Bromine (from Ancient Greek: βρῶμος, brómos, meaning "stench") is a chemical element with symbol Br and atomic number 35. It is a halogen. The element was isolated independently by two chemists, Carl Jacob Löwig (in 1825) and Antoine Jérôme Balard (in 1826). Elemental bromine is a fuming red-brown liquid at room temperature, corrosive and toxic, with properties between those of chlorine and iodine. Bromine does not occur free in nature, but in colorless soluble crystalline mineral halide salts, analogous to table salt. Bromine is rarer than about three-quarters of elements in the Earth's crust. The high solubility of bromide ions has caused its accumulation in the oceans, and commercially the element is easily extracted from brine pools, mostly in the United States, Israel and China. About 556,000 tonnes were produced in 2007, an amount similar to the far more abundant element magnesium. At high temperatures, organobromine compounds readily convert to free bromine atoms, a process that stops free radical chemical chain reactions. This effect makes organobromine compounds useful as fire retardants and more than half the bromine produced worldwide each year is put to this purpose. Unfortunately, the same property causes sunlight to convert volatile organobromine compounds to free bromine atoms in the atmosphere, causing ozone depletion. As a result, many organobromide compounds that were formerly in common use—such as the pesticide methyl bromide—are discontinued. Bromine compounds are still used in well drilling fluids, in photographic film, and as an intermediate in the manufacture of organic chemicals. Bromine has long been considered to be possibly essential in humans, but with the support of only limited circumstantial evidence, and no clear biological role. However, one recent study has purportedly identified an essential role in a key protein. Bromine is used preferentially to chlorine by one antiparasitic enzyme in the human immune system. Organobromides are needed and produced from bromide by some lower life forms in the sea, particularly algae, and the ash of seaweed was one source of bromine's discovery. 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.