4.1. CHEMICAL IDENTITY

Information regarding the chemical identity, chemical synonyms, and identification numbers for wood creosote, coal tar creosote, and coal tar is provided in Tables 4-1 through 4-3. Coal tar pitch is similar in composition to coal tar creosote and is not presented separately. Coal tar pitch volatiles are compounds given off from coal tar pitch when it is heated. The volatile component is not shown separately because it varies with the composition of the pitch. Creosotes and coal tars are complex mixtures of variable composition containing primarily condensed aromatic ring compounds (coal-derived substances) or phenols (wood creosote). Therefore, it is not possible to represent these materials with a single chemical formula and structure. The sources, chemical properties, and composition of coal tar creosote, coal tar pitch, and coal tar justify treating these materials as a whole. Wood creosote is discussed separately because it is different in nature, use, and risk. The partitioning behavior of PAHs and other semi-volatile substances between the vapor and particulate phase in air is well understood (Eisenreich et al. 1981; Xie et al. 2014). In general, several of the low molecular weight constituents are semi-volatile and exist in air in the vapor-phase, while larger PAHs are less volatile and tend to exist in the particulate phase; this affects atmospheric transport, degradation, and deposition into the lungs (Volckens and Leith 2003).

Table 4-1

Chemical Identity of Wood Creosote.

Table 4-2

Chemical Identity of Coal Tar Creosote.

Table 4-3

Chemical Identity of Coal Tar.

4.2. PHYSICAL AND CHEMICAL PROPERTIES

Wood creosote, coal tar creosote, coal tar, and coal tar pitch differ from each other with respect to their composition. Descriptions of each mixture are presented below.

Wood Creosote. Wood creosotes are derived from either beechwood (referred to herein as beechwood creosote) or the resin from leaves of the creosote bush (Larrea, referred to herein as creosote bush resin). Beechwood creosote consists mainly of phenol, cresols, guaiacols, and xylenols. It is a colorless or pale yellowish liquid, and it has a characteristic smoky odor and burnt taste (Miyazato et al. 1981). It had therapeutic applications in the past as a disinfectant, laxative, and stimulating expectorant, but it is not a major pharmaceutical ingredient today in the United States. Beechwood creosote is obtained from fractional distillation (200–220°C at atmospheric pressure) of beechwood or related plants. The mixture has been characterized by Ogata and Baba (1989). Phenol, p-cresol, and guaiacols (guaiacol and 4-methylguaiacol) comprise the bulk of beechwood creosote. Xylenols, other methylated guaiacols, and trimethylphenols account for virtually all the remaining phenolics in the material. Since beechwood creosote is obtained from different sources using no standardized procedures, its composition may vary to some degree. For the sample analyzed by Ogata and Baba (1989), more than two-thirds of the more than 20 compounds identified (see Table 4-4) were represented by just four components (phenol, p-cresol, guaiacol, and 4-methylguaiacol). Information regarding selected chemical and physical properties of wood creosote are shown in Table 4-5.

Table 4-4

Identity of Major Components of Beechwood Creosote.

Table 4-5

Physical and Chemical Properties of Wood Creosote.

Creosote bush resin consists of phenolics (e.g., flavonoids and nordihydroguaiaretic acid), neutrals (e.g., waxes), basics (e.g., alkaloids), and acidics (e.g., phenolic acids). The phenolic portion comprises 83–91% of the total resin. Nordihydroguaiaretic acid accounts for 5–10% of the dry weight of the leaves (Leonforte 1986). No other relevant chemical/physical data are available for creosote bush resin; the substance is therefore not addressed further in this profile.

Coal Tar Creosote, Coal Tar, and Coal Tar Pitch. These three substances are very similar mixtures obtained from the distillation of coal tars. The physical and chemical properties of each are similar, although limited data are available for coal tar, and coal tar pitch. Chemical Abstracts Service (CAS) Registry Numbers are associated with coal tar creosote (8001-58-9), coal tar pitch (67996-93-2), and coal tar (8007-45-2). Literature searches for coal tar pitch produce data identical to that obtained for coal tar creosote. A distinction between these materials is provided in the following discussion.

