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Polycyclic aromatic hydrocarbons in the environment.

Polyclinic aromatic hydrocarbons (PAHs, or polynuclear aromatic hydrocarbons) are widespread in the environment. They are characterized by the presence of three or more fused five- or six-membered aromatic rings (see Figure 1), relatively high-molecular weights, and hydrophobicity. Their nomenclature is complex, and the terminology used here is that recommended by IUPAC (1979). The major environmental concer about PAHs focusses on their role in mutagenesis and carcinogenesis in various animal phyla. Oxygen, sulphur, or nitrogen may substitute for ring carbons to yield heterocyclic compounds. These arise from the same sources as PAHs, occur with them in the environment, and include carcinogenic and mutagenic members, but will not be considered further here.

PAH Sources

PAHs are formed during various natural and synthetic combustion and pyrolytic processes. Major natural sources include volcanoes, forest fires, biosynthesis (of certain congeners), and geothermal processes. Synthetic sources include the use of fossil fuels and other carbonaceous materials as fuels and chemical feedstocks.

PAH formation is associated with incomplete combustion, and as the oxygen : fuel raio decreases, PAH formation increases. Pyrolytic processing, eg. the destructive distillation of bituminous coal, or the catalytic cracking of petroleum, can lead to products extremely rich in PAHs. Higher temperatures lead to simpler, non-alkylated PAHs, while cooler processes, such as those involved in petroleum formation, lead to more complex, alkylated PAHs. All processes result in complex PAH mixtures. For example, approximately 1000 PAHs, from indene to pentamethylchrysene, have been identified in cigarette smoke. Marijuana smoke contains higher PAH levels, because through selective breeding tobacco contains smaller amounts of the wax which is believed to be the major contributor to PAH formation during burning.

Some specific combustion sources are space heating, power generation, internal and external combustion engines, garbage burning, and cooking. Some pyrolytic products containing PAHs in appreciable concentrations are coal tars, coal tar creosote, anthracene soils, coal tar pitch, used crankcase lubricants, coal gasification oils and tars, automotive tires, certain asphalts, and carbon black. Special sources of human exposure are the smoking of tobacco and other plants, and the consumption of improperly prepared foodstuff.

PAH Analysis

It is generally necessary to measure several PAH compounds. For example, 16 PAH compounds are routinely measured in sediments and dredge spoils under the Canadian Environmental Protection Act (CEPA). Matrices of environmental interest are diverse and include foodstuffs, biota, water, air, sediments, sludges, etc. PAH concentrations range from residual to percent levels.

Three major problems bedevil PAH measurement:

1. specimens contain complex PAH mixtures; thus efficient separation methods coupled with highly specific detection systems are required for determination of PAH compounds;

2. concentrations of interest are often in the low [mu]g . [kg.sup.-1] range;

3. PAHs are often tightly bound within the sample; thus, certain PAH formation processes also lead to formation of highly sorptive materials (carbons).

Popular analytical methods are capillary gas chromatography-mass spectroscopy, and reversed-phase liquid chromatography with UV absorption or fluorescence. Sample treatment is variable. Saponification and extraction followed by adsorption or gel filtration chromatography are generally employed for the analysis of foodstuffs and biota. Photodegradation may be a problem with certain PAHs during analysis.

Several intercomparative studies have demonstrated large inter-laboratory differences in measurements of common non-alkylated PAHs. Fortunately, experience in food regulatory laboratories has demonstrated that with careful quality management procedures inter-laboratory relative standard deviations of about 10% can be achieved for benzo[a]pyrene at a concentration of 10 [mu]g . [kg.sup.1-] wet wt. A few reference materials for common PAHs are available from the National Research Council of Canada and elsewhere.

Environmental Inputs of PAHs

Due to both the complex nature of PAH-containing materials and to PAH formation depending upon process efficiency, estimates of PAH inputs to the environment are imprecise. The global consumption of fossil and other fuels is massive. The major PAH input to the environment comes from the mishandling and inefficient combustion of these fuels. It has been estimated that the annual global release of PAHs to the atmosphere in the mid-1970s was approximately 1 x [10.sup.5] tonnes; this included 5 x [10.sup.3] tonnes of benzo[a]pyrene. In terms of residential space heating, the relative annual hydrocarbon emission contributions from gas, kerosene or fuel oil, and wood were approximately 5%, 5%, 2%, and 88%, respectively, in the US. Approximately 6 x [10.sup.9] tonnes of asphalt (containing 2-3 [mu]g . [g.sup.-1] PAHs) are present on American road surfaces.

