Coal tar

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] It may be applied to the affected area to treat psoriasis and seborrheic dermatitis (dandruff).[5] It may be used in combination with ultraviolet light therapy.[5] Industrially it is a railway tie preservative and used in the surfacing of roads.[6]

Coal tar
Clinical data
Trade namesBalnetar, Cutar, others
Other namesliquor carbonis detergens (LCD)
liquor picis carbonis (LPC)[1]
AHFS/Drugs.comMultum Consumer Information
Pregnancy
category
  • US: C (Risk not ruled out)
    Routes of
    administration    		Topical
    ATC code
    Legal status
    Legal status
    • US: OTC / Rx-only
    Identifiers
    CAS Number
    ChemSpider
    • none
    UNII
    CompTox Dashboard (EPA)
    ECHA InfoCard100.029.417

    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.[7] The exact mechanism of action is unknown.[8] 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.[8]

    Coal tar was discovered around 1665 and used for medical purposes as early as the 1800s.[6][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]

    Uses

    Medicine

    Coal tar is used in medicated shampoo, soap and ointment. It demonstrates antifungal, anti-inflammatory, anti-itch, and antiparasitic properties.[8] It may be applied topically as a treatment for dandruff and psoriasis, and to kill and repel head lice.[5] It may be used in combination with ultraviolet light therapy.[5]

    Pine tar has historically also been used for this purpose. Though it is frequently cited online as having been banned as a medical product by the FDA due to a "lack of evidence having been submitted for proof of effectiveness", pine tar is included in the Code of Federal Regulations, subchapter D: Drugs for Human Use, as an OTC treatment for "Dandruff/seborrheic dermatitis/psoriasis".[11]

    Coal tar may be used in two forms: crude coal tar (Latin: pix carbonis) or a coal tar solution (Latin: liquor picis carbonis, LPC) also known as liquor carbonis detergens (LCD).[8][12][13] Named brands include Denorex, Balnetar, Psoriasin, Tegrin, T/Gel, and Neutar. When used in the extemporaneous preparation of topical medications, it is supplied in the form of coal tar topical solution USP, which consists of a 20% w/v solution of coal tar in alcohol, with an additional 5% w/v of polysorbate 80 USP; this must then be diluted in an ointment base such as petrolatum.

    Construction
    Main article: Sealcoat

    Coal tar was a component of the first sealed roads. In its original development by Edgar Purnell Hooley, tarmac was tar covered with granite chips. Later the filler used was industrial slag. Today, petroleum derived binders and sealers are more commonly used. These sealers are used to extend the life and reduce maintenance cost associated with asphalt pavements, primarily in asphalt road paving, car parks and walkways.

    Coal tar is incorporated into some parking-lot sealcoat products used to protect the structural integrity of the underlying pavement.[14] Sealcoat products that are coal-tar based typically contain 20 to 35 percent coal-tar pitch.[14] Research[15] shows it is used throughout the United States of America, however several areas have banned its use in sealcoat products, [16][17][18] including the District of Columbia; the city of Austin, Texas; Dane County, Wisconsin; the state of Washington; and several municipalities in Minnesota and others.[19][20]

    Industry

    Being flammable, coal tar is sometimes used for heating or to fire boilers. Like most heavy oils, it must be heated before it will flow easily.

    A large part of the binder used in the graphite industry for making "green blocks" is coke oven volatiles (COV), a considerable portion of which is coal tar. During the baking process of the green blocks as a part of commercial graphite production, most of the coal tar binders are vaporised and are generally burned in an incinerator to prevent release into the atmosphere, as COV and coal tar can be injurious to health.

    Coal tar is also used to manufacture paints, synthetic dyes (notably tartrazine/Yellow #5), and photographic materials.

    In the coal gas era, there were many companies in Britain whose business was to distill coal tar to separate the higher-value fractions, such as naphtha, creosote and pitch. A great many industrial chemicals were first isolated from coal tar during this time. These companies included:[21][22]

    Safety

    Side effects of coal tar products 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.[23]

    According to the National Psoriasis Foundation, coal tar is a valuable, safe and inexpensive treatment option for millions of people with psoriasis and other scalp or skin conditions.[24] According to the FDA, coal tar concentrations between 0.5% and 5% are considered safe[25] and effective for psoriasis.