Coal tars are byproducts of the carbonization of coal to produce coke and/or natural gas. Physically, they are usually viscous liquids or semi-solids that are black or dark brown with a naphthalene-like odor. The coal tars are complex combinations of PAHs, phenols, heterocyclic oxygen, sulfur, and nitrogen compounds. By comparison, coal tar creosotes are distillation products of coal tar. They have an oily liquid consistency and range in color from yellowish-dark green to brown. The coal tar creosotes consist of aromatic hydrocarbons, anthracene, naphthalene, and phenanthrene derivatives. At least 75% of the coal tar creosote mixture is PAHs. Unlike the coal tars and coal tar creosotes, coal tar pitch is a residue produced during the distillation of coal tar. The pitch is a shiny, dark brown to black residue, which contains PAHs and their methyl and polymethyl derivatives, as well as heteronuclear compounds (AWPA 1988). Coal tar creosote is defined by the latter organization as:

A distillate derived from coal tar. As used in the wood preserving industry, creosote denotes a distillate of coal tar produced by the high temperature carbonization of bituminous coal. Coal tar creosote consists principally of liquid and solid aromatic hydrocarbons and contains some tar acids and tar bases; it is heavier than water and has a continuous boiling range beginning at about 200°C (AWPA 1988).

Coal tar creosote is now commonly defined by function and refers to “the fractions or blends of fractions specifically used for timber preservation” (IARC 1987). The substance is a complex mixture typically composed of approximately 85% PAHs and 2–17% phenolics (Bedient et al. 1984). The composition of the creosote mixture is dependent on the sources and preparation parameters of the coal tar, and as a result the creosote components are rarely consistent in their type and concentration. An example of the composition variability among creosote samples was presented by Weyand et al. (1991). In that study, the concentrations of several PAHs were analyzed in four coal tars. All of the PAHs identified exhibited 2-fold to nearly 20-fold differences in concentration among the four samples. Benzo[a]pyrene, a component whose individual toxicity has been examined extensively, ranged from nondetectable levels (detection limit 0.3 g/kg) to 1.7, 6.4, and 3.9 g/kg of coal tar.

The International Programme on Chemical Safety (IPCS) Concise International Chemical Assessment Document (CICAD) for coal tar creosote lists some common constituents of some coal tar creosotes that were analyzed for their chemical identity (IPCS 2004). These are summarized in Table 4-6.

Table 4-6

Some Constituents and Weight Percentage of Eight Coal Tar Creosote Mixtures.

Coal tar itself is produced by the carbonization, or coking, of coal. Coal tar is defined by Hawley (1977) as:

A black, viscous liquid (or semi-solid), naphthalene-like odor, sharp burning taste; obtained by the destructive distillation of bituminous coal, as in coke ovens; 1 ton of coal yields 8.8 gallons of coal tar. Combustible. Specific gravity 1.18–1.23 (66/60°F). Soluble in ether, benzene, carbon disulfide, chloroform; partially soluble in alcohol, acetone, methanol, and benzene; only slightly soluble in water.

The composition of the mixture will vary across lots and across manufacturers. Gallacher et al. (2017a, 2017b) performed an analysis of 16 coal tar samples obtained from five different production processes. They identified a total of 2,369 unique compounds. This included 948 aromatics, 196 aliphatics, 380 sulfur-containing compounds, 209 oxygen-containing compounds, 262 nitrogen-containing compounds, and 865 heterocyclic compounds (15 mixed heterocycles); of all the PAHs, 359 were hydroxylated. The contents of both heterocyclic and hydroxylated PAHs varied greatly with the production process used. Of the 2,369 compounds identified, 173 were found to be present in all samples (the majority of these were PAHs). A full list of these compounds can be obtained (Gallacher et al. (2017c). Properties of coal tar creosote are shown in Table 4-7.

Table 4-7

Physical and Chemical Properties of Coal Tar Creosote.

Coal tar pitch is the tar distillation residue produced during coking operations (NIOSH 1977). The grade of pitch thus produced is dependent on distillation conditions, including time and temperature. The fraction consists primarily of condensed ring aromatics, including 2–6 ring systems, with minor amounts of phenolic compounds and aromatic nitrogen bases. The number of constituents in coal tar pitch is estimated to be in the thousands (EPA 2015). A list of the components comprising the PAH fraction of coal tar pitch is shown in Table 4-8. Table 4-9 summarizes physical/chemical data for coal tar. Properties for this substance are similar or identical to those shown in Table 4-7 for coal tar creosote. Because these substances are all complex mixtures, physical-chemical properties such as log K_ow and Henry’s Law constants cannot be represented by a single value. Ranges of values for several physical-chemical properties for the chemical classes of coal tar creosote have been published (IPCS 2004). Because of the variability in feedstock and manufacturing processes, presentation of exact values for various properties presented in Tables 4-7 and 4-9 is not possible.

Table 4-8

Identity of PAH Components of Coal Tar Pitch.

Table 4-9

Physical and Chemical Properties of Coal Tar.