Coal tar

History

Coal tar was discovered circa 1665. Much later, it was an undesirable byproduct in the production of coke and illumination gases ("town gas"). Soon it was found that the tar could be a source of solvents. Around 1850, it was discovered that it could be used as a source of many organic compounds. Its use as a precursor to dyes engendered an entire industry. In 1854 Frederick Crace Calvert, "an eminent English chemist," made the extraordinary statement before the Society of Arts that ere long, some valuable dyeing substances would be prepared from coal." Coal tar was used for medical purposes as early as the 1800s.

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.

Coal tar is a mixture of approximately 10,000 chemicals, of which only about 50% have been identified. Most of the chemical compounds are polycyclic aromatic hydrocarbon:

• 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)
• methylated and polymethylated derivatives, mono- and polyhydroxylated derivatives, and heterocyclic compounds.

Others: benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene and methyl-naphthalenes, acenaphthene, fluorene, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene. Many of these constituents are known carcinogens.

Derivatives

Various phenolic coal tar derivatives have analgesic (pain-killer) properties. These included acetanilide, phenacetin, and paracetamol aka acetaminophen. Paracetamol may be the only coal-tar derived analgesic still in use today. 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.

Mechanism of action

The exact mechanism of action is unknown. Coal tar is a complex mixture of phenols, polycyclic aromatic hydrocarbons, and heterocyclic compounds.

It is a keratolytic agent, which reduces the growth rate of skin cells and softens the skin's keratin.

Uses

Medicinal

Coal tar is on the World Health Organization's List of Essential Medicines. Coal tar is generally available as a generic medication over the counter for topical use, typically as a shampoo.

Coal tar may be used in two forms: crude coal tar () or a coal tar solution () also known as liquor carbonis detergens. When used as a topical medication, it is supplied in the form of coal tar 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.

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

Although coal tar topical products have been used to treat childhood psoriasis, this is an uncommon clinical practice, with risk of cancer development existing from long-term treatment. Coal tar as a monotherapy for childhood psoriasis has not been adequately studied.

Adverse effects

Possible adverse effects of using topical coal tar to treat psoriasis include folliculitis, skin irritation, contact dermatitis, phototoxicity, and changes in skin pigmentation and odor.

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 often 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. Sealcoat products that are coal-tar based typically contain 20 to 35 percent coal-tar pitch. Research shows that it is used throughout the United States of America, but several areas have banned its use in sealcoat products, including the District of Columbia; Austin, Texas; Dane County, Wisconsin; the state of Washington; and several municipalities in Minnesota and others. From 2025, Canada has also banned its use in sealants.

Industry

In modern times, coal tar is mostly traded as fuel and as an application for tar, such as roofing. The total value of the trade in coal tar is around US$20 billion per year (2023).

• Historically as a chemical feedstock for dyes.

• As a fuel.

• In the manufacture of paints, synthetic dyes (notably tartrazine/Yellow #5), and photographic materials.

• For heating or to fire boilers. Like most heavy oils, it must be heated before it will flow easily.

• As a source of carbon black.

• As a binder in manufacturing graphite; a considerable portion of the materials in "green blocks" is coke oven volatiles (COV). 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.

• As a main component of the electrode paste used in electric arc furnaces. Coal tar pitch act as the binder for solid filler that can be either coke or calcined anthracite, forming electrode paste, also widely known as Söderberg electrode paste.

• As a feed stock for higher-value fractions, such as naphtha, creosote and pitch. In the coal gas era, companies distilled coal tar to separate these out, leading to the discovery of many industrial chemicals. Some British companies included:

  • Bonnington Chemical Works
  • British Tar Products
  • Lancashire Tar Distillers
  • Midland Tar Distillers
  • Newton, Chambers & Company (owners of Izal brand disinfectant)
  • Sadlers Chemicals

Chemicals from coal tar

At one time, coal tar was a major source of organic compounds. This application has almost disappeared with the growth of the petrochemical industry. Coal tar is produced by thermolysis of coal, usually with the objective of obtaining coke, which is heavily used in the production of iron and steel. The tar is further processed, generating the following fractions of chemical interest: a mixture of benzene, toluene and xylenes ("BTX", phenolics, and polycyclic aromatics, especially naphthalene).

Safety

Side effects include skin irritation, sun sensitivity, allergic reactions, and skin discoloration. It is unclear if use during pregnancy is safe for the baby and its use during breastfeeding is not typically recommended. The exact mechanism of action is unknown. It is a complex mixture of phenols, polycyclic aromatic hydrocarbons (PAHs), and heterocyclic compounds. It demonstrates antifungal, anti-inflammatory, anti-itch, and antiparasitic properties.

Cancer

Long-term, consistent exposure to coal tar likely increases the risk of non-melanoma skin cancers. Evidence is inconclusive as to whether medical coal tar, which does not remain on the skin for the long periods seen in occupational exposure, causes cancer, because there is insufficient data to make a judgment. While coal tar consistently causes cancer in cohorts of workers with chronic occupational exposure, animal models, and mechanistic studies, the data on short-term use as medicine in humans has so far failed to show any consistently significant increase in rates of cancer.

Coal tar contains many polycyclic aromatic hydrocarbons, and it is believed that their metabolites bind to DNA, damaging it. The PAHs found in coal tar and air pollution induce immunosenescence and cytotoxicity in epidermal cells. It is possible that the skin can repair itself from this damage after short-term exposure to PAHs but not after long-term exposure. Long-term skin exposure to these compounds can produce "tar warts", which can progress to squamous cell carcinoma.

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. 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.

The International Agency for Research on Cancer lists coal tars as Group 1 carcinogens, meaning they directly cause cancer. The U.S. Department of Health and Human Services lists coal tars as known human carcinogens.

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.

Other

Coal tar causes increased sensitivity to sunlight, 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 13 January 2010 as a substance of very high concern by the European Chemicals Agency.

Regulation

The U.S. 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 U.S. 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.

References

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