CHAPTER ONE

1.1 INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds consisting of two or more fused benzene rings (linear, cluster or angular arrangement), or compounds made up of carbon and hydrogen atoms grouped into rings containing five or six carbon atoms. They are called ÒPAH derivativesÓ when an alkyl or other radical is introduced to the ring, and heterocyclic aromatic compounds (HACs) when one carbon atom in a ring is replaced by a nitrogen, oxygen or sulphur atoms. PAHs originate mainly from anthropogenic processes particularly from incomplete combustion of organic fuels. PAHs are distributed widely in the atmosphere. Natural processes, such as volcanic eruptions and forest fires, also contribute to an ambient existence of PAHs (Suchanova et al., 2008). PAHs can be present in both particulate and gaseous phases, depending on their volatility. Low molecular weight PAHs (LMW PAHs) that have two or three aromatic rings (molecular weight from 152 to 178g/mol) are emitted in the gaseous phase, while high molecular weight PAHs (HMW PAHs), molecular weight ranging from 228 to 278g/mol, with five or more rings, are emitted in the particulate phase, (ATSDR, 1995) . In the atmosphere, PAHs can undergo photo-degradation and react with other pollutants, such as sulfur dioxide, nitrogen oxides, and ozone. Due to widespread sources and persistent characteristics, PAHs disperse through atmospheric transport and exist almost everywhere. There are hundreds of PAH compounds in the environment but in practice PAH analysis is restricted to the determination of six (6) to sixteen (16) compounds. Human beings are exposed to PAH mixtures in gaseous or particulate phases in ambient air. Long term exposure to high concentration of PAHs is associated with adverse health problems. Since some PAHs are

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considered carcinogens, inhalation of PAHs in particulates is a potentially serious health risk linked to lung cancer (Philips, 1999).

1.2. Physical and Chemical Characteristics of PAHs.

PAHs are a group of several hundred individual organic compounds which contain two or more aromatic rings and generally occur as complex mixtures rather than single compounds. PAHs are classified by their melting and boiling points, vapour pressure, and water solubility, depending on their structure. Pure PAHs are usually coloured, crystalline solids at ambient temperature. The physical properties of PAHs vary with their molecular weight and structure (Table1). Except for naphthalene, they have very low to low water solubilities, and low to moderately high vapour pressures. Their octanol-water partition coefficients (Kow) are relatively high, indicating a relatively high potential for adsorption to suspended particles in the air and in water, and for bioconcentration in organisms (Sloof et al., 1989). Table 1 shows physical and chemical characteristics of few selected PAHs from the sixteen (16) priority PAHs, listed by the US EPA. (see appendix). Most PAHs, especially as molecular weight increases, are soluble in non-polar organic solvents and are barely soluble in water (ATSDR, 1995).

Most PAHs are persistent organic pollutants (POPs) in the environment. Many of them are chemically inert. However, PAHs can be photochemically decomposed under strong ultraviolet light or sunlight, and thus some PAHs can be lost during atmospheric sampling. Also, PAHs can react with ozone, hydroxyl radicals, nitrogen and sulfur oxides, and nitric and sulfuric acids which affect the environmental fate or conditions of PAHs (Dennis et al., 1984; Simko, 1991).

PAHs possess very characteristic UV absorbance spectra. Each ring structure has a unique UV spectrum, thus each isomer has a different UV absorbance spectrum. This is especially useful in

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the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). Generally, PAHs only weakly absorb light of infrared wavelengths between 7 and 14µm, the wavelength usually absorbed by chemical involved in global warning (Ramanathan, 1985).

