Editing Halogen (section)

== Biological role ==
Fluoride anions are found in ivory, bones, teeth, blood, eggs, urine, and hair of organisms. Fluoride anions in very small amounts may be essential for humans.<ref name="who.int">{{cite web|last1=Fawell|first1=J. |title=Fluoride in Drinking-water|url=https://www.who.int/water_sanitation_health/dwq/chemicals/fluoride.pdf|publisher=World Health Organisation|access-date=10 March 2016}}</ref> There are 0.5 milligrams of fluorine per liter of human blood. Human bones contain 0.2 to 1.2% fluorine. Human tissue contains approximately 50 parts per billion of fluorine. A typical 70-kilogram human contains 3 to 6 grams of fluorine.<ref name = "Nature's Building Blocks"/>

Chloride anions are essential to a large number of species, humans included. The concentration of chlorine in the [[dry weight]] of cereals is 10 to 20 parts per million, while in [[potato]]es the concentration of chloride is 0.5%. Plant growth is adversely affected by chloride levels in the [[soil]] falling below 2 parts per million. Human blood contains an average of 0.3% chlorine. Human bone typically contains 900 parts per million of chlorine. Human tissue contains approximately 0.2 to 0.5% chlorine. There is a total of 95 grams of chlorine in a typical 70-kilogram human.<ref name = "Nature's Building Blocks"/>

Some bromine in the form of the bromide anion is present in all organisms. A biological role for bromine in humans has not been proven, but some organisms contain [[organobromine compound]]s. Humans typically consume 1 to 20 milligrams of bromine per day. There are typically 5 parts per million of bromine in human blood, 7 parts per million of bromine in human bones, and 7 parts per million of bromine in human tissue. A typical 70-kilogram human contains 260 milligrams of bromine.<ref name = "Nature's Building Blocks"/>

Humans typically consume less than 100 micrograms of iodine per day. Iodine deficiency can cause [[intellectual disability]]. [[organoiodine chemistry|Organoiodine compound]]s occur in humans in some of the [[gland]]s, especially the [[thyroid gland]], as well as the [[stomach]], [[epidermis (skin)|epidermis]], and [[immune system]]. Foods containing iodine include [[Cod (food)|cod]], [[oyster]]s, [[Shrimp (food)|shrimp]], [[Herring (food)|herring]], [[lobster]]s, [[sunflower seed]]s, [[seaweed]], and [[Edible mushroom|mushrooms]]. However, iodine is not known to have a biological role in plants. There are typically 0.06 milligrams per liter of iodine in human blood, 300 parts per billion of iodine in human bones, and 50 to 700 parts per billion of iodine in human tissue. There are 10 to 20 milligrams of iodine in a typical 70-kilogram human.<ref name = "Nature's Building Blocks"/>

[[Astatine]], although very scarce, has been found in micrograms in the earth.<ref name = "Nature's Building Blocks"/> It has no known biological role because of its high radioactivity, extreme rarity, and has a half-life of just about 8 hours for the most stable isotope. 

Tennessine is purely man-made and has no other roles in nature.