Selenium

Selenium is a nonmetallic element that belongs to the group VIa of the periodic table, which is located directly under sulfur. Selenium is a naturally occurring trace element that is found in most rocks and soils. It exists as metallic gray to black hexagonal crystals in its pure form, but in nature, it is usually combined with sulfide, silver, copper, lead, or nickel minerals.

Most processed selenium is used in the electronics industry. Selenium is also used: as a nutritional supplement; in the glass industry; as a component of pigments used in plastics, paints (one common product is used to stain metal a gun-metal blue color), enamels, inks, and rubber; in the preparation of pharmaceuticals; as a nutritional feed additive for poultry and livestock; in pesticide formulations; in rubber production; as an ingredient in anti-dandruff shampoos; and as a constituent of fungicides. Radioactive selenium is used in diagnostic medicine.

Selenium can be released through both natural and manufacturing processes. The general population is exposed to very low levels of selenium in air, food and water. Food is the most common route for daily intake. Selenium may also accumulate up the food chain. Selenium enters water from rocks and soil, as well as agricultural or industrial waste.  Selenium can enter the air from burning coal and oil.

People working in or living near areas where selenium is produced, processed or converted into commercial products may be exposed to higher levels of selenium in the air. People living close to hazardous waste sites or coal burning plants may also be exposed to higher levels of selenium in the air or water. Selenium may enter drinking water from the erosion of natural deposits and people using privately owned well water can be exposed to higher levels.

Common exposure thresholds

The EPA restricts the amount of selenium allowed in public water supplies to 50 parts total selenium per billion parts of water (50 ppb).

ATSDR and the EPA have determined that 5 micrograms of selenium per kilogram of body weight taken daily would not be expected to cause any adverse health effects over a lifetime of such intake.

According to the ATSDR, urinary excretion rates of 20-200 ug selenium/day are generally not associated with either selenium deficiency or toxicity.

The 4CSBC uses the National Health and Nutrition Examination Survey (NHANES) 95th percentile level as a screening threshold. Only five percent of the NHANES participants (representing the American population) have levels that exceed the 95th percentile level. Everyone else is below that level. Thus, that level is considered elevated. It may or may not be harmful. The 95th percentile screening threshold for selenium is 40 μg/L

Health concerns

Because selenium is a naturally occurring trace element, it is nutritionally essential to maintain good health. It plays a critical role in reproduction, thyroid hormone metabolism, DNA synthesis, and protection from oxidative damage and infection. Selenium, however, may be toxic at high concentrations and can cause adverse health effects. Different chemical forms of selenium have very different toxic potentials (e.g. selenium selenite versus selenium sulfides). Symptoms of toxicity depend on the route of exposure (i.e. inhalation or ingestion).

Short-term oral exposure to high concentrations of selenium may cause:

Chronic oral exposure to high concentrations of selenium compounds can produce a disease called Selenosis. Selenosis first appears as a garlicky odor in the breath, and a metallic taste in the mouth. Symptoms include:

Short-term exposure to high levels of elemental selenium in the air can result in:

Chronic exposure to air-borne forms can cause:

Studies of laboratory animals and people have found that most selenium compounds do not cause cancer. In fact, studies in humans suggest that lower-than-normal selenium levels in the diet might increase the risk of cancer. The EPA, however, has determined that one specific form of selenium, selenium sulfide, is a probable human carcinogen. Selenium sulfide is not present in foods and is a very different chemical from the organic and inorganic selenium compounds found in food and in the environment. Selenium sulfide has been linked with the occurrence of liver and lung tumors in mice and rats following oral exposure, and is a possoble (Group B2) carcinogen as per EPA classification.

Sources of additional information

Agency for Toxic Substances and Disease Registry (ATSDR)

National Institutes of Health

U.S. Environmental Protection Agency (EPA)

Food and Drug Administration (FDA)

Papers

Last Update: May 5, 2016