Polycyclic aromatic hydrocarbons are present in the environment as complex mixtures that are difficult to characterize and measure. They are generally analyzed using gas chromatography coupled with mass spectrometry (GC-MS) or by using high pressure liquid chromatography (HPLC) with ultraviolet (UV) and fluorescence dectetors (Slooff et al., 1989)

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Table 1 Physical and Chemical Characteristics of Some Popular PAHs

s/n Names of PAHs Chemical structure/formula Mol Vapour pressure Partition
weight coefficient
(kow)
1 Naphthalene
C10H8 128.17 0.087mmHg 3.29

2 Fluorine

166.2 3.2×10-4mmHg4.18

C13H10

3 Fluoranthene

202.26 5.0 x10-6mmHg4.90

C16H10

4 Pyrene

202.3 2.5 x10-6mmHg4.88
C16H10
5 Benzo(a)anthracene 228.29 5.61
C20H12 2.5 x10-6mmHg
6 Benzo(k)fluoranthene 6.06
C20H12 252.3 9.59×10-11mmHg

7 Benzo(a)pyrene

C20H12 252.3 5.6×10-9mmHg6.06

8 Indeno(1,2,3-c,d)pyrene

276.3 10×10-16mmHG 6.58

C22H12

Sources: (ATSDR, 1995)

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Source and Emission of PAHs

PAHs are mainly derived from anthropogenic activities related to pyrolysis and incomplete combustion of organic matter. Sources of PAHs affect their characterization and distribution, as well as their toxicity. Major sources of PAH emissions may be divided into four classes: stationary sources (including domestic and industrial sources), mobile emission, agriculture activities, and natural sources (Wania et al, 1996).

1.3. Stationary Sources

Some PAHs are emitted from point sources and this is hardly shifted (moved) for a long period of time. Stationary sources are further subdivided into two main sources: domestic and industrial.

1.3.1. Domestic Sources

Heating and cooking are dominant domestic sources of PAHs. The burning and pyrolysis of coal, oil, gas, garbage, wood, or other substances are the main domestic sources. Domestic sources are important contributors to the total PAHs emitted into the environment. Difference in climate patterns and domestic heating systems produce large geographic variations in domestic emission. PAH emissions from these sources may be a major health concern because of their prevalence in indoor environments (Ravindra et al., 2008). According to a recent World Health Organization (WHO) report, more than 75% of people in China, India, and South East Asia and 50-75% of people in parts of South America and Africa use combustion of solid fuel, such as wood, charcoal for daily cooking.

Main indoor PAH sources are cooking and heating and infiltration from outdoors. PAH emissions from cooking account for 32.8% of total indoor PAHs (Zhu et al., 2009). LMW PAHs which originate from indoor sources are the predominant proportion of the total PAHs identified

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in residential non-smoking air. Toxicity of PAH mixtures from indoor sources is lower than mixtures which contain large amounts of high molecular weight PAHs. Cigarette smoke is also a dominant sources of PAHs in indoor environments. In many studies, PAHs in the indoor air of smoking residences tend to be higher than those of non-smoking residences.

1.3.2. Industrial Sources

Sources of PAHs include emission from industrial activities, such as primary aluminum and coke production, petrochemical industries, rubber tire and cement manufacturing, bitumen and asphalt industries, wood preservation, commercial heat and power generation, and waste incineration (Fabbri and Vassura , 2006).

1.3.3. Mobile Sources

Mobile sources are major causes of PAHs emissions in urban areas. PAHs are mainly emitted from exhaust fumes of vehicles, including automobile, railways, ships, aircrafts, and other motor vehicles. PAHs emissions from mobile sources are associated with use of diesel, coal, gasoline, oils, and lubricant oil. Exhaust emissions of PAHs from motor vehicles are formed by three mechanisms: (1) synthesis from smaller molecules and aromatic compounds in fuel; (2) storage in engine deposits and in fuel; (3) pyrolysis of lubricants (Baek et al., 1991). One of the major influences on the production of PAHs from gasoline automobiles is the air-to-fuel ratio. It has been reported that the amount of PAHs in engine exhaust decreases with leaner mixtures (Ravindra et al., 2006b). A main contribution to PAH concentrations in road dust as well as urban areas is vehicle exhaust. Abrantes et al., (2009) reported that the total emissions and toxicities of PAHs released from light-duty vehicles using ethanol fuels are less than those using

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gasohol. Low molecular weight PAHs are the dominant PAHs emitted from light duty vehicles and helicopter engines.

