Selenium in the environment

Selenium is one of the most widely distributed elements of the earth's crust. Much of the selenium in the earth's crust occurs associated with sulfide minerals. The presence or absence of selenium in any soil is dependent upon the composition of the parent material, and is also dependent upon leaching or processes subsequent to soil formation, that may have added selenium. Selenium can be easily oxidized from Se 0 to Se +4 or Se +6. Selenium is usually recovered as a by-product of the refining of the sulfide ores of other metals such as copper. The greatest amounts of selenium are used for the manufacture of the photoelectric cell. Selenium is taken up by plants as selenate, selenite or organic selenium. Se 75 selenite in 30 minutes was translocated primarily to selenomethionine. In sheep or pigs the duodenum is the main site of Se 75 absorption. Selenium is excreted in the feces, the urine, and the expired air, the amounts and proportions depending upon the level and form of the intake, the nature of the rest of the diet, and the species. A dietary intake of 0.1 μg/g Se provides a satisfactory margin of safety for grazing sheep and cattle. In humans, the recommended daily allowance for selenium is between 100 to 200 μg/day. The toxicity of selenium to animals varies with the amounts of chemical forms of selenium ingested, with the duration and continuity of intake, and with the type and nature of the diet, especially its protein and sulfate content. Deficiency of selenium results in selenium responsive diseases in various animal species, such as muscular dystrophy, exudative diathesis and hepatosis dietetica. Selenium also prevents several type of chemically induced cancer in animals, and, where more selenium occurs in the environment, human cancer death rates are lower. Selenium deficient rats and lambs develop abnormal electrocardiograms accompanied by blood pressure changes. Human heart disease mortality is also lower in the high selenium areas. In China, a large clinical trial is underway showing that selenium prevents a congestive heart failure in children from severely selenium deficient areas.