Selenium (Se) is one of the many elements that is necessary for human life and health in very small amounts, but in larger amounts becomes harmful. As a result, when it comes to drinking water, selenium is generally regarded as a toxic contaminant.

Because selenium is useful in many industrial settings , it is used to build electronics such as photocopier parts but can also be found in glass, rubber, fabrics, and medical therapies ;runoff is possible and this is how selenium would be most likely to enter the water supply.

The MCL for selenium has been set by the EPA at 0.05 ppm. The concern about selenium is not about when it's consumed in the natural process of life, but when it's absorbed at artificially high levels; in these situations it is important for selenium to be removed from drinking water. Selenium, if ingested at toxic levels, will cause damage to the nervous system at worst; in less dramatic situations it causes variation in hair and fingernail development. Selenium can also cause respiratory problems and stomach pains. Though studies have shown that selenium can affect reproductive ability in animals, as yet it is unclear if human exposure results in similar consequences.

Reverse Osmosis will reduce Selenium in water.