1.3.4 Agricultural Sources

Open burning of bush wood, straw, moorland heather, and stubble are agricultural sources of PAHs. All of those activities involve burning organic materials under suboptimum combustion conditions. Thus it is expected that a significant amount of PAHs are produced from the open burning of biomass. PAH concentrations released from wood combustion depend on wood type, kiln type, and combustion temperature. Between 80 Ð 90% of PAHs emitted from biomass burning are low molecular weight PAHs, including naphthalene acenaphthylene, phenanthene, fluoranthene and pyrene. Lu et al., (2009) reported that PAHs emitted from the open burning of rice and bean straw are influenced by combustion parameters. Total emissions of 16 PAHs from the burning of rice and bean straw varied from 9.29 to 23.6µg/g and from 3.13 to 49.9µg/g
respectively. PAH emissions increased with increasing temperature from 200 to 7000c.

Maximum emissions of PAHs were observed at 40% O2 content in supplied air. However,

emission of PAHs released from the open burning of rice straw negatively correlate with the moisture content in the straw (Lu et al., 2009).

1.3.5. Natural Sources

Accidental burning of forests, woodland, and moorland due to lightning strikes are natural sources of PAHs. Furthermore, volcanic eruptions and decaying organic matter are also important natural sources, contributing to the levels of PAHs in the atmosphere. The degree of PAH production depends on meteorological conditions such as wind, temperature, humidity, and

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fuel characteristics and type; such as moisture content, green wood, and seasonal wood (Wild and Jones, 1995).

1.3.6 Uses of PAHs

PAHs are not synthesized chemically for industrial purposes. Rather than industrial sources, the major source of PAH is the incomplete combustion of organic material such as coal, oil, and wood. However, there are a few commercial uses for many PAHs. They are mostly used as intermediaries in pharmaceuticals, agricultural products, photographic products, thermosetting plastics, lubricating materials, and other chemical industries. Acenaphthene, Anthracene, Fluoranthene, Fluorene, Phenanthrene and Pyrene are used in the manufacture of dyes, plastics, pigments, pharmaceutical and agrochemicals such as pesticides, wood preservatives resins and so on.

Other PAHs may be contained in asphalt used for the construction of roads, as well as roofing tar. Precise PAHs, specific refined products, are used also in the field of electronics, functional plastics, and liquid crystals. (Katarina, 2011).

1.4 Routes of Exposure for PAHs

PAH exposure through air, water, soil, and food sources occurs on a regular basis. The routes of exposure include ingestion, inhalation, and dermal contact in both occupational and non-occupational settings. Some exposure may involve more than one route simultaneously, affecting the total absorbed dose (such as dermal and inhalation exposure from contaminated air). All non-workplace source of exposure such as diet, smoking, and burning of coal and wood should be taken into consideration (ATSDR, 1995).

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1.4.1 Air

PAHs concentrations in air can vary from less than 5 to 200,000 (ng/m3) (Cherng et al., 1996; Georgiadis and Kyrtopoulos, 1999). Although environmental air levels are lower than those associated with specific occupational exposure, they are of public health concern when spread over large urban populations (Zmirou et al., 2000).

The background levels of the Agency for Toxic Substances and Disease RegistryÕs toxicological

priority for PAHs in ambient air have been reported to be 0.02 Ð 1.2 ng/m3 in rural areas and

0.15 Ð 19.3 ng/m3 in urban areas (ATSDR, 1995).

Cigarette smoking and environmental tobacco are other sources of air exposure. Smoking one cigarette can yield an intake of 20-40ng of benzo (a) pyrene (Philips, 1996; OÕNeill et al., 1997). Smoking one pack of unfiltered cigarette per day yields 0.7µg/day benzo (a) pyrene exposure. Smoking a pack of filtered cigarette per day yields 0.4 µg/day (Sullivan and Krieger 2001).

