POLYCYCLIC AROMATIC HYDROCARBONS (PAHs)

POLYCYCLIC AROMATIC HYDROCARBONS (PAHS):

2.0 POLYCYCLIC AROMATIC HYDROCARBONS (PAHS):

The major PAH compounds as given in the Table : 1 are present in the most of pollution sources : area , point & mobile sources .

Table : 1 List of Priority PAHs

	Molecular Formula	Molecular Weight
Naphthalene	C₁₀H₈	128
Phenanthrene	C₁₄H₁₀	178
Anthracene	C₁₄H₁₀	178
Fluoranthene	C₁₆H₁₀	202
Pyrene	C₁₆H₁₀	202
Chrysene	C₁₈H₁₂	228
Benzo(a) anthracene	C₁₈H₁₂	228
Benzo(b) f luoranthene	C₂₀H₁₂	252
Benzo(k) f luoranthene	C₂₀H₁₂	252
Benzo(e) pyrene	C₂₀H₁₂	252
Benzo(a) pyrene	C₂₀H₁₂	252
Perylene	C₂₀H₁₂	252
Benzo(ghi) perylene	C₂₂H₁₂	276
Dibenzo(ah) anthracenes	C₂₂H₁₄	278
Indeno(cd) pyrene	C₂₂H₁₂	276
Coronene	C₂₄H₁₂	300

2.1 Sources of Polycyclic Aromatic Hydrocarbons (PAHs) :

The sources of atmospheric PAHs are both natural and anthropogenic. The natural sources are forest fire, volcanic activities and bacterial decay of organic materials. The anthropogenic sources may be divided into the following four categories.

· Industrial

Coke oven, aluminium production, iron and steel foundries, coal gasification and coke production are the main industrial sources of PAHs. Petroleum refining, thermal power plant, occupations where coaltars, pitch, asphalt (bitumen), shell-oil and creosotes are used, emit much of PAH to make air polluted. Exposures of coke oven workers to PAH and BaP in a case study is given in Table : 2

Table : 2 Exposures of Coke Oven Workers to PAH and BaP

	Total PAH (m g/m³)		Benzo (a)pyrene(m g/m³)
	Range	Arithmetic mean value	Range	Arithmetic mean value
Stationary sampling Battery top^a	270-2423	1000	16-69	37
Personal sampling Larry car operator^b Coke car operator^c Push car operator^d	168-1044 4.8-73 9-62	370 37 33	12-43 0.5-5.8 0.9-4.4	22 3.1 2.9

^aaverage of 25 samples, ^baverage of 7 samples , ^caverage of 3 samples , ^daverage of 4 samples

Source : A. Bjorseth, in Polynuclear Aromatic Hydrocarbons (P. W. Jones and P. Leber, Eds.), Ann Arbor Science Publishers, Ann Arbor, Michigan, pp. 371-381 ( 1979).

Size fraction of the airborne particulate matter has been conducted together with the concentration of PAH associated with each size fraction. The bulk of the PAH is in those size fractions that are of a respirable size : 98% and 72% of the particulate PAH is attached to particles below 7 µm and 3 µm, respectively. This finding explains the epidemiologically proven dangers of the tarry fumes that can be inhaled inside the coke oven plants. Levels of PAHs as high as few hundreds of microgram (10^-6g) of PAHs per cubic meter of coke oven emissions in the area of Dhanbad (Bihar, India) & few hundred nanogram (10^-9g) of PAH per cubic meter.in ambient air in their vicinity have been measured by Central Pollution Control Board .

Automobile

Motor vehicle emissions (especially diesel vehicles) make a considerable contribution to PAH concentration in air due to burning and incomplete combustion of diesel or gasoline. Air craft engine as a source of PAH in the atmosphere has also been identified.

