Facts About Selenium
Brazil nuts are an important source of dietary selenium.
Credit: Madlen | Shutterstock

Selenium is a nutritionally essential element. People need selenium for healthy joints, heart and eyes. It plays a critical role in DNA synthesis, the immune system and the reproductive system. It also helps fight cancer and other diseases. 

The body cannot make this element, though, and needs it from outside sources. However, a little selenium goes a long way, and too much selenium in the diet can be toxic. Selenium supplements may be harmful to people who get enough selenium in their diet. 

Selenium has several allotropic forms, but only three are generally recognized. Amorphous selenium is either red, in powder form, or black, in vitreous, or glassy, form. The most stable form of the element, crystalline hexagonal selenium, is a metallic gray, while crystalline monoclinic selenium is a deep red. The element resembles sulfur in its makeup.

Selenium is black in its vitreous (glassy) form and is usually sold as beads. In powder form, amorphous selenium is red.
Selenium is black in its vitreous (glassy) form and is usually sold as beads. In powder form, amorphous selenium is red.
Credit: W. Oelen/Creative Commons

Here are the properties of selenium, according to the Jefferson Lab:

  • Atomic number (number of protons in the nucleus): 34
  • Atomic symbol (on the Periodic Table of Elements): Se
  • Atomic weight (average mass of the atom): 78.96
  • Density: 4.809 grams per cubic centimeter
  • Phase at room temperature: Solid
  • Melting point: 429 degrees Fahrenheit (220.5 degrees Celsius)
  • Boiling point: 1,265 F (685 C)
  • Number of stable isotopes: 6

Selenium was discovered in 1817 by Swedish chemist Jöns Jacob Berzelius. Berzelius also determined the atomics weights of many elements and developed a system of chemical symbols. History

Berzelius discovered selenium after analyzing an impurity in sulfuric acid being produced at a factory in Sweden. He thought it was tellurium at first, and then realized it was unknown element, according to the Jefferson Lab.

At one time, selenium was thought to be a toxin. In the 1960s doctors began researching selenium's ability to fight tumors. It turns out both trains of thought were right. Some animal studies have shown that selenium may help protect against cancer, but that taking a supplement is not a good idea, according to the American Cancer Society. There is very little room for error in dosage and just a little more than needed can be toxic. 

High doses can cause these side effects, according to the University of Maryland:

  • Irritability
  • Fingernail loss
  • Weight loss
  • Skin rash
  • Fatigue

Selenium is found in many different foods. Seafood and organ meats are the richest sources, according to the National Institutes of Health (NIH). Other foods that have selenium include muscle meats, grains and dairy products. It can also be found in drinking water in some places.

Electron configuration and elemental properties of selenium.
Electron configuration and elemental properties of selenium.
Credit: Greg Robson/Creative Commons, Andrei Marincas Shutterstock

The amount of selenium found in the soil affects the amount of selenium found in the food that is grown in it, though soil PH and other things factor into the selenium level, as well. Soil in the Central Northern states are particularly rich in selenium, according to the National Geochemical Survey

Some plants, such as garlic and Brazil nuts, tend to accumulate selenium if they are grown in selenium-rich soil. One ounce of Brazil nuts — about six kernels — can provide as much as 543 micrograms of selenium. People only need trace amounts, 55 micrograms or millionths of a gram, each day. The American Cancer Society recommends that the maximum dose in a supplement not exceed 200 micrograms of selenium per day. 

Other than internal human uses, selenium is also used in manufacturing. It is used to color and decolorize glass. When used in glass and enamels it can create a deep red. It is also used to make photographic toner, photo cells, electric eyes and light meters for cameras. It is also added to stainless steel.

A 2008 study by the Johns Hopkins University Bloomberg School of Public Health showed that low levels of selenium were associated with a lower risk of cancer death. The study included 13,887 adult participants. The study also found that higher serum selenium levels may be associated with increased mortality.

While there are studies that suggest that selenium may prevent cancer, a study published in the Journal of the National Cancer Institute in 2014 suggests that selenium may double prostate cancer risk. 

In addition, some studies, such as this 2014 one by Department of Clinical Pharmacy on 60 patients, have shown that selenium supplementation in patients with type 2 diabetes may be associated with adverse effects on blood glucose. 

  • "Selenium's name comes from the Greek word for moon — which is ironic because it sits right above Tellurium, whose name comes from the Greek word for Earth," said Amanda Simson, an assistant professor of chemical engineering at the University of New Haven.
  • At room temperature selenium is a solid, but not a metal. 
  • Selenium is in the gaseous odor you smell when an unhappy skunk is nearby, said Simson.
  • When light is shown on selenium, it conducts electricity better. The brighter the light, the better the better the conduction is, according to Jefferson Lab.

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