The environmental effect of this massive amount of PAHs is unknown, since their release may be too slowly to be significant. Approximately 9.5 x [10.sup.5] tonnes of creosote/coal tar, containing 5-10% PAHs, are used annually in the US as a wood preservative. It is estimated that the annual input of crude and processed oil (1-3% PAHs) to the oceans of the world is 1.1 x [10.sup.6] tonnes.

Environmental Occurrence of PAHs

The qualitative distribution of non-alkylated PAHs is about the same in most environmental samples. The fluoranthene : pyrene ratio is near unity with relative abundances of phenanthrene, fluoranthene, and pyrene of 12%, 16%, and 15%, respectively. Benzo[a]pyrene, the PAH of greatest toxicological interest, generally represents 5% or less of the total. Most environmental PAHs are of terrestrial anthropogenic origin. Studies of dated sedimentary cores demonstrate that the qualitative PAH distribution has been constant over the past 125 years with respect to distribution of alkylated PAHs. The total PAH concentration has increased since the start of this period, indicating anthropogenic input consistent with fossil fuel use. PAH concentrations in deeper (older) cores are much lower and their composition is more characteristic of forest fire input.

Average US urban air levels of PAHs are approximately 60 ng . [m.sup3] with levels tending to increase in the winter due to increased space heating. European air levels are somewhat higher due to the greater use of coal. Controls on space heating equipment use have led to decreases in air levels in recent years. However, the growing popularity of residential wood stoves has caused increased PAH levels in certain areas. Most PAHs in air are associated with particulates and PAH deposition is characteristic of particulate deposition.

As expected from their hydrophobicity, PAH concentrations in drinking water are rarely very high. One estimates is that individuals receive only 0.1% of their total PAH intake from drinking water.

Because of their hydrophobicity, PAHs tend to bind to sediments in aquatic systems. PAH concentrations are highest in inshore surface sediments and levels decrease offshore, indicating terrestrial inputs as their major source. PAH-containing substances, such as coal tars and pitches, are heavier than water. Situations exist (eg. the in south arm of Sydney Harbour, NS, which received discharge from a coal-coking facility, and the 'chemical book' draining a wood preservative plant at Newcastle, NB) where sediment PAH concentrations reach parts-per-thousand levels.

Plants may contain appreciable PAH concentrations, particularly when grown in areas of substantial PAH input. Direct drying (where the goods are in contact with combustion products) can increase total PAH concentrations in the final product. Oils prepared from PAH-containing starting materials are enriched with respect to their PAH content. However, levels of carcinogenic PAHs in processed Canadian vegetable and dairy foodstuffs are in the low parts-per-billion range and are similar to levels reported elsewhere.

Animals exposed to environmental PAHs may or may not accumulate them depending upon exposure, uptake, and degradation rates. Many higher animals metabolize PAHs to water-soluble excretory products and thus do not accumulate them. Lower animals, most notably shellfish such as crustaceans and bivalves, do not metabolise PAHs appreciably and can accumulate startling high concentrations. This is especially true if they live in industrial harbours which contain creosoted structures or receive various PAH-containing inputs.

Crustaceans such as the lobster (Homarus americanus) which has a large, lipid-rich digestive gland (the mig-gut gland, or 'hepatopancreas') do not readily depurate PAHs when moved into a clean environment. Lobsters captured offshore Nova Scotia had only trace concentrations of benzo[a]pyrene in tail meat, while lobsters taken nearshore contained 0.2 - 0.9 [mu]g . [kg.sup.-1] wet wt. of benzo[a]pyrene. Tail meat from lobsters held for a few months in a tidal storage pound partially constructed of creosote-treated lumber contained 7.4 - 281 [mu]g . [kg.sup.-1] wet wt. of benzo[a]pyrene. Digestive gland benzo[a]pyrene concentrations were approximately 10 times higher, the highest value recorded being 2300 [mu]g . [kg.sup.-1] wet wt. benzo[a]pyrene.

