Selenium is a naturally occurring element in the environment (chemical symbol Se, atomic number 34). It is rarely found in its elemental form but rather found in compound with other elements. For humans, selenium is a key mineral  required for healthy living and an appropriate amount is usually consumed through food or water. In other uses, selenium is used to manufacture glass, to fight disease in livestock, to make lubricants, and is an ingredient in anti-dandruff shampoo. With regards to electronics, selenium is found in various electronic components such as circuit boards and photosensitive drums in photocopiers.

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As mentioned above, selenium is important in human health, playing a key role in preventing oxidative damage to tissues in the human body, which helps to prevent cancer. Having an insufficient amount of selenium as part of the diet can lead to muscle pain and cardiac problems. For most people, selenium levels are not issue- adequate levels are found in the diet, and environmental exposure to harmful selenium compounds is rare. However, higher levels of exposure (usually 5x the recommended allowance) can lead to hair and nail deformations, and a condition called selenosis may result, which manifests as a loss of feeling and control in the limbs. Depending on the method of entry into the body, selenium poisoning can also result in damage to lung tissue, increase blood pressure, headaches, diarrhea, and other disruptions in normal body function.

Impact on Humans of Dietary Selenium

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For organisms that are not humans, adverse health impacts are more severe. Testing on respiratory function in rats has resulted in hemorrhage and edema in the lungs. Depending on method of exposure, nearly all body organs can be affected- from the brain to the liver and kidneys. Chronic exposure also leads to disruption of reproductive function, resulting in irregular menstrual cycles, lower fertility, and lower birth weight in offspring. In aquatic organisms, selenium also tends to bioaccumulate and biomagnify the higher the organism is on the trophic ladder. Finally, although death due to selenium poisoning is rare in humans, it can occur and has been observed in other organisms.

Further  Resources:

Department of Health and Human Services Toxicological Profile: