Polycyclic aromatic hydrocarbons (PAHs) are organic compounds consisting of three or more condensed aromatic rings, where certain carbon atoms are common to two or three rings. Such a structure is also referred to as a fused ring system. The rings can be arranged in a straight line, angled or in a cluster formation. Furthermore, the name hydrocarbon indicates that the molecule contains only carbon and hydrogen. The simplest fused structure, containing only two condensed aromatic rings, is naphthalene. To the aromatic rings, other types of rings can be fused such as five-carbon rings or rings containing other atoms (oxygen, nitrogen or sulphur) substituted for carbon. The latter compounds are referred to as heteroaromatic or heterocyclic compounds and will not be considered here. In the PAH literature many other notations are found: PNA (polynuclear aromatics), PAC (polycyclic aromatic compounds), POM (polycyclic organic matter). The last notation often includes heteroaromatic compounds. PAHs include hundreds of compounds which have attracted much attention because many of them are carcinogenic, especially those PAHs containing four to six aromatic rings.
The nomenclature is not uniform in the literature, which can confuse the reader of papers from different countries and ages. IUPAC (International Union of Pure and Applied Chemistry) has adopted a nomenclature which nowadays is commonly used. A very brief summary of the system follows:
Some parent PAHs are selected and their trivial names are retained. As many rings as possible are drawn in a horizontal line and the greatest number of remaining rings are placed in the upper right quadrant. The numbering starts with the first carbon atom not common to two rings in the ring to the right in the top line. The following carbon atoms binding a hydrogen are numbered clockwise. The outer sides of the rings are given letters in alphabetical order, beginning with the side between C 1 and C 2.
To elucidate the nomenclature of PAHs, the name for benzo(a)pyrene is taken as an example. Benzo(a)— indicates that an aromatic ring is fused to pyrene in the a position. A ring can be fused also in positions b, e, and so on. However, positions a, b, h and i are equivalent, and so are e and l. Accordingly, there are only two isomers, benzo(a)pyrene and benzo(e)pyrene. Only the first letter is used, and the formulas are written according to the rules above. Also in positions cd, fg, and so on, of pyrene a ring can be fused. However, this substance, 2H-benzo(cd)pyrene, is saturated in position 2, which is indicated by an H.
Physico-chemical properties of PAHs. The conjugated II-electron systems of the PAHs account for their chemical stability. They are solids at room temperature and have very low volatility. Depending on their aromatic character, the PAHs absorb ultraviolet light and give characteristic fluorescence spectra. The PAHs are soluble in many organic solvents, but they are very sparingly soluble in water, decreasing with increasing molecular weight. However, detergents and compounds causing emulsions in water, or PAHs adsorbed on suspended particles, can increase the content of PAHs in wastewater or in natural waters. Chemically, the PAHs react by substitution of hydrogen or by addition reactions where saturation occurs. Generally the ring system is retained. Most PAHs are photo-oxidized, a reaction which is important for the removal of PAHs from the atmosphere. The most common photo-oxidation reaction is formation of endoperoxides, which can be converted to quinones. For steric reasons an endoperoxide cannot be formed by photo-oxidation of benzo(a)pyrene; in this case 1,6-dione, 3,6-dione and 6,12-dione are formed. It has been found that the photo-oxidation of adsorbed PAHs can be greater than that of PAHs in solution. This is of importance when analysing PAHs by thin-layer chromatography, especially on layers of silica gel, where many PAHs very rapidly photo-oxidize when illuminated by ultraviolet light. For the elimination of PAHs from the occupational environment the photo-oxidation reactions are of no importance. PAHs rapidly react with nitrogen oxides or HNO3. For example anthracene can be oxidized to anthraquinone by HNO3 or give a nitro derivative by a substitution reaction with NO2. PAHs can react with
SO2, SO3 and H2SO4 to form sulphinic and sulphonic acids. That carcinogenic PAHs react with other substances does not necessarily mean that they are inactivated as carcinogens; on the contrary, many PAHs containing substituents are more powerful carcinogens than the corresponding parent compound. A few important PAHs are considered individually here.
Formation. PAHs are formed by pyrolysis or incomplete combustion of organic material containing carbon and hydrogen. At high temperatures the pyrolysis of organic compounds yields molecule fragments and radicals which combine to give PAHs. The composition of the resulting products of the pyrosynthesis is dependent on the fuel, the temperature and the residence time in the hot area. Fuels found to yield PAHs include methane, other hydrocarbons, carbohydrates, lignins, peptides, lipids and so on. However, compounds containing chain branching, unsaturation or cyclic structures generally favour the PAH yield. Evidently PAHs are emitted as vapours from the zone of burning. Due to their low vapour pressures most PAHs will immediately condense on soot particles or form very small particles themselves. PAHs entering the atmosphere as vapour will be adsorbed on existing particles. Aerosols containing PAHs are thus spread in the air and may be transported great distances by winds.
Occurrence and Uses
Many PAHs can be prepared from coal tar. The pure substances have no significant technical use, except for naphthalene and anthracene. However, they are used indirectly in coal tar and petroleum, which contain mixtures of various PAHs.
PAHs can be found almost everywhere, in air, soil and water originating from natural and anthropogenic sources. The contribution from natural sources such as forest fires and volcanoes is minute compared to the emissions caused by humans. The burning of fossil fuels causes the main emissions of PAHs. Other contributions come from the combustion of refuse and wood, and from the spillage of raw and refined petroleum which per se contains PAHs. PAHs also occur in tobacco smoke and grilled, smoked and fried food.
The most important source of PAHs in the air of the occupational environment is coal tar. It is formed by pyrolysis of coal in gas and coke works where emissions of fumes from the hot tar occurs. The workers in the vicinity of the ovens are highly exposed to these PAHs. Most investigations of PAHs in work environments have been made in gas and coke works. In most cases only benzo(a)pyrene has been analysed, but there are also some investigations on a number of other PAHs available. Generally, the benzo(a)pyrene content in the air above the ovens shows the highest values. The air above the flues and the tar precipitator is extremely rich in benzo(a)pyrene, up to 500 mg/m3 has been measured. By personal air sampling, the highest exposure has been found for truck drivers, wharf workers, chimney sweeps, lid workers and tar chasers. Naphthalene, phenanthrene, fluoranthene, pyrene and anthracene dominate among the PAHs isolated from air samples taken on the battery top. It is evident that some of the workers in the gas and coke industry are exposed to PAHs at high levels, even in modern installations. Certainly, in these industries, it would not be unusual for a large number of workers to have been exposed for many years. Epidemiological investigations have shown an elevated risk of lung cancer for these workers. Coal tar is used in other industrial processes, where it is heated, and thereby PAHs are liberated to the ambient air.
The poly aryl hydrocarbons are primarily used in the manufacture of dyes and chemical sythesis. Anthracene is used for the production of anthraquinone, an important raw material for the manufacture of fast dyes. It is also used as a diluent for wood preservatives and in the production of synthetic fibres, plastics and monocrystals. Phenanthrene is used in the manufacture of dye-stuffs and explosives, biological research, and the synthesis of drugs.
Benzofuran is employed in the manufacture of coumarone-indene resins. Fluoranthene is a constituent of coal tar and petroleum-derived asphalt used as lining material to protect the interior of steel and ductile-iron potable water pipes and storage tanks.
Aluminium is manufactured in an electrolytic process at a temperature of about 970 °C. There are two types of anodes: the Söderberg anode and the graphite (“prebaked”) anode. The former type, which is the most commonly used, is the main cause of PAH exposure in aluminium works. The anode consists of a mixture of coal-tar pitch and coke. During electrolysis it is graphitized (“baked”) in its lower, hotter part, and finally consumed by electrolytic oxidation to carbon oxides. Fresh anode paste is added from above to keep the electrode running continuously. PAH components are liberated from the pitch at the high temperature, and they escape to the work area in spite of ventilation arrangements. In many different occupations in an aluminium smelter such as stud-pulling, rack-raising, mounting of flaints and adding of anode paste, the exposure can be considerable. Also ramming of cathodes causes exposure to PAHs, as pitch is used in rodding and slot mixes.
Graphite electrodes are used in aluminium reduction plants, in electric steel furnaces and in other metallurgical processes. The raw material for these electrodes is generally petroleum coke with tar or pitch as a binder. The baking is done by heating this mixture in ovens to temperatures above 1,000 °C. In a second heating step up to 2,700 °C the graphitization occurs. During the baking procedure large quantities of PAHs are liberated from the electrode mass. The second step involves rather little PAH exposure, since the volatile components are given off during the first heating.
