Selenium is a naturally occurring substance that is widely but unevenly distributed in the earth's crust and is commonly found in sedimentary rock. Selenium is not often found in its pure form but is usually combined with other substances. Much of the selenium in rocks is combined with sulfide minerals or with silver, copper, lead, and nickel minerals. When rocks change to soils, the selenium combines with oxygen to form several substances, the most common of which are sodium selenite and sodium selenate. Some selenium compounds are gases; but hydrogen selenide is probably the only gaseous selenium compound that might pose a health concern in occupational settings.
Plants easily take up selenate compounds from water and change them to organic selenium compounds such as selenomethionine. Selenium is an essential element for humans and animals. Humans and animals can use both inorganic and organic selenium compounds. In the body, selenium helps prevent damage by oxygen to tissues, as does vitamin E. Selenium, however, is harmful to humans and animals when eaten in amounts that are not much higher than the amounts needed for good nutrition.
Humans are exposed daily to selenium in their food. Estimates of the average intake of selenium from food for the U.S. population range from 0.071 to 0.152 milligrams of selenium per person per day (mg selenium/day). Generally, the levels in food are enough to protect against diseases that may result from too little selenium. Most of the daily intake of selenium comes from eating grains, cereals, and meat. Selenium amounts in food are usually between 0.01 and 0.3 parts per million (ppm) in vegetables and dairy products, between 0.4 and 0.7 ppm in many grains and wheat bread, and between 0.1 and 0.7 ppm in poultry, meat, and fish. Seafood and organ meats, such as liver and kidney, contain the highest selenium levels of this group.
In some parts of the United States, especially in the western states, the soils contain rather high levels of soluble selenium compounds such as selenate and some organic forms of selenium. Some plants can build up selenium to levels that are harmful to livestock feeding on these plants. In these areas, humans can be exposed to too much selenium if they eat locally grown grains and vegetables that have built up high levels of selenium. In fresh water containing high levels of selenium (e.g., agricultural water drainage basins in the San Joaquin Valley in California), fish may contain selenium at levels of more than 5 ppm. At one time in the Kesterson Reservoir in the San Joaquin Valley, some fish were found to contain 50-370 ppm selenium. The Kesterson Reservoir received subsurface runoff water containing selenium from nearby agricultural areas, but has now been drained and filled.
Humans can also be exposed to selenium in drinking water. Most of the water sources in the United States contain levels of selenium that are very low compared with levels found in food. Selenium levels are less than 0.01 ppm in most drinking water sources (99.5%). Less than 1% of the daily intake of selenium is estimated to come from drinking water. Occasionally, water containing selenium can seep from abandoned uranium or coal mining areas into ground water in which selenium can reach dangerous levels. At hazardous waste disposal sites, selenium can be washed from the soil into surface water or can flow into groundwater. At this time, selenium has been identified in at least 54 of the 1177 hazardous waste sites on the National Priorities List.
Humans are normally not exposed to large amounts of selenium in the air, unless selenium dust or volatile selenium compounds are formed in their workplace. Occupations in which humans may be exposed to airborne selenium exist in metal industries, selenium-recovery processes, painting, and special trades. Selenium dioxide and uncombined selenium can be released into the air during the burning of coal and oil.
Selenium enters the body when food containing selenium is eaten. The selenium contained in grains and meat is associated with proteins. The human body easily absorbs the organic selenium compounds that are eaten and makes them available where they are needed in the body. 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.
The normal intake of selenium in food, about 50-150 micrograms (ug)/day, is enough to meet the daily need for this essential nutrient. 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. Swallowing a lot of sodium selenate or sodium selenite (for example, part of a bottle of sodium selenate prepared for a flock of sheep, or large numbers of selenium supplement pills) could be life-threatening without immediate treatment. 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. No populations in the United States have been reported with symptoms of serious, long-term selenium poisoning.
The U.S. Department of Health and Human Services has determined that selenium sulfide may reasonably be anticipated to be a carcinogen. Although some early studies with rats suggested that some selenium compounds might cause cancer, more recent and more thorough studies did not show that selenium compounds cause cancer (with the exception of selenium sulfide). Studies of human populations also did not show that the common selenium compounds caused cancer in humans. In fact, some studies of human populations and experiments with laboratory animals showed that lower-than-normal selenium levels in the diet might increase the risk of cancer. But higher-than-normal selenium levels in the diet do not reduce the risk of developing cancer and may increase the risk of selenium poisoning.
The only selenium compound shown to be carcinogenic in animals is selenium sulfide, which is used in some anti-dandruff shampoos. Selenium sulfide is not present in foods and is very different chemically from the organic and inorganic selenium compounds found in foods and in the environment. The rats and mice that developed cancer were fed the compound daily. Because selenium sulfide is not absorbed through the skin, use of shampoos containing this compound is considered safe unless a person has open cuts or sores on the scalp or hands.
Although exposure to high levels of inorganic selenium compounds has been shown to cause birth defects in birds, selenium compounds have not been shown to cause birth defects in humans or in other mammals. Because birds and mammals develop very differently, it is not surprising that some chemicals can cause effects in one group but not in the other.
Although people exposed to selenium dust and airborne selenium compounds in the workplace reported dizziness, fatigue, irritation of mucous membranes, and, in extreme cases, fluid in the lungs (pulmonary edema) and severe bronchitis, it is not possible to identify the exact exposure levels at which these effects are expected to occur.
Humans who have accidentally eaten large amounts of selenium had upset stomachs, muscular weakness, difficulty in breathing, and pulmonary edema. Unfortunately, the amount of selenium that these people swallowed is not known. Information about the health effects from eating or drinking too much selenium over long periods of time has come from areas in China with very high selenium levels in the soil and in the rice and vegetables people eat. These people had loss of hair, loss of and poorly formed nails, problems with walking, reduced reflexes, and some paralysis when exposed to levels of 1.64 ppm or higher selenium in their food over months to years.
Information excerpted from:
Toxicological Profile for Selenium December 1989