Uses of Coal tar

Coal tar is a complex hydrocarbon mixture produced by thermal destruction (pyrolysis) of coal, typically a dark viscous liquid or semisolid with a smoky or naphthenic odor. The composition of coal tar will be influenced by the process used for pyrolytic distillation as well as by the original composition of the coal; however all coal tars will be comprised of a variable mixture of organic compounds including benzene, toluene, xylenes, cumenes, coumarone, indene, benzofuran, naphthalene, acenaphthene, methylnaphthalenes, fluorine, phenol, cresols, pyridine, picolines, phenanthracene, carbazole, quinolines, fluoranthene, and pyrene. The number of specific chemical constituents is in the thousands. Coal tar creosotes and coal tar distillates, oily liquids generally lighter in color and of lower viscosity than coal tar, are fractions produced by additional distillation of crude coal tar. Coal tar pitch is a highly viscous dark semisolid byproduct of coal pyrolysis. Coal tar volatiles are the vapors produced from heated coal tar or coal tar pitch, containing lower molecular weight (smaller ring number) polycyclic aromatic hydrocarbons (PAHs).

Coal tar is noteworthy as one of the first – if not the first – chemical substances documented to cause cancer through occupational exposures. In the eighteenth century, Sir Percival Pott, a British surgeon, noticed a higher incidence of cancers in chimney sweeps chronically exposed to soot and coal tar. He then demonstrated excess cancers occurring in laboratory animals when coal tar is applied to the ears and skin. In the early twentieth century, polycyclic aromatic compounds isolated from coal tar were identified as chemical carcinogens.

Uses

Coal tar is primarily used as a raw material in the manufacture of plastics, solvents, dyes, and in the manufacturing of other chemicals. Most coal tar undergoes further distillation. Industries that use coal tar include road paving, roofing, smelting, and coking. Coal tar creosote is used as a wood preservative. Coal tar products are also ingredients in medicine (Coal Tar United States Pharmacopeia) used to treat skin diseases such as psoriasis or eczema.

Environmental Fate

Environmental partitioning will vary dependent upon the chemical characteristic of various constituents of coal tar. Photochemical degradation may occur in the atmosphere. If entered into aquatic systems, light hydrocarbon constituents such as benzene will volatilize in the air. Biodegradation in aquatic ecosystems will occur at various rates for different constituents. Large molecules such as PAHs are likely to adsorb to soil and sediment, undergoing slow degradation. Other hydrocarbons, such as phenols, may be readily degraded under aerobic conditions.

Mechanism of Toxicity

Due to the variability in composition, it is not possible to describe all potential mechanisms of toxicity for coal tar. The reader is recommended to review additional references for individual constituents found in the Table of Contents. It is likely that acidity of some constituents, such as phenols, and the defatting potential of some hydrocarbons, contribute to the irritancy of coal tar. Phototoxicity of PAHs is likely to be a main cause of contact irritation.
The carcinogenicity of PAH constituents is believed to lie in their potential for their reactive metabolites to be bound to macromolecules such as DNA. The mechanism of therapeutic value as a topical agent in the treatment of skin diseases is unknown but is thought to involve decreased epidermal proliferation.