In iron and steel works and foundries exposure occurs to PAHs originating from coal tar products in contact with molten metal. The tar preparations are used in furnaces, runners and ingot moulds.
The asphalt used for paving streets and roads mainly comes from the distillation residue of petroleum crude oils. The petroleum asphalt in itself is poor in higher PAHs. In some cases, however, it is mixed with coal tar, which increases the possibility of exposure to PAHs when working with hot asphalt. In other operations where tar is melted and spread on a large area, the workers may be heavily exposed to PAHs. Such operations include pipeline coating, wall insulation and roof tarring.
In 1775 an English surgeon, Sir Percival Pott, first described occupational cancer. He associated scrotal cancer in chimney sweeps with their prolonged exposure to tar and soot under conditions of bad personal hygiene. One hundred years later, skin cancer was described in workers exposed to coal tar or shale oil. In the 1930s, lung cancer in workers at steel works and coke works was described. Experimentally developed skin cancer in laboratory animals after repeated application of coal tar was described at the end of the 1910s. In 1933 it was shown that a polycyclic aromatic hydrocarbon isolated from coal tar was carcinogenic. The isolated compound was benzo(a)pyrene. Since then hundreds of carcinogenic PAHs have been described. Epidemiological studies have indicated an elevated frequency of lung cancer of workers in the coke, aluminium and steel industries. Approximately a century later, several of the PAHs have been regulated as occupational carcinogens.
The long latency between first exposure and symptoms, and many other factors, have made the establishment of threshold limit values for PAHs in the work atmosphere an arduous and drawn out task. A long latency period also has existed for standards-making. Threshold limit values (TLVs) for PAHs were practically non-existent until 1967, when the American Conference of Governmental Industrial Hygienists (ACGIH) adopted a TLV of 0.2 mg/m3 for coal tar pitch volatiles. It was defined as the weight of the benzene-soluble fraction of the particulates collected on a filter. In the 1970s, the USSR issued a maximum allowable concentration (MAC) for benzo(a)pyrene (BaP) based upon laboratory experiments with animals. In Sweden a TLV of 10 g/m3 was introduced for BaP in 1978. As of 1997, the US Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for BaP is 0.2 mg/m3. The ACGIH has no time-weighted average (TWA) since BaP is a suspected human carcinogen. The US National Institute for Occupational Safety and Health (NIOSH) recommended exposure limit (REL) is 0.1 mg/m3 (cyclohexane extractable fraction).
Occupational sources of PAHs other than coal tar and pitch are carbon black, creosote, mineral oils, smoke and soot from various types of burning, and exhaust gases from vehicles. Mineral oils contain low levels of PAHs, but many types of usage cause considerable increase of the PAH content. Some examples are motor oils, cutting oils and oils used for electric discharge machining. However, since the PAHs remain in the oil, the risk of exposure is mainly limited to skin contact. Exhaust gases from vehicles contain low levels of PAHs compared to fumes from coal tar and pitch. In the following list, measurements of benzo(a)pyrene from various types of workplaces has been used to range them according to the degree of exposure:
- very high benzo(a)pyrene exposure (more than 10 mg/m3)— gas and coke works; aluminium works; graphite electrode plants; handling of hot tar and pitch
- moderate exposure (0.1 to 10 g/m3)—gas and coke works; steel works; graphite electrode plants; aluminium works; foundries
- low exposure (less than 0.1 g/m3)—foundries; asphalt manufacturing; aluminium works with prebaked electrodes; automobile repair shops and garages; iron mines and construction of tunnels.
Hazards associated with selected PAHs
Anthracene is a polynuclear aromatic hydrocarbon with condensed rings, which forms anthraquinone by oxidation and 9,10-dihydroanthracene by reduction. The toxic effects of anthracene are similar to those of coal tar and its distillation products, and depend on the proportion of heavy fractions contained in it. Anthracene is photosensitizing. It can cause acute and chronic dermatitis with symptoms of burning, itching and oedema, which are more pronounced in the exposed bare skin regions. Skin damage is associated with irritation of the conjunctiva and upper airways. Other symptoms are lacrimation, photophobia, oedema of the eyelids, and conjunctival hyperaemia. The acute symptoms disappear within several days after cessation of contact. Prolonged exposure gives rise to pigmentation of the bare skin regions, cornification of its surface layers, and telangioectasis. The photodynamic effect of industrial anthracene is more pronounced than that of pure anthracene, which is evidently due to admixtures of acridine, carbazole, phenanthrene and other heavy hydrocarbons. Systemic effects manifest themselves by headache, nausea, loss of appetite, slow reactions and adynamia. Prolonged effects may lead to inflammation of the gastrointestinal tract.
It has not been established that pure anthracene is carcinogenic, but some of its derivatives and industrial anthracene (containing impurities) have carcinogenic effects. 1,2-Benzanthracene and certain monomethyl and dimethyl derivatives of it are carcinogens. The dimethyl and trimethyl derivatives of 1,2-benzanthracene are more powerful carcinogens than the monomethyl ones, especially 9,10-dimethyl-1,2-benzanthracene, which causes skin cancer in mice within 43 days. The 5,9- and 5,10- dimethyl derivatives are also very carcinogenic. The carcinogenicity of 5,9,10- and 6,9,10-trimethyl derivatives are less pronounced. 20-Methylcholanthrene, which has a structure similar to that of 5,6,10-trimethyl-1,2-benzanthracene, is an exceptionally powerful carcinogen. All dimethyl derivatives which have methyl groups substituted on the additional benzene ring (in the 1, 2, 3, 4 positions) are non-carcinogenic. It has been established that the carcinogenicity of certain groups of alkyl derivatives of 1,2-benzanthracene diminishes as their carbon chains lengthen.
Benz(a)anthracene occurs in coal tar, up to 12.5 g/kg; wood and tobacco smoke, 12 to 140 ng in the smoke from one cigarette; mineral oil; outdoor air, 0.6 to 361 ng/m3; gas works, 0.7 to 14 mg/m3. Benz(a)anthracene is a weak carcinogen, but some of its derivatives are very potent carcinogens—for example, 6-, 7-, 8- and 12-methylbenz(a)anthracene and some of the dimethyl derivatives such as 7,12-dimethylbenz(a)anthracene. Introducing a five-membered ring at the 7 to 8 position of benz(a)anthracene results in cholanthrene (benz(j)aceanthrylene), which, together with its 3-methyl derivative, is an extremely powerful carcinogen. Dibenz(a,h)anthracene was the first pure PAH shown to have carcinogenic activity.
Chrysene occurs in coal tar pitch up to 10 g/kg. From 1.8 to 361 ng/m3 has been measured in air and 3 to 17 mg/m3 in diesel engine exhaust. Smoke from a cigarette can contain up to 60 ng of chrysene. Dibenzo(b,d,e,f)-chrysene and dibenzo(d,e,f,p)-chrysene are carcinogenic. Chrysene has weak carcinogenic activity.
Diphenyls. Little information is available about the toxic effects of diphenyl and its derivatives, with the exception of the polychlorinated biphenyl (PCBs). Owing to their low vapour pressure and smell, exposure by inhalation at room temperature does not usually entail a serious risk. However, in one observation, workers engaged in impregnating wrapping paper with a fungicide powder made of diphenyl experienced bouts of coughing, nausea and vomiting. In repeated exposure to a solution of diphenyl in paraffin oil at 90 °C and airborne concentrations well above 1 mg/m3, one man died of acute yellow atrophy of the liver, and eight workers were found suffering from central and peripheral nervous damage and liver injury. They complained of headache, gastrointestinal disturbances, polyneuritic symptoms and general fatigue.
Molten diphenyl can cause serious burns. Skin absorption is also a moderate hazard. Eye contact produces mild to moderate irritation. Processing and handling of diphenyl ether in ordinary use involves little health hazard. The odour may be very unpleasant, and excessive exposure results in eye and throat irritation.
Contact with the substance can produce dermatitis.
The mixture of diphenyl ether and diphenyl at concentrations between 7 and 10 ppm does not seriously affect experimental animals in repeated exposure. However, in humans it can cause eye and airways irritation and nausea. Accidental ingestion of the compound resulted in severe impairment of liver and kidney.