Diesel Exhaust Particles and Their Health Effects

The popularity of the diesel engine in heavy duty applications in trucking, rail road, marine transport, DG sets and construction industry is due to both its fuel efficiency and long service life relative to the gasoline engine. Compared with gasoline engine, diesel emissions are lower in carbon monoxide (CO), hydrocarbon (HC) and carbon dioxide (CO₂), but higher in oxides of nitrogen (NOx) and particulate matter (PM). Diesel exhaust is a complex mixture of both particulate and gaseous phase.

Diesel exhaust has particulate with mass median diameter of 0.05 to 1.00 micrometer, a size rendering them easily respirable and capable of depositing in the airways and alveoli. The particles consist of a carbonaceous core with a large surface area to which various hydrocarbons are absorbed, including carcinogenic polycyclic aromatic hydrocarbons (PAHs) and Nitro-PAHs that have elicited the most concern with respect to human health. The gaseous phase contains various products of combustion and hydrocarbons including some of the PAHs present in the particle phase. Once emitted, components of diesel exhaust undergo atmospheric transformation in ways that may be relevant to human health. For example, nitro-PAHs, created by the reaction of directly emitted PAHs with hydroxyl radicals in the atmosphere can be more potent mutagens and carcinogens and more bio available than their precursors. A study undertaken by a Swedish Consultancy, Ecotraffic (Peter Ahlvik and Ake Branberg,1999) shows that the cancer potency of diesel vehicles is more than two times than that of petrol vehicles in India. If only the most harmful of the exhaust emissions, that is particulate emission is considered, the carcinogenic effect of one new diesel car is equivalent to 24 petrol cars and 84 new CNG cars on the road. The Honourable Supreme Court of India has restricted the use of commercial diesel driven vehicles in Delhi due to its harmful effects.

Domestic

Cooking (fuel burning) and waste refuse incineration.

(Household Women Cooking on Chulha Using Cow Dung & Wood as Fuel)

Human Habitats

Smoking cigarettes, cigar and tobacco.

Mobile sources are however likely to be the major PAH contributors in urban or suburban areas, where major stationary sources are not present.

Major Sources of PAHs in the Environment
» Combustion of fossil fuels	» Coal gasification and liquefaction process
» Automobile engine exhausts	» Creosote and other wood preservative wastes
» Atmospheric fallout of fly ash particulate	» Petrochemical industrial effluents
» Coal tar and other coal processing wastes	» Forest and prairie fires
» Aluminium Plants	» Rural and urban sewage sludge
» Refinery and oil storage wastes	» Municipal wastewater discharges run off
» Accidental spills from oil tankers and other ships	» Smoke, charcoal broiled, or pan fried foods
» Tobacco and cigarette smoke	» River borne pollution
» Refuse and waste incineration	» Commercial and pleasure boating activities

2.2 Formation of PAHs :

PAH can be formed mainly as a result of incomplete combustion (pyrolysis) or high temperature pyrolytic process during combustion of fossil fuels /organic materials, as well as in natural processes such as carbonisation ( pyrosynthesis) . Thus, PAHs are the constituents of the products of incomplete combustion (PIC). PAHs on reaction with other atmospheric pollutants viz., NOx, SO₂, O₂, etc. may form hetro-PAHs. The carcinogenecity and mutagencity of many of these hetero-PAHs compounds is greater than their parent compounds. Emission of Benzo (a) Pyrene (BaP) produced by 1 kg fuel of different kinds is given in Table-3. .

Table: 3 Emission of Benzo (a) Pyrene (BaP) Produced by 1 kg Fuel

Sources of BaP	BaP emission(mg/kg)	Consumption^a per year (tons)
Hard coal briquets (domestic heating)	5-380	1128000
Oil heating (light) (domestic heating)	0.0001	426000000
Passengers Cars	0.10	20400000
Cars with diesel engines	0.03	7650000
Power plants	0.005	268000000

^aIn the Federal Republic of Germany

Source : A. Bjorseth, in Polynuclear Aromatic Hydrocarbons (P. W. Jones and P. Leber, Eds.), Ann Arbor Science Publishers, Ann Arbor, Michigan, pp. 371-381 ( 1979).

