Petroleum, Coal Tar, and Related Products
Authors: Niemeier Richard W. 
Published: August 2012

The chemical composition of coal tar, coal tar pitch, and related materials is complex and variable. The estimated number of compounds present in these complex mixtures is in thousands. Because of variation in source materials and manufacturing processes, including different temperatures and times of carbonization, no two coal tars or pitches are chemically identical, and their toxicity may differ with their origin. In general, however, approximately 80% of the total carbon present in coal tars exists in aromatic form.

Benzo[a]pyrene (B[a]P) is probably the most potent, widespread occupational carcinogen in coal tar, coal tar pitch and its volatiles, coke oven emissions, and creosote, all of which have corresponding work exposure standards; however, there is no occupational workplace standard for B[a]P in the United States. B[a]P may account for more than 75% of the carcinogenic activity of coal tar pitch fume condensate. Individuals who work in tarring facilities, roofing operations, power plants, and asphalt and coke manufacturing facilities may be exposed to benzo[a]pyrene and related PAHs. These mixtures may differ qualitatively and quantitatively.

Coal tar is completely or nearly completely soluble in benzene and nitrobenzene and it is partially soluble in acetone, carbon disulfide, chloroform, diethyl ether, ethanol, methanol, petroleum ether, hexane, and sodium hydroxide solution, and slightly soluble in water. It has a characteristic naphthalene‐like odor. Coal tar is heavier than water and on ignition it burns with a reddish, luminous, and very sooty flame. Coal tar fumes are highly flammable and are easily ignited by heat, sparks, or flames. Vapors are heavier than air. They may travel to a source of ignition and flash back and may form explosive mixtures with air. Vapors will spread along ground and collect in low or confined areas (sewers, basements, tanks). Vapor explosion is a potential hazard indoors, outdoors, or in sewers. Some may polymerize explosively when heated or involved in a fire. Runoff to a sewer may create a fire or explosion hazard. Containers may explode when heated. Coal tar may be transported hot.

The greatest complexity occurs when toxicity is based on the effects of a class of compounds or of a material of a certain physical description. Some polynuclear aromatic hydrocarbons (PNAs) and polycyclic aromatic hydrocarbons (PAHs) are carcinogens of varying potency, and they usually exist in mixtures with other PNAs/PAHs and with compounds (activators, promoters, and inhibitors) that modify their activity. Analysis of each individual compound is very difficult and when done does not yield a clear answer. Given the complexity of the mixture of biologically active agents and their interactions, a calculated equivalent dose would have little accuracy. In these instances, it is common to measure some quantity related to the active agents and to base the occupational exposure limit on that index. An occupational exposure limit for PNAs has been based on the total weight of benzene‐ or hexane‐soluble airborne material. This limit may be appropriate for coal tar pitch volatiles (CTPV) for which it was developed, but it may not work for other PNA/PAH containing materials. Crude oil, asphalt fumes, and cracked petroleum stocks may contain PNA/PAH. The coal dust particles mixed in with coal tar pitch volatiles are not soluble in benzene, but almost all of the petroleum‐derived materials admixed with PNAs/PAHs are soluble in benzene.

Some of the highest measured levels of coal tar pitch volatiles have occurred in the aluminum reduction industry, especially in the Soderberg process pot rooms.

The problem of differentiating the several classes of compounds in a mixed atmosphere such as coal tar pitch volatiles adds complexity to sampling method selection, and it is sometimes necessary to make, and clearly state alongside the results, certain simplifying assumptions. It is commonly assumed when measuring the more toxic soluble form of an element that the “safe” assumption may be made that the entire element present was soluble.

In a study of bioremediation effectiveness, the ability of indigenous soil microorganisms to remove these contaminants from aqueous solutions was determined by GC analysis of organic extracts of biotreated groundwater. Changes in potential environmental and human health hazards associated with the biodegradation of this material were determined at intervals by “Microtox” assays and fish toxicity and teratogenicity tests.