Fluoranthene occurs in coal tar, tobacco smoke and airborne PAHs. It is not a carcinogen whereas the benzo(b)-, benzo(j)- and benzo(k)- isomers are.
Naphthacene occurs in tobacco smoke and coal tar. It causes colouration of other colourless substances isolated from coal tar, such as anthracene.
Naphthalene is readily flammable and, in particulate or vapour form, will form explosive mixtures with air. Its toxic action has been observed primarily as a result of gastrointestinal poisonings in children who mistook mothballs for sweets, and is manifested by acute haemolytic anaemia with hepatic and renal lesions and vesical congestion.
There have been reports of serious intoxication in workers who had inhaled concentrated naphthalene vapours; the most common symptoms were haemolytic anaemia with Heinz bodies, hepatic and renal disorders, and optic neuritis. Prolonged absorption of naphthalene may also give rise to small punctiform opacities in the periphery of the crystalline lens, with no functional impairment. Eye contact with concentrated vapours and condensed micro-crystals may result in punctiform keratitis and even chorioretinitis.
Skin contact has been found to cause erythemato-exudative dermatitis; however, such cases have been attributed to contact with crude naphthalene which still contained phenol, which was the causative agent of the foot dermatitis encountered amongst workers who discharge naphthalene crystallization trays.
Phenanthrene is prepared from coal tar and can be synthesized by passing diphenylethylene through a red-hot tube. It occurs also in tobacco smoke and is found among airborne PAHs. It does not appear to have carcinogenic activity, but some alkyl derivatives of benzo(c)phenanthrene are carcinogenic. Phenanthrene is a recommended exception to systematic numbering; 1 and 2 are indicated in the formula.
Pyrene occurs in coal tar, tobacco smoke and airborne PAHs. From 0.1 to 12 mg/ml is found in petroleum products. Pyrene has no carcinogenic activity; however, its benzo(a) and dibenzo derivatives are very potent carcinogens. Benzo(a)pyrene (BaP) in outdoor air has been measured from 0.1 ng/m3 or lower in unpolluted areas to values several thousand times higher in polluted urban air. BaP occurs in coal tar pitch, coal tar, wood tar, automobile exhaust, tobacco smoke, mineral oil, used motor oil and used oil from electric discharge machining. BaP and many of its alkyl derivatives are very potent carcinogens.
Terphenyl vapours cause conjunctival irritation and some systemic effects. In experimental animals p-terphenyl is poorly absorbed by oral route and appears to be only slightly toxic; meta- and especially ortho-terphenyls are dangerous to the kidney, and the latter can also impair liver functions. Morphologic alterations of mitochondria (the small cellular bodies performing respiratory and other enzymatic functions essential to biological synthesis) have been reported in rats exposed to 50 mg/m3. Heat transfer agents made of hydrogenated terphenyls, terphenyl mixture and isopropyl-meta-terphenyl produced functional changes of nervous system, kidney and blood in experimental animals, with some organic lesions. A carcinogenic risk has been demonstrated for mice exposed to the irradiated coolant, while the non-irradiated mixture appeared to be safe.
Health and Safety Measures
PAHs are found mainly as air contaminations in a great variety of workplaces. Analyses always show the highest content of PAHs in air samples taken where visible smoke or fumes occur. A general method to prevent exposure is to diminish such emissions. In coke works this is done by tightening leaks, increasing ventilation or using cabs with filtered air. In aluminium works similar measures are taken. In some instances, fume and vapor clearance systems will be necessary. Use of prebaked electrodes almost eliminates PAH emissions. In foundries and steel works PAH emissions can be decreased by avoiding preparations containing coal tar. Special arrangements are not needed to remove PAHs from garages, mines and so on, where exhaust gases from automobiles are emitted; ventilation arrangements necessary to remove other more toxic substances simultaneously decrease the PAH exposure. Skin exposure to used oils containing PAHs is avoidable by using gloves and changing contaminated clothes.
Engineering, personal protective, training and sanitary facilties described elsewhere in this Encyclopaedia are to be applied. Since so many members of this family are known or suspected carcinogens, particular care must be given to adherence to the precautions required for the safe handling of carcinogenic substances.
Polyaromatic hydrocarbons tables
Table 1 - Chemical information.
Table 2 - Health hazards.
Table 3 - Physical and chemical hazards.
Table 4 - Physical and chemical properties.