Selenium is an essential element found in many over the counter vitamin preparations because its anti-oxidant activity is thought to be anti-carcinogenic. Selenium deficiency targets cardiac muscle leading to Cardiomyopathy. Selenium accumulates in biological tissue. The normal concentration in adult human blood serum is 70 to 150 ng/mL (0.15 parts per million) with a population mean value of 98 ng/mL. Optimal selenium concentration is age dependent (see Reference Values); children require less circulating selenium than do adults. In the state of selenium deficiency associated with loss of glutathione peroxidase activity, the serum concentration is usually below 40 ng/mL.