Selenium is a non-metallic element and generally does not occur as a pure element naturally. Instead, selenium is found in rocks alongside heavy metals such as copper or lead. When these metallic ores are refined industrially to purify the metal, selenium is produced as a biproduct. Other sources of selenium in the environment are volcanic eruptions and emissions from coal-fired power plants.
Selenium is mostly used in the electronics industry, the glass industry, and as a pigment. Other niche uses include medicated shampoos because it is toxic to fungi that contribute to dandruff. Selenium is biologically important to almost all living things. Plants concentrate selenium from the soil and water that they grow in. This includes food crops ‐ our diet is our main source of selenium.
As with many things, having some selenium is good for you, and having lots is bad for you. Humans need selenium to correctly produce thyroid hormones, which regulate the body's metabolism. Selenium is also important for your body to protect itself from toxins. For example, it is is used by your body to detoxify heavy metals and as an antioxidant. The recommended reference nutrient intake in the UK for adults is 75 micrograms a day for males, and 60 micrograms a day for females. (For context, one small grain of salt weighs approximately 60 micrograms).
Not having enough selenium in your food and water can directly cause health problems. These can include issues with the immune system, depression, and fatigue. A severe lack of selenium can also contribute to more dangerous diseases. However, these only occur in a few parts of the globe that have extremely low soil selenium. Too much selenium can also be toxic but there is some disagreement about the exact amount that is problematic. The World Health Organization specifies a tolerable upper intake level of 400 μg per day for adults, while the EU's Food Safety Authority is more conservative and gives a safe upper limit of 255 μg per day for adults.
Happily, these limits are both quite high, and it is unlikely that a person could consume more selenium than this through their diet alone, and in fact, selenium supplement tablets are available to buy. The effects of selenium overdose can include garlicky breath, gastrointestinal upset, fatigue, joint pain, and the loss of hair and nails.
Groundwater can pick up selenium after flowing through soil or rock layers that are rich in selenium. This is partially dependent on the other chemicals present in the water, so high levels of selenium in the rock don't always translate to high levels in the water. The Drinking Water Inspectorate (DWI) sets the limit for selenium at 10 ppb, however Scottish legislation sets it higher at 20 ppb. This then rises to 30 ppb where geological conditions can lead to higher levels.
Despite this, it is generally rare to find water that has much selenium in. For example, the DWI reported that 0 out of 12,296 English municipal water samples tested by the water companies in 2021 breached the 10 ppb legal limit.
Selenium in your drinking water can be detected by the following test.
Selenium can be reduced or removed from your drinking water using the following methods.