Similarly, digestive gland from lobsters captured in the south arem of Sydney Harbour, Nova Scotia, contained 433-2240 [mu]g . [kg.sup.-1] wet wt. benzo[a]pyrene. As expected, other PAHs were present in all samples, resulting in higher total PAH concentrations. In 1982, the federal Department of Fisheries and Oceans required pound owners to remove creosote-treated lumber and closed the south arm of Sydney Harbour to lobsters fishing.

The magnitude of this contamination can be contrasted with that in other foodstuffs where benzo[a]pyrene was detected in only 32 of 60 food products, and then to a maximum of only 7 [mu]g . [kg.sup.-1] wet wt. benzo[a]pyrene.

Degradation of PAHs

PAHs are destroyed in the environment by various chemical and biochemical mechanisms. Photo-oxidation is probably the most important pathway quantitatively. Microorganisms can metabolize PAHs, in particular, lower molecular-weight ones. PAHs are also susceptible to oxidation by ozone, peroxides, and nitrogen and sulphur oxides. PAHs on airborne particulate matter are less readily oxidised than non-particulate airborne PAHs.

The metabolism of PAHs in higher organisms is complex, and varies with species and strain, tissue, age, sex, and nutritional status. Most higher animals have some basal level of the enzyme system responsible for the initial steps of PAH metabolism (aryl hydrocarbon hydroxylase, or more specifically cytochrome P45IA1), which catalyses the addition of oxygen to the aromatic structure to yield epoxides. These rapidly hydrate to dihydrols and are oxidised further to diol epoxides that are hydrated to tetrahydrols. Several aryl hydrocarbon hydroxylases are known. Oxidation of a single PAH compound such as benzo[a]pyrene leads to an array of stereo- and positional isomeric oxidised products.

Conjugating enzymes promote further reaction with hydrophilic compounds to form polar derivatives such as glucuronides, glycosides, and mercapturates, which are excreted in the bile and urine. Pretreatment of the animal with PAHs (or other inducers) leads to high levels of enzyme activity. Such induction may result in compounds like benzo[a]pyrene being rapidly metabolised by the pre-exposed animal. However, some conversion products of aromatic hydrocarbons tend to accumulate in tissues and their concentrations may remain high following exposure.

Toxicology of PAHs

PAHs, particularly those with larger numbers of rings, are usually not acutely toxic to fish, which presumably reflects their low water solubility. The mutagenic effects of PAHs have been demonstrated in mammals and in Ames tests, and are greatest in fractions containing four- and five-membered rings.

The ability of certain PAH compounds and PAH mixtures to induce cancers has been the subject of research since the original observation by Pott of increased scrotal cancer incidence among chimney sweeps in 1775, and the first induction of skin tumours in rabbits by dermal application of coal tar for 150 days. Induction of tumours in animals is not a straightforward indication of human health hazard since, besides species and strain differences, questions can be raised regarding dosing levels and dosing procedures, exacerbating and ameliorating factors, routes of administration, and target organs.

The first step leading to cancer is believed to be the reaction of a PAH metabolic intermediate with a cellular constituent such as DNA. The concentration of this 'active' intermediate depends, of course, on its rate of formation and degradation, reaction with other non-cancer inducing cellular constituents, and the extent to which its precursor is metabolized by other routes. Given the complexity of the metabolizing system and the variety of intracellular reactions to which 'active' intermediates are subject, it is not surprising that cancer-related chemicals have been divided into several types:

1. carcinogens that induce cancer by themselves;

2. co-carcinogens that are synergistic with carcinogens;

3. tumour initiators that lead to cancer only when used as a pretreatment followed by a low dose of another compound;

4. promoters that are generally non-carcinogens, but act with initiators to cause cancer;

5. anti-cancer agents that are antagonists to carcinogens.

Various PAH compounds have been demonstrated to have one or more of these activities, including -- though infrequently -- even anti-cancer activity. Some common PAH compounds that have been identified as carcinogens are benzo[a]pyrene, 3-methyl-cholanthrene, benz[a]anthracene and dibenz[a,h]anthracene.

Probably because of the complexity of both PAH exposure and metabolism, very few countries have imposed fixed tolerances for total PAH or benzo[a]pyrene concentrations in foodstuffs. However, all national health agencies recommend minimal exposure to PAH-containing materials because of their cancer-inducing roles.
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Author:Uthe, John F.
Publication:Canadian Chemical News
Date:Aug 1, 1991
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