2.3 Predominant PAH Source Profile/ Markers:

The following PAH have been identified as markers for various sources in urban atmospheres:

Coal combustion	:	Phenenthrene, fluorathene and pyrene;
Coke production	:	Anthracene, phenenthrene and benzo(a)pyrene;
Incineration	:	Pyrene, phenenthrene and fluoranthene;
Wood combustion	:	Benzo(a)pyrene and fluoranthene;
Industrial – oil burning	:	Fluoranthene pyrene and chrysene;
Petrol powered vehicles	:	Fluoranthene and pyrene with higher ratios of benzo(b)Fluoranthene and benzo(k)fluoranthene, plus thiophene compounds.
Diesel powered vehicles	:	Fluoranthene and pyrene with higher ratios of benzo(b)Fluoranthene and benzo(k)fluoranthene, plus thiophene compounds.

As may be noted from the markers listed above, there is much similarity and overlap between profiles from different source categories.

Table - 4 Source Distribution of the Percentage of PAHs to the Total mass of 20 PAHs

PAH*	Tunnel	Diesel engines	Gasoline engines	Coke oven	Wood combustion
2-ring	76	8.7	55	89	11
3-ring	16	56	18	8.9	69
4-ring	4.3	10	12	0.97	6.6
5-ring	3.1	18	13	0.22	13
6-ring	0.38	5.2	0.053	0.014	bd
7-ring	bd	0.18+	0.082	bd	bd

*2-ring: naphthalene; 3-ring: acenaphthylene, acenaphthene, fluorine, phenanthrene, anthracene and retene; 4-ring: fluoranthene , pyrene, benz(a)anthracene, chrysene and triphenylene; 5-ring: cyclopenta(c, d)pyrene, benzo(b, k)Fluoranthene, benzo(a, e)pyrene, di benzo(ghi)perylene; 6-ring: indeno(1,2,3,cd)pyrene and benzo(ghi)pyrene; 7-ring: coronene.+ One measurement ; bd: below the detection limit of this study.

Joop H. Van Wijnern et. al. determined the urinary 1-hydroxypyrene (1-HP) concentration and the cretinine-adjusted 1-HP concentration in 644 randomly selected Dutch children, aged 1-6 years and living in five areas with roughly different levels of polycyclic aromatic hydrocarbon (PAHs) in soil and ambient air. The mean urinary 1-HP content of the total study population was 2.06 nmol/l. This varied from 1.58 nmol/l in the reference area (Flevoland) to 2.71 nmol/l in the valley of the Geul. Only indoor sources of PAHs showed a small , positive association with urinary 1-HP. In one neighbourhood built on coal –mine tailings, the urinary 1-HP content in children was weakly but positively associated with the PAH content in the upper soil layer of the garden of their homes.

2.4 Nitro PAHs :

Both petrol and diesel fueled vehicles produce PAHs and nitro - PAHs. Diesel exhaust tend to contain higher concentration of carcinogenic nitro-PAHs and low toxicity 2, 3 and 4 ring PAHs. Petrol engine exhaust gases tend to produce higher concentrations of the 5 and 6 rings PAHs (Benzo(a)pyrene, Benzo (g, h, I) perylene, Indeno (1,2,3-cd) pyrene coronene and Anthanthrene) which are more carcinogenic than the 2 and 3 ring PAHs . Nitro-PAHs is either formed from combustion process or via nitro substituion of the parent PAH. In later case these are initiated by chemical reactions in the atmosphere with OH radicals in the presence of NOx and N₂O₅ and sunlight. Although the total concentration of nitro-PAHs emitted from diesel exhaust is lower than the total concentration of PAHs, their toxicity is much greater. Non-toxic primary PAHs such as naphthalene can undergo photochemical reactions to form highly toxic nitro-PAHs such 1-nitronaphthalene.