Environmental tobacco smoke contains a variety of PAHs, such as benzo (a) pyrene, and more than 40 known or suspected human carcinogens. Side-stream smoke (smoke emitted from a burning cigarette between puffs) contains PAHs and other cytotoxic substances in quantities much higher than those found in mainstream smoke (exhaled smoke of smoker) (Jinot and Bayard, 1996; Nelson, 2001).

1.4.2. Water

PAHs can leach from soil into ground water. Water contamination also occurs from industrial effluents and accidental spills during oil shipment at sea. Concentrations of benzo (a) pyrene in

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drinking water are generally lower than those in untreated water and about 100 fold lower than the US Environmental Protection AgencyÕs (EPA) drinking water standard. (EPAÕs maximum contaminant level (MCL) for benzo (a) pyrene in drinking water is 0.2 parts per billion {ppb}(US EPA, 1995).

1.4.3 Soil

Soil contains measurable amounts of PAHs primarily from airborne fallout. Documented level of PAHs in soil near oil refineries have been as high as 200,000 micrograms per kilogram (µg/kg) of dried soil. Levels in soil samples obtained near cities and areas with heavy traffic were typically less than 2,000 µg/kg (IARC, 1973).

1.4.4 Food Stuffs

In non-occupational settings, up to 70% of PAH exposure for non-smoking person can be associated with diet (Skupinska et al., 2004). PAH concentrations in foodstuffs vary. Charring meat or barbecuing food over a charcoal, wood, or other type of fire greatly increase the concentration of PAHs. For example, the PAH level for charring meat can be as high as 10-20 µg/kg (Philips, 1999). Charbroiled and smoked meats and fish contain more PAHs than do uncooked products, with up to 2.0 µg/kg of benzo (a) pyrene detected in smoked fish. Tea, roasted peanuts, coffee, refined vegetable oil, cereals, spinach, and many other foodstuffs contain PAHs. Some crops such as wheat, rye and lentils, may synthesize PAHs or absorb them via water, air, or soil (Grimmer, 1968; Shabad and Cohan 1972; IARC, 1973).

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1.4.5 Other Sources of Exposure

PAHs are found in prescription and non-prescription coal tar products used to treat dermatologic disorders such as psoriasis and dandruff (Van Schooten, 1996). PAHs and their metabolites are excreted in breastmilk, and they readily cross the placenta.

Antracene laxative use has been associated with melanosis of the colon and rectum (Badiali et al., 1985).

1.5 Individuals at Risk of Exposure

Workers in industries or trades using or producing coal or coal products are at highest risk for PAHs exposure. Those workers include, but are not limited to Aluminum workers, Asphalt workers, Carbon black workers, Chimney sweeps, Coal-gas workers, Fishermen (coal tar on nets), Graphite electrode workers, Machinists, Mechanics (auto and disel engine), Printers, Road (pavement) workers, Roofers, Steel foundry workers, Tire and rubber manufacturing workers, and Workers exposed to creosote, such as Carpenters, Farmers, railroad workers, Tunnel construction workers, and Utility workers

Exposure is almost always to mixtures that pose a challenge in developing conclusions (Samet, 1995). Fetuses may be at risk for PAH exposure. PAH and its metabolites have been shown to cross the placenta in various animal studies (ATSDR, 1995). Because PAH are excreted in breast milk, nursing infants of exposed mothers can be easily exposed.

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1.6 Standard and Regulations of PAHs Exposure.

The United States Government Agencies have established standards that are relevant to PAHs exposure in the workplace and the environment. There is a standard relating to PAHs in the workplace, and also a standard for PAHs in drinking water.

Occupational safety and health administrations (OSHA) have not established a substance-specific standard for occupational exposure to PAHs. Exposures are regulated under OSHAÕs Air contaminants standard for substances termed coal tar pitch volatiles (CTPVs) and coke oven emission. Employees exposed to CTPVs in the coke oven industry are covered by the coke oven emissions standard.