, and, in extreme cases, loss of feeling and control in arms and legs. These health effects were seen in several villages in the People's Republic of China where people were exposed to foods high in selenium for months to years.