2.5 Vapour / Particle Phase Distribution of PAHs :

PAHs and nitro - PAHs may exist in vapour or particle phase. The lower molecular weight PAHs and nitro-PAHs with a ring number of 2 and 3 tend to be associated with the vapour phase. The larger molecular weight PAHs tend to be associated with particulate in the atmosphere. PAH species with a molecular weight below that of below pyrene exist to a large extent in the gas phase. On an average 47% of the total PAH were reported in gas phase. Three ring PAH are predominantly gaseous, five ring PAH mixture of both phases and five six ring PAH primarily particulate.

Pierce and Katz also studied size distribution of PAH containing particulates, which showed approximately a log-normal relationship for suburban and rural sampling sites with majority of PAH content (50-78%) associated with particles below 3.0 um diameter. Total PAHs were higher for winter period by a factor of 65-75% and concentration range vary between lowest 2 ug/g particulate (anthracene) to 20 m g/g particulate (BghiP). For all the sites, approximately 85-90% of the PAH content with respect to volume of air sampled was associated with particles less than 5.0 um diameter for the winter sampling period, while 70-85% was associated with the same size fraction for the summer sampling period (1972-1973) in Toronto, Canada.

PAH species with a molecular weight below that of below pyrene exist to a large extent in the gas phase (R.M. Hoff et.al., 1987, M.L. Lee et.al., 1976). According snook et al., 1976, on average 47% of the total PAH were reported in gas phase. Three ring PAH are predominantly gaseous, five ring PAH mixture of both phases and five, six ring PAH primarily particulate.

The cumulative fractions of total PAHs in different particle size ranges in the ambient air of traffic intersections in Southern Taiwan and at rural sites are presented in Table - 5 .

Table - 5 Particle Size Distribution of PAHs in Ambient Air in Taiwan

Particle Size	Fraction of Total PAH (Percent)
Particle Size	Traffic Intersections	Rural Sites
<1.0 µm	50.9%	38.3%
<2.5 µm	74.2%	56.4%
<10 µm	90.8%	85.75%
<35 µm	95.9%	90.4%

Source : Sheu H-L, Lee W-J, Tsai J-H, Fan Y-Ch, Su Ch-Ch, Chao R-R. Particle size
distribution of polycyclic aromatic hydrocarbon in the ambient air of a traffic intersection.
J. Environ Sci Health 1996; 31 : 1293-1316.

The results indicate that PAH exposure is widespread in Taiwan. Atmospheric concentrations of PAHs are strongly dependent upon the size of the carbon particulate matter, with the highest concentration being in the particle size range. About 95% of total PAH is associated with a size class less than 3 µm in diameter.

Distribution of PAH in the Particulate and gas phase from a stratified- charge engine is given in Table - 6 .

Table – 6: Distribution of PAH in the Particulate and gas Phase
from a Stratified- Charge Engine PAH (micrograms/sample)

PAH	Filter	XAD trap
Phenanthrene	40	16
Anthracene	8	30
Fluoranthene	34	30
Pyrene	36	40
Chrysene+Benzo(a)anthracene	70	50
Benzo(e) pyrene	28	0.1
Benzo(a) pyrene	9	0.1
Benzo(ghi) perylene	31	0.2
Picene+dibenzoanthracenes	9	0.2
Anthanthrenes	13	0.2
Dibenzopyrene	3	0.2
Coronene	4	0.2

Source : F.S.C.Lee, T.J. Prater, and F. Ferris, PAH emissions from a stratified charge vehicle with and without oxidation catalyst, in Polynuclear Aromatic Hydrocarbons, P.W. Jones and P.Leber (Eds.) , Ann Arbor Science Publs., Ann Arbor, Mich., 1979.

