A very reactive and highly toxic green, gaseous element, belonging to the halogen family of substances. It is one of the most widespread elements, as it occurs naturally in sea-water, salt lakes and underground deposits, but usually occurs in a safe form as common salt (NaCl). Commercially it is used in large quantities by the chemical industry both as an element to produce chlorinated organic solvents, like polychlorinated biphenyls (PCBs), and for the manufacture of polyvinyl chloride plastics, thermoplastic and hypochlorite bleaches. Chlorine was the basis for the organochlorine pesticides, like DDT and other agricultural chemicals that have killed wildlife. The reactivity of chlorine has proved disastrous for the ozone layer and has been the cause of the creation of the ozone hole, which was first detected in the Southern Hemisphere over Antarctica and then over the Northern Hemisphere.

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