    Cancer

    Evidence is inconclusive whether the coal tar in the concentrations seen in non-prescription treatments causes cancer, because there is insufficient data to make a judgment.[26] While coal tar consistently causes cancer in animal studies,[27] short-term treatments of humans have shown no significant increase in rates of cancer.[26] It's possible that the skin can repair itself after short-term exposure to PAHs, but not after long-term exposure.[26]

    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.[28] 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.[27]

    Coal tar contains many polycyclic aromatic hydrocarbons, and it is believed that their metabolites bind to DNA, damaging it.[29] Long-term skin exposure to these compounds can produce "tar warts", which can progress to squamous cell carcinoma.[28]

    The International Agency for Research on Cancer lists coal tars as Group 1 carcinogens, meaning they directly cause cancer.[27][30][31] Both the U.S. Department of Health and Human Services and the state of California list coal tars as known human carcinogens.[32]

    In response to public health concerns regarding the carcinogenicity of PAHs some municipalities, such as the city of Milwaukee, have banned the use of common coal tar-based road and driveway sealants citing concerns of elevated PAH content in groundwater.[33]

    Other

    Coal tar causes increased sensitivity to sunlight,[34] so skin treated with topical coal tar preparations should be protected from sunlight.

    The residue from the distillation of high-temperature coal tar, primarily a complex mixture of three or more membered condensed ring aromatic hydrocarbons, was listed on 28 October 2008 as a substance of very high concern by the European Chemicals Agency.

    Mechanism of action

    The exact mechanism of action is unknown.[8] 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.[35][28]

    Composition

    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.[28]

    Coal tar contains approximately 10,000 chemicals, of which only about 50% have been identified.[36] 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.[27][37] Others include benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene and methyl-naphthalenes, acenaphthene, fluorene, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene.[28] Many of these constituents are known carcinogens.[38][29]

    Derivatives

    Various phenolic coal tar derivatives have analgesic (pain-killer) properties. These included acetanilide, phenacetin, and paracetamol (acetaminophen).[39] Paracetamol is the only coal-tar derived analgesic still in use today, but industrial phenol is now usually synthesized from crude oil rather than coal tar. 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.[40]

    Society and culture

    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] In the United Kingdom 125 ml of 5% shampoo costs the NHS about £1.89.[41] In the United States a month of treatment costs less than US$25.[4]

    Regulation

    Exposure to coal tar pitch volatiles can occur in the workplace by breathing, skin contact, or eye contact. The Occupational Safety and Health Administration (OSHA) has set the permissible exposure limit) to 0.2 mg/m3 benzene-soluble fraction over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.1 mg/m3 cyclohexane-extractable fraction over an 8-hour workday. At levels of 80 mg/m3, coal tar pitch volatiles are immediately dangerous to life and health.[42]

    When used as a medication in the United States, coal tar preparations are considered over-the-counter drug pharmaceuticals and are subject to regulation by the Food and Drug Administration (FDA).

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    See also

    References
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    3. Vallee, Yannick (1998). Gas Phase Reactions in Organic Synthesis. CRC Press. p. 107. ISBN 9789056990817.
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    11. "Title 21 – Food and Drugs. CHAPTER I – FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED): SUBCHAPTER D—DRUGS FOR HUMAN USE". United States Government Publishing Office. 1 March 2008. Archived from the original on 2016-03-10. Retrieved 3 March 2016.
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    28. Roberts, L. (2014). "Coal Tar". In Wexler, Philip (ed.). Encyclopedia of Toxicology (Third Edition). Oxford: Academic Press. pp. 993–995. doi:10.1016/b978-0-12-386454-3.00012-9. ISBN 9780123864550. composition of coal tar will be influenced by the process used for pyrolytic distillation as well as by the original composition of the coal ... He then demonstrated excess cancers occurring in laboratory animals when coal tar is applied to the ears and skin ... [therapeutic effect] is thought to involve decreased epidermal proliferation ... Coal tar is classified as a human carcinogen ... Both inhalation and dermal routes of exposure are considered hazardous.
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    36. Heinz-Gerhard Franck (May 1963). "The Challenge in Coal Tar Chemicals". Industrial & Engineering Chemistry. 55 (5): 38–44. doi:10.1021/ie50641a006.
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    38. "EUR-Lex - 32013R1272 - EN - EUR-Lex". eur-lex.europa.eu. Archived from the original on 2015-10-19. Retrieved 2017-06-10. ...are classified as carcinogens of category 1B in accordance with Annex VI to Regulation (EC) No 1272/2008 of the European Parliament
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