Halogens

Most of the organic halogen compounds mentioned are made synthetically. However, there are also natural sources. In 1968 there were 30 known naturally occurring compounds. By 1994 around 2,000 had been discovered, and many biological organisms, especially marine species, those in the oceans, had not been looked at as yet. Halogenated compounds were found in ocean water, in marine algae, in corals, jelly fish, sponges, terrestrial plants, soil microbes, grasshoppers, and ticks. Volcanoes are another natural source of halogens, and they release significant amounts into the air during eruptions. Chlorine and fluorine are present in largest quantities, mostly as hydrogen chloride and hydrogen fluoride.

In the 1980s depletion of the layer of ozone (O₃) high in Earth's atmosphere was observed. Ozone absorbs much of the high energy ultraviolet radiation from the Sun that is harmful to biological organisms. During September and October, in the atmosphere over the Antarctic, ozone concentration in a roughly circular area, the "ozone hole," drops dramatically.

Chlorine-containing compounds, especially CFCs, undergo reactions releasing chlorine atoms, which can catalyze the conversion of ozone to ordinary oxygen, O₂. Bromine and iodine-containing carbon compounds may also contribute to ozone depletion. Countries signing the Montreal Protocol on Substances that Deplete the Ozone Layer have pledged to eliminate manufacture and use of halocarbons. However, natural sources, such as volcanic eruptions and fires, continue to add halogen compounds to the atmosphere. Finding substitutes that work as well as the banned compounds and do not also cause problems is a current chemical challenge.