Regulations Introduction 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. 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. Exposure Pathways Humans are exposed daily to selenium in their food. 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.