Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis.
This 400 microgram Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992.
The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries.
The Chinese people who suffered from selenium toxicity ingested selenium by eating corn grown in extremely selenium-rich stony coal (carbonaceous shale).
This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature. A dose of selenium as small as 5 mg per day can be lethal for many humans.
Symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage.
Extreme cases of selenosis can result in cirrhosis of the liver, pulmonary edema, and death. Elemental selenium and most metallic selenides have relatively low toxicities because of their low bioavailability.
By contrast, selenates and selenites are very toxic, having an oxidant mode of action similar to that of arsenic trioxide.
The chronic toxic dose of selenite for human beings is about 2400 to 3000 micrograms of selenium per day for a long time. Hydrogen selenide is an extremely toxic, corrosive gas.
Selenium also occurs in organic compounds such as dimethyl selenide, selenomethionine, selenocysteine and methylselenocysteine, all of which have high bioavailability and are toxic in large doses. Nano-size selenium has equal efficacy, but much lower toxicity.
On April 19, 2009, twenty-one polo ponies began to die shortly before a match in the United States Polo Open. Three days later, a pharmacy released a statement explaining that the horses had received an incorrect dose of one of the ingredients used in a vitamin compound, with which the horses had been injected. Such vitamin injections are common to promote recovery after a match.
The pharmacy did not initially release the name of the specific ingredient due to ongoing law-enforcement and other investigations.
Analysis of inorganic compounds of the vitamin supplement indicated that selenium concentrations were ten to fifteen times higher than normal in the horses' blood samples and 15 to 20 times higher than normal in their liver samples. It was later confirmed that selenium was the ingredient in question.
Selenium poisoning of water systems may result whenever new agricultural runoff courses through normally dry undeveloped lands.
This process leaches natural soluble selenium compounds (such as selenates) into the water, which may then be concentrated in new "wetlands" as the water evaporates.
High selenium levels produced in this fashion have been found to have caused certain congenital disorders in wetland birds.
Further Reading
- Selenium - What is Selenium?
- Selenium History
- Sources of Selenium
- Selenium Production
- Selenium Non-Biologic Uses
- Selenium Biologic Applications
- Selenium Evolution
- Selenium Controversial Health Effects
- Selenium Deficiency
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