Bromine

Bromine has sometimes been considered to be possibly essential in humans, but with the support of only limited circumstantial evidence, and no clear biological role. As a pharmaceutical, the simple bromide ion (Br⁻) has inhibitory effects on the central nervous system, and bromide salts were once a major medical sedative, before replacement by shorter-acting drugs. They retain niche uses as antiepileptics.

Biological role and toxicity

Unlike its neighbours in the halogen column (chlorine and iodine), bromine is not known to have a confirmed biological role in humans. A single 2014 study suggests that it may be a necessary catalyst to make collagen IV, making it essential to basement membrane architecture and tissue development, but these findings have not been confirmed and the precise role posited for bromine is not certain.^[56] Aside from this, only circumstantial evidence in the form of hypothesised deprivation symptoms exists for a trace role for bromine.^[57] Nevertheless, even if bromine is not an essential element, it can be pharmacologically beneficial. For example, in the presence of hydrogen peroxide, H₂O₂, formed by the eosinophil, and either chloride or bromide ions, eosinophil peroxidase provides a potent mechanism by which eosinophils kill multicellular parasites (such as, for example, the nematode worms involved in filariasis) and some bacteria (such as tuberculosis bacteria). Eosinophil peroxidase is a haloperoxidase that preferentially uses bromide over chloride for this purpose, generating hypobromite (hypobromous acid), although the use of chloride is possible.^[58]

Marine organisms are the main source of organobromine compounds, and it is in these organisms that the essentiality of bromine is on much firmer ground. More than 1600 such organobromine compounds were identified by 1999. The most abundant is methyl bromide (CH₃Br), of which an estimated 56,000 tonnes is produced by marine algae each year.^[59] The essential oil of the Hawaiian alga Asparagopsis taxiformis consists of 80% bromoform.^[60] Most of such organobromine compounds in the sea are made by the action of a unique algal enzyme, vanadium bromoperoxidase.^[61]

The bromide anion is not very toxic: a normal daily intake is 2 to 8 milligrams.^[57] However, high levels of bromide chronically impair the membrane of neurons, which progressively impairs neuronal transmission, leading to toxicity, known as bromism. Bromide has an elimination half-life of 9 to 12 days, which can lead to excessive accumulation. Doses of 0.5 to 1 gram per day of bromide can lead to bromism. Historically, the therapeutic dose of bromide is about 3 to 5 grams of bromide, thus explaining why chronic toxicity (bromism) was once so common. While significant and sometimes serious disturbances occur to neurologic, psychiatric, dermatological, and gastrointestinal functions, death is rare from bromism.^[62] Bromism is caused by a neurotoxic effect on the brain which results in somnolence, psychosis, seizures and delirium.^[63]

Elemental bromine is toxic and causes chemical burns on human flesh. Inhaling bromine gas results in similar irritation of the respiratory tract, causing coughing, choking, and shortness of breath, and death if inhaled in large enough amounts. Chronic exposure may lead to frequent bronchial infections and a general deterioration of health. As a strong oxidising agent, bromine is incompatible with most organic and inorganic compounds.^[64] Caution is required when transporting bromine; it is commonly carried in steel tanks lined with lead, supported by strong metal frames.^[45] The Occupational Safety and Health Administration (OSHA) of the United States has set a permissible exposure limit (PEL) for bromine at a time-weighted average (TWA) of 0.1 ppm. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of TWA 0.1 ppm and a short-term limit of 0.3 ppm. The exposure to bromine immediately dangerous to life and health (IDLH) is 3 ppm.^[65] Bromine is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.^[66]