Halocarbon

A large amount of the naturally occurring halocarbons, such as dioxine, are created by wood fire and volcanic activity.

Six mg of iodide a day can be used to treat patients with hyperthyroidism due to its ability to inhibit the organification process in thyroid hormone synthesis, the so-called Wolff–Chaikoff effect.

In large doses, iodides inhibit proteolysis of thyroglobulin, which permits TH to be synthesized and stored in colloid, but not released into the bloodstream.

The major disadvantage of iodide treatment lies in the fact that excessive stores of TH accumulate, slowing the onset of action of thioamides (TH synthesis blockers).

Before they became strictly regulated, the general public often encountered haloalkanes as paint and cleaning solvents such as trichloroethane (1,1,1-trichloroethane) and carbon tetrachloride (tetrachloromethane), pesticides like 1,2-dibromoethane (EDB, ethylene dibromide), and refrigerants like Freon-22 (duPont trademark for chlorodifluoromethane).

Haloalkenes have also been used as solvents, including perchloroethylene (Perc, tetrachloroethene), widespread in dry cleaning, and trichloroethylene (TCE, 1,1,2-trichloroethene).

A few halocarbons, including acid halides like acetyl chloride, are highly reactive; these are rarely found outside chemical processing.

The stability of halocarbons tended to encourage beliefs that they were mostly harmless, although in the mid-1920s physicians reported workers in polychlorinated naphthalene (PCN) manufacturing suffering from chloracne (Teleky 1927), and by the late 1930s it was known that workers exposed to PCNs could die from liver disease (Flinn & Jarvik 1936) and that DDT would kill mosquitos and other insects (Müller 1948).

In 1956, for example, after testing hydraulic oils containing polychlorinated biphenyl (PCB)s, the U.S. Navy found that skin contact caused fatal liver disease in animals and rejected them as "too toxic for use in a submarine" (Owens v. Monsanto 2001).

Within a few years, ozone depletion was being observed above Antarctica, leading to bans on production and use of chlorofluorocarbons in many countries.

[7] Since the 1970s there have been longstanding, unresolved controversies over potential health hazards of trichloroethylene (TCE) and other halocarbon solvents that had been widely used for industrial cleaning (Anderson v. Grace 1986) (Scott & Cogliano 2000) (U.S. National Academies of Science 2004) (United States 2004).

More recently perfluorooctanoic acid (PFOA), a precursor in the most common manufacturing process for Teflon and also used to make coatings for fabrics and food packaging, became a health and environmental concern starting in 2006 (United States 2010), suggesting that halocarbons, though thought to be among the most inert, may also present hazards.