2.6 Photolysis, Half Life & Fate of PAHs in the Environment :

Submicron aerosol has a half life of about 5-30 days in the atmosphere thus PAH may be transported and deposited at other surface in very remote region at highly reduced concentration as a result of the effects of atmospheric dispersion and chemical reaction. PAH laden aerosol is transported from air to soil and water via physical processes involving impaction surfaces, gravitational settling and scavenging by rain and snow. Transfer rates are also highly sensitive to particle size. The physical removal or transport of airborne particles is a function of the particles size and meteorological conditions. The occurrence of some PAH in remote areas such as Arctic and Marine atmospheres (B.D. Veety Mc et.at. 1988) was mainly by aerial transport from distant anthropogenic sources.

A number of research workers have demonstrated that may PAH are susceptible to photo chemical and or chemical oxidation under simulated atmospheric conditions. There is however, potential for chemical transformation of PAH by gas-particle interactions in emission plumes, exhaust systems or even during atmospheric transport according NAS. Some research workers (Atkinson,1987) have studied oxy, nitro, hydroxy and hydroxy nitro PAH reaction products present in the gas phase as well as particulate.

Half lives of PAH under simulated atmosphere conditions (expressed in hours) are given in Table – 7 .

Table –7 Half Lives of PAH under Simulated Atmosphere Conditions
(Expressed in Hours)

PAHs	Simulated sunlight	Simulated sunlight+ozone (0.2ppm)	Dark reaction ozone (0.2ppm)
Anthracene	0.20	0.15	1.23
Benzo(a)Anthracene	4.20	1.35	2.88
Dibenzo (a,h) Anthracene	9.60	4.80	2.71
Dibenzo(a,c) Anthracene	9.20	4.60	3.82
Pyrene	4.20	2.75	15.72
Benzo(a)Pyrene	5.30	0.58	0.62
Benzo (e) Pyrene	21.10	5.38	7.60
Benzo(b)Fluroanthene	8.70	4.20	52.70
Benzo(k)Fluroanthene	14.10	3.90	34.90

Source: M. Katz, C. Chan, H. Tosine, and T. Sakuma, in Polynuclear Aromatic Hydrocarbons,
P. W. Jones and P. Leber (Eds.) , Ann Arbor Science Publs., Ann Arbor, Mich., 1979.

FATE OF PAHs IN THE ENVIRONMENT

» PAHs enter the air mostly as releases from burning coal, cokeoven plant,      automobile exhaust and wood fire.
» PAHs remains in air attached to dust particles.
» Most PAHs do not dissolve easily in water. They stick to solid particles and     settle to the bottom of lakes or rivers.
» Microorganisms break down PAHs in soil or water after a period of weeks to      months.
» In soil, PAHs are most likely to stick tightly to particles. Certain PAHs move     through soil to contaminate ground water.
» PAHs in remote areas such as arctic & marine atmosphere are result of long     range transport.

2.7 Mode of Exposure and Daily Intake of PAHs :

Human exposure to PAH can occur through several environmental pathways due to their numerous sources. However, the occurrence of PAH in urban air has caused particular concern because of the continuous nature of the exposure and the size of the population at risk . The urban atmosphere is a very complex and dynamic system containing a large variety of interacting chemical species in both the gas and particulate phases. PAHs compounds can reach to the human body by four different mode of exposure :

(a) Direct inhalation of polluted air

(b) Direct inhalation of tobacco

(c) Ingestion of contaminated and processed food and water

(d) Dermal contact

Air Inhalation -The intake of BaP by inhalation of polluted ambient air depends on the occurrence of PAHs in air. For example, an exposure to relatively high concentration of 50 ng BaP/M³ and a deposition rate of 40% from 20m³ air inhaled per day, the daily intake would be 400 ng of BaP.

