Coal tar is a thick dark liquid which is a by-product of the production of coke and coal gas from coal.[2][3] It has both medical and industrial uses.[2][4] Medicinally it is a topical medication applied to skin to treat psoriasis and seborrheic dermatitis (dandruff).[5] It may be used in combination with ultraviolet light therapy.[5] Industrially it is a railroad tiepreservative and used in the surfacing of roads.[6] Coal Tar was listed as a known human carcinogen in the first Report on Carcinogens from the U.S. Federal Government.[7]
Coal tar was discovered circa 1665 and used for medical purposes as early as the 1800s.[6][8] Circa 1850, the discovery that it could be used as the main ingredient in synthetic dyes engendered an entire industry.[9] It is on the World Health Organization's List of Essential Medicines.[10] Coal tar is available as a generic medication and over the counter.[4]
Side effects include skin irritation, sun sensitivity, allergic reactions, and skin discoloration.[5] It is unclear if use during pregnancy is safe for the baby and use during breastfeeding is not typically recommended.[11] The exact mechanism of action is unknown.[12] It is a complex mixture of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds.[2] It demonstrates antifungal, anti-inflammatory, anti-itch, and antiparasitic properties.[12]
Coal tar contains many polycyclic aromatic hydrocarbons, and it is believed that their metabolites bind to DNA, damaging it.[33] The PAHs found in coal tar and air pollution induce immunosenescence and cytotoxicity in epidermal cells.[34][35] It's possible that the skin can repair itself from this damage after short-term exposure to PAHs but not after long-term exposure.[31] Long-term skin exposure to these compounds can produce "tar warts", which can progress to squamous cell carcinoma.[36]
Coal tar was one of the first chemical substances proven to cause cancer from occupational exposure, during research in 1775 on the cause of chimney sweeps' carcinoma.[36] Modern studies have shown that working with coal tar pitch, such as during the paving of roads or when working on roofs, increases the risk of cancer.[32]
Mechanism of action[edit]
The exact mechanism of action is unknown.[12] Coal tar is a complex mixture of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds.[2]
It is a keratolytic agent, which reduces the growth rate of skin cells and softens the skin's keratin.[42][36]
Composition[edit]
Coal tar is produced through thermal destruction (pyrolysis) of coal. Its composition varies with the process and type of coal used – lignite, bituminous or anthracite.[36]
Coal tar contains approximately 10,000 chemicals, of which only about 50% have been identified.[43][better source needed] Components include polycyclic aromatic hydrocarbons (4-rings: chrysene, fluoranthene, pyrene, triphenylene, naphthacene, benzanthracene, 5-rings: picene, benzo[a]pyrene, benzo[e]pyrene, benzofluoranthenes, perylene, 6-rings: dibenzopyrenes, dibenzofluoranthenes, benzoperylenes, 7-rings: coronene), as well as methylated and polymethylated derivatives, mono- and polyhydroxylated derivatives, and heterocyclic compounds.[32][44] Others include benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene and methyl-naphthalenes, acenaphthene, fluorene, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene.[36] Many of these constituents are known carcinogens.[45][33]
Derivatives[edit]
Various phenolic coal tar derivatives have analgesic (pain-killer) properties. These included acetanilide, phenacetin, and paracetamol aka acetaminophen.[46]Paracetamol may be the only coal-tar derived analgesic still in use today.[47] Industrial phenol is now usually synthesized from crude oil rather than coal tar.[48]
Coal tar derivatives are contra-indicated for people with the inherited red cell blood disorder glucose-6-phosphate dehydrogenase deficiency (G6PD deficiency), as they can cause oxidative stress leading to red blood cell breakdown.[49]
Society and culture[edit]
Coal tar is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system.[10] Coal tar is generally available as a generic medication and over the counter.[4]
https://en.wikipedia.org/wiki/Coal_tar
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