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No other essential nutrient is as two-faced as chlorine (Cl). In its elemental form, it is a poisonous pale-green gas; in the body, it is necessary to support life, but its excess can easily harm it.

Chlorine is found mostly in the form of chloride, a negatively charged element (anion) which with positively charged elements (cations) forms compounds necessary for fluid and electrolyte balance, acid-alkaline balance, and other important body functions. With sodium, it forms salt, and with with hydrogen it forms hydrochloric acid. It is necessary for absorption of proteins and metallic minerals, as well as vitamin B12.

Chlorine DRI (Dietary Reference Intakes, the most recent set of dietary recommendations set by the government) for an average healthy adult is 2.3g, and slightly less after the age of 50. Most people get as much or more just from the salt intake. However, deficiencies are possible with low salt intake, as well as due to significant loss of fluid (prolonged heavy sweating, diarrhea).

High levels of calcium - chloride's antagonist (mutual antagonism) - can also suppress body's chloride level.

In adults, chlorine deficiency may result in dehydration and associated symptoms. Children are more at risk, since metabolic consequences of low chlorine can cause impaired physical development.

Excessive chloride levels, on the other side, can result in water retention and the associated elevated blood pressure, as well as increased risk of developing cancer. Also, excess chloride becomes free-radical initiator, resulting in damage to the arterial walls, leading to arteriosclerosis.

Chlorine has many industrial uses, from disinfectant (water supply, household, industrial) to production of plastics, pesticides and bleaches, to name a few. There is a growing body of evidence that its presence in drinking water, as a common water disinfectant, leads to formation of carcinogenic substances. In males, it can also cause lower sperm count and infertility.

Due to the presence of chlorine in water supply, significant exposure factor is also its high concentration in the water vapor (showering, cooking). Inhalation can cause irritation and a number of symptoms, from acute breathing difficulties and chest pain to headache and vomiting.