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Vol. 20 No. 1 - January 2014

Lichens as Sentinels of Atmospheric Polycyclic Aromatic Hydrocarbons (PAHs) in India

By: Vertika Shukla and D.K. Upreti*

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds with two or more fused aromatic rings. PAHs are widespread environmental contaminants resulting from incomplete combustion of organic materials. Anthropogenic activities such as fossil fuel-burning, motor vehicle, waste incinerator and oil refining are the major sources of PAHs in environment (Table 1). PAHs are hydrophobic having a relatively low solubility in water, but are highly lipophilic. PAHs can undergo photodecomposition when exposed to ultraviolet light from solar radiation. In the atmosphere, PAHs may react with pollutants such as ozone, nitrogen oxides and sulfur dioxide, yielding secondary compounds like diones, nitro- and dinitro- PAHs, and sulfonic acids, respectively.

Among different derivatives, nitrated polycyclic aromatic hydrocarbons (N-PAHs) are an important category of derivatives of PAHs. N-PAHs have been recognized as direct-acting mutagens and carcinogens to mammalian systems. Thus the N-PAHs are considered to have far greater toxicity than non substituted PAHs. Nitro-PAHs are formed mainly from incomplete combustion processes or by the reaction of PAH with atmospheric oxidants, such as dinitrogen pentoxide, nitrogen trioxide, and oxygen radicals in the presence of nitrogen oxides. Nitro-PAHs occur as a mixture with parent PAHs in the vapor phase or adsorbed onto particulate matter in the atmosphere. Two-ring N-PAHs, such as nitro-naphthalene, are the dominant N-PAHs in the vapor phase. However, N-PAHs, which include nitro derivatives of pyrene, fluoranthene, anthracene, chrysene, and others, tend to condense on particle surfaces because of their low vapor pressure. Atmospheric lifetimes of N-PAHs are affected by photolysis and gas-phase reactions with hydroxyl and nitrate radicals and with ozone under atmospheric conditions.

In urban areas, N-PAH pollution is predominantly caused by diesel engine, vehicle traffic and residential heating. Indoor human exposure to nitro-PAHs is from kerosene heating and use of cooking oil. Effects of N-PAHs on human health have been estimated based on the data of carcinogenic effects for 28 N-PAHs.

PAHs have received increased attention in recent years in air pollution studies because some of these compounds are highly carcinogenic or mutagenic. The Stockholm Convention on Persistent Organic Pollutants (POPs) which was adopted in 2001 and revised in 2009 was called in response to the urgent need for global action to protect human health and the environment from chemicals that are highly toxic, persistent, bio accumulate and move long distance in the environment. The Convention seeks the elimination or restriction of production and use of all unintentionally produced PAHs. In absence of strict EIA norm developing countries are more prone towards emission and direct environmental exposure to these hazardous compounds. Thus the exposure assessments of PAHs in the developing world are important.

Table 1. Predominant PAH as markers of its anthropogenic source

Health risk associated with PAHs

Air with high concentrations of PAHs causes many adverse effects on different types of organisms, including plants, birds, and mammals. Some studies reported that there is a significant positive correlation between mortality by lung cancer in humans and exposure to PAHs from exhaust from coke ovens, roofing-tar, and cigarette smoke. Some PAHs have been demonstrated to be carcinogenic in humans and experimental animals, and they are classified as carcinogenic materials by many environmental protection agencies as “priority pollutants”. Eight PAHs (Car-PAHs) typically considered as possible carcinogens are: benzo(a)anthracene, chrysene, benzo(b) fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene (B(a)P), dibenzo(a,h)anthracene, indeno(1,2,3-cd)pyrene and benzo(g,h,i)perylene. In particular, benzo(a)pyrene has been identified as being highly carcinogenic. Low molecular weight PAHs, except naphthalene, usually are associated with relatively lower toxicity (cancer risk) than High molecular weight PAHs with 5 or 6 aromatic rings. Many toxicity studies reported that benzo[a]pyrene (BaP) has the highest carcinogenic potency with long-term persistency in the environment. Significant increase in all lung tumors and a dose-dependent increase in malignant lung tumors for mice exposed to PAH-enriched exhausts containing 0.05 or 0.09 mg/m3 BaP has been observed. BaP is often used as an indicator of human exposure to PAHs, and the toxicity of other PAHs is converted into toxicity equivalency factors (TEFs) to BaP to evaluate their relative toxicities. Methods using TEFs and the BaP as a surrogate are more or less similar to each other, except for not requiring expensive monitoring

Health Risk Estimation

Toxicity equivalency factors (TEFs) evaluation is the most popular method used to identify the toxicity of PAHs. TEFs of individual PAHs have been reported by many researchers. Toxicity equivalency concentrations (TEQs) are calculated as the product of summing up the values obtained by TEF values and concentrations of PAHs, as follows:

TEQ = Σ (C_i x TEF_i)

Where, TEQ: toxic equivalent concentration; C_i: concentration of PAH_i.

TEFs has been successfully employed in assessing occupational and environmental health risks associated with exposure to airborne mixtures of PAHs. Information of the ratio between airborne concentrations of BaP equivalents to the concentrations of BaP alone, can indicate the variation of risk for the different environments. 1.73% of the cancer sufferers of Beijing inhabitants in 2007 were found to be related to inhalation of PAHs in ambient air. There is an increasing trend of the cancer risk of residents by inhalation of ambient air containing hazardous air pollutants (HAPs), such as PAHs.

The BaP is the highest carcinogenic contributor, followed by DahA, Ind and BbF. However, DahA was suggested as a new surrogate compound to measure the toxicity of particle phase-PAHs because its toxicity is almost equal to that of BaP. While estimating the toxicity of PAHs in road dust of Ulsan, Korea, significant correlation coefficient was found between TEQ and total PAH concentrations.

Another index for calculation of PAHs associated health risk is BaPE index. B(a)P, the classical chemical carcinogen, is considered to be the useful indicator for cancer risk assessment. According to World Health Organization (WHO), B(a)P is considered to be reliable index for assessment of total PAHs carcinogenicity. Since B(a)P is easily oxidized and photodegraded therefore PAHs carcinogenic character could be underestimated. For better quantification of carcinogenicity related to whole PAH factor, BaP-equivalent potency (BaPE) index after Yassaa et al., is calculated. High BaPE index indicates high cancer risk is associated with high vehicular activity. In a study carried out in Mahabaleshwar (India) it was observed that BaPE was higher at sites with heavy traffic load. Thus urban population appears to be exposed to significantly higher cancer risk. Distribution and uptake by plants PAHs are semi volatile organic compounds (SOCs) in nature that are found in air, soil, vegetation, water and ice, i.e., multi-compartmental substances. PAHs are mostly distributed in the source regions but reach the Arctic and the Antarctic. The gas/particle partitioning in air influences the atmospheric cycling and the total environmental fate (compartmental distributions). Re-volatilization is significant phenomenon for semi volatile PAHs for its long range transport (LRT) potential.

Partition between the vapor and particulate phases, affects the deposition, degradation, transportation, and subsequent fate of these environmentally significant constituents. Distribution between the vapor and particulate phases can also be important to plant exposure to these potentially harmful chemicals.

The amount of uptake of PAHs by plants varies significantly and depends on many factors, including plant species, initial soil concentrations and microbial population. Several studies have demonstrated that vegetables grown in soil contaminated with PAHs may uptake PAHs. Laboratory experiments of PAH uptake in plants grown in spiked soil, or directly in contaminated water, also show that uptake occurs. Several mechanisms may be responsible for the transfer of organic contaminants from soil to plant tissue, including uptake in the transpiration stream, volatilization and subsequent re-deposition on leaves, and sorption from direct contact with soil particles. Atmospheric deposition has been identified as a predominant pathway of PAHs uptake in many studies including lichens.

Perspective of lichen biomonitoring of PAHs

As PAHs pose significant potential health and environmental risks at varying spatial scales, ranging from localized to global scales. Thus, there is increasing interest in monitoring the levels and distribution of pollutants. Such monitoring programs implicitly involve sampling ambient air, but this is often hampered by problems associated with sampling air in remote areas. A potentially useful approach is to use biomonitors such as lichens, which concentrate a variety of pollutants in their tissues.

In particular, lichens function as an efficient bioindicator and bioaccumulator organisms, because of several factors, as they are perennial, maintain uniform morphology over time, grow slowly, and are dependent on atmospheric deposition for their mineral nutrition. Lichens have no roots and adsorb water and nutrients directly from the air; consequently, they may co-adsorb/absorb other substances from the atmosphere, including pollutants. Thus, lichen data can be used to identify areas that may require more intensive or quantitative monitoring using devices.

Biomonitoring studies utilizing lichens carried out globally show that lichens can accumulate PAHs of all sizes. Lichens accumulate substances in the gas phase more easily, but can also accumulate compounds bound to particulate matter. The highest concentrations were observed for three-ring and four-ring PAHs molecules, whereas concentrations of five- and six-ring chemicals were the lowest.

In India PAHs accumulation studies (Table 2) with lichens have been recently initiated in the Himalayan region of Uttrakhand. The first baseline data on the distribution and origin of polycyclic aromatic hydrocarbons (PAHs) in Garhwal Himalayan region has been prepared which exhibit the presence of all 16 USEPA PAHs in the region. Significantly higher concentration of phenanthrene, pyrene and acenapthalene indicates road traffic as major source of PAH pollution in the area. The probable mechanism of bioaccumulation may be attributed to the donor-acceptor complex has been reported to be formed between polycyclic aromatic hydrocarbons (carcinogenic and noncarcinogenic), and compounds of biological importance.

Table 2. Total PAHs concentration in various lichen species sampled from different regions of India.

Apart from quantification of PAHs, lichen biomonitoring has been successfully employed to distinguish industrial PAHs from urban PAHs, which reveal that 5 and 6 ringed PAHs are predominant in industrial emissions while lower molecular weight PAH are of urban origin (vehicular activity, cooking and coal combustion etc.). Spatial distribution of PAH profile reveals that fluoranthene (4-ringed PAH) has highest spatial continuity, this has been established by modeling studies which has been further affirmed by lichen biomonitoring studies carried out in high altitude Himalayan ecosystem of Uttarakhand.

In India PAHs profile in lichens considerably varies from site to site. Diagnostic molecular ratio has been applied to the biomonitoring data and the results were found to be in conformity with the pollution source, dominant mode of transport. As in Hardwar city, commercial and tourist activity encourages more and more diesel driven vehicles, has been affirmed by diagnostic ratios at industrial and city center, an important holy pilgrimage, having combustion being predominant source.

Growth form of lichens may also play a significant role in the accumulation of PAHs. The crustose and squamulose species growing (thallus is small) on rocks mostly accumulate uniform concentration of low molecular weight 2 and 3 ringed compounds, as these PAHs are prominently in gas phase while foliose lichens are considered better accumulator and indicator profile of PAHs as the larger thallus allows adsorption as well as absorption of particulate bound and gas phased PAHs respectively in the lichen thalli. Ultra structural study of lichen Pyxine subcinerea shows that thallus surface is not uniform but has hexagonal compartments which act as sink for particulate matter to get adsorbed and remain adhered to the thallus surface. Studies carried out till now in India, establishes the utility of member of the Physciaceae family (Pyxine subcinerea and Phaeophyscia hispidula) as an excellent biomonitoring organism in monitoring of PAHs from foot hill to sub-temperate area of the country and may be effectively utilized in the other part of the country with transplant studies.

Conclusion

As PAHs and its derivative pose health risk, therefore for regulation and monitoring, of its emission and dispersal, is required to be carried out regularly and at wider spatial scale. As lichen species absorb airborne contaminants and accumulated them at rates that are generally proportional to pollution load. Therefore lichen biomonitoring data may be well utilized in regulatory management practices. Lichen biomonitoring can supplement the air sampling studies for detection and quantification of PAH pollution especially in remote areas where installation of heavy equipments is not possible. Environmental protection and ensuring healthy environment is an integrated effort which needs to be enforced, implemented and realized for future generations.

*Lichenology Laboratory, CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow 226 001, India e-mail [email protected]

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.