Tobacco/Cigarette Smoking - Tobacco alone accounts for 30% of total morality due to cancer every year. More than 70 PAH compounds have been analyzed in cigarette smoke. Smokers have eight times more probability of cancer attack than non smoker.In developing countries approximately 30% smokers are young in the age group to 10-29 year. About 30-40% of them fall victim of premature death than expected life.The average total BaP content in the main stream smoke of 1 cigarette was 35 ng before 1960 and 18 ng in 1978-1979. Modern low tar cigarettes deliver 10 nag BaP. The concentration of BaP in a room extremely polluted with cigarette smoke was 22 ng/m3 (WHO 1587).

Drinking Water- Examination of number of drinking a range from 0.1 to 23.4 µg/l, while for other PAHs the concentration were between 0.001 to 0.01µg/l.

Food- American sources indicate an intake of total PAHs from food in order of 1.6-16 g per day. The contents of BaP in various processed food was repeatedly found to measure up to 50 µg/kg.

Menzie et al (1992) estimated that potential doses of carcinogenic PAHs by inhalation range between about 0.02 µg/day and 3 µg/day with median value of 0.16 µg/day. This nearly 20 times less than calculated food dose and about 25 times more than the potential dose with drinkable water.

Table – 8 Intake of Potential Carcinogenic PAHs by Non Smokers and Smokers

Source of PAHs	Intake
	Non Smoker		Smoker
	µg/day	Total %	µg/day	Total %
Food	3	93.0	3	44.6
Air	0.16	4.9	0.16	2.4
Water	0.006	0.2	0.006	<0.01
Soil (Accidental Injestion)	0.06	1.9	0.06	1.0
Cigarette	-	-	3.5	52
Total	3.22	100	6.72	100

Research shows that air contributes 3-20% of total human exposure to PAH and comes in second after food. In certain cases, where urban / industrial atmosphere contains high ambient concentration of PAHs, air is a predominant exposure route. Cigarette smoke can significantly contribute to potential PAH doses via inhalation (over 50% of total exposure). Smokers consuming one pack of non filtered cigarettes per day had an estimated additional intake of 1-5 µg/day.

The inhalation intake of total PAH and BaP during different seasons calculated on the basis of average inhalation of 15 m3 air /day . The inhalation intake of total PAH was as high as 5.8 ug/day in industrial area during winter. The inhalation intake of BaP ranged from 0.14 – 0.27 ug/day in industrial area, which is equivalent to smoking 7 to 14 cigarettes/day (Bridbord et. al., 1976), where as in residential area it ranged from 0.03 – 0.08 ug/day equating to maximum of 4 cigarettes/day. The yearly exposure levels of total PAH and BaP in the industrial area were higher by a factor of about 4 as compared to those in residential area. The levels of PAHs in the industrial area (141.8 ng/m3 for total PAH and 12.7 ng/m3 for BaP) of Headband were higher than those found in residential area i.e. 38.5 nag/m3 for total PAH and 3.4 ng/m3 for BaP in industrial town of Kokolla in Finland. The total PAH concentration in Mumbai city at Saki Naka was 38.8 nag/m3 and that at IIT was 24.5 ng/m3 with individual PAH species concentration ranging from 1-13 nag/m3. The total PAH fraction as percentage of particulate (i.e. associated with respirable particulates) under 10 um aerodynamic diameter) at Saki Naka was 2.5 times than observed at IIT site. At the IIT site, primarily vehicular emission alone with cooking fuel emission are likely contributors while industrial oil burning is additional contributor at Saki Naka accounting for higher concentration of Pyrene, Benzo(a)anthracene, Chrysene.

ROUTE OF EXPOSURE OF PAHS

Breathing air containing PAHs in the workplace of coking coal, tar and asphalt production plants, smoke houses and municipal incinerators.

Breathing air containing PAHs from cigarette smoke, wood smoke, vehicle exhaust and asphalt

Coming in contact with air, water or soil , near hazardous waste site.

Eating contaminated cereals, flour,. Bread, vegetables, fruits, meats.

Drinking contaminated water or milk.

Nursing infants of mothers living near hazardous waste sites may be exposed to PAHs through their mother's milk.

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