ReviewA review of airborne polycyclic aromatic hydrocarbons (PAHs) and their human health effects
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are a large group of chemicals with 2 to 7 fused aromatic rings (Arey and Atkinson, 2003, Di-Toro et al., 2000). Some PAHs are well known as carcinogens, mutagens, and teratogens and therefore pose a serious threat to the health and the well-being of humans (Boström et al., 2002). The physico-chemical properties of PAHs makes them highly mobile in the environment, allowing them to distribute across air, soil, and water bodies where their presence is ubiquitous (Baklanov et al., 2007, Latimer and Zheng, 2003, Sverdrup et al., 2002).
Polycyclic aromatic hydrocarbons are released into the environment from both natural and anthropogenic sources (WHO, 2003). The widespread occurrence of PAHs is due to their production by virtually all types of combustion of organic materials. The anthropogenic sources of PAHs and their derivatives are diverse and include: incomplete burning of fuels, garbage, or other organic substances such as tobacco and plant material. (Zhang and Tao, 2009). Likewise, forest fires and volcanic eruptions can also contribute to the natural budget of the PAH inventory (Zhang and Tao, 2009).
PAHs are widely distributed in the atmosphere and were one of the first atmospheric pollutants designated as a suspected carcinogen. The PAHs entering the atmosphere can be transported over long distances before deposition through atmospheric precipitation onto soils, vegetation or waters (Ravindra et al., 2008). As molecular weight increases, the carcinogenicity of PAHs also increases with reducing acute toxicity. The most potent PAH carcinogens have been identified to include benzo[a]anthracene, benzo[a]pyrene, and dibenz[ah]anthracene (Armstrong et al., 2004, Bach et al., 2003, CCME (Canadian Council of Ministers of the Environment), 2010). Given the ubiquitous presence of PAHs in the environment and the health risk associated with their exposure, the aim of this paper is to review contemporary information on the properties, fate, and risk associated with the presence of these compounds in the atmosphere.
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Chemical characteristics of PAHs
PAHs are organic substances made up of carbon and hydrogen atoms grouped into at least two condensed or fused aromatic ring structures (CCME, 2010). They can be divided into two categories: low molecular weight compounds consisting of fewer than four rings and high molecular weight compounds of four or more rings. Pure PAHs are usually colored, crystalline solids at ambient temperature (Masih et al., 2012). The physical properties of PAHs vary with their molecular weight and structure. The
Fate and transformations of PAHs in atmosphere
The behavior of PAHs in the atmosphere depends on complex physico-chemical reactions, interactions with other pollutants, photochemical transformations, and dry and wet deposition (Delgado Saborit et al., 2010, Zhong and Zhu, 2013, Zhu et al., 2009). PAHs in the ambient air exist in vapor phase or adsorb into airborne particulate matter depending on the atmospheric conditions (ambient temperature, relative humidity, etc.), the nature (i.e., origin and properties) of the aerosol, and the
Atmosphere
The presence of PAHs in the environment is due primarily to emissions from incomplete combustion of carbon containing fuels from natural, industrial, commercial, vehicular and residential sources. Although low molecular weight PAHs are found more commonly as vapor in the troposphere, they can also exist in the particulate phase through condensation after emission. In contrast, the high molecular weight PAHs are dominantly found in the particulate phase.
Detailed estimates of PAH emissions in the
Human exposure to polycyclic aromatic hydrocarbons
The major route of exposure to PAHs in the general population is from breathing ambient (and indoor) air, eating food containing PAHs, smoking cigarettes, or breathing smoke from open fireplaces (ACGIH, 2005). A variety of PAHs from tobacco smoke are suspected human carcinogens (Lannerö et al., 2008). For non-smokers, the main route of exposure is through food. Processing (such as drying and smoking) and cooking of foods at high temperatures (grilling, roasting, and frying) are major sources of
Acute or short-term health effects
The acute effects of PAHs on human health will depend mainly on the extent of exposure (e.g., length of time), the concentration of PAHs during exposure, the toxicity of the PAHs, and the route of exposure, e.g., via inhalation, ingestion, or skin contact (ACGIH, 2005). Many other factors may also affect health impacts. These include factors such as pre-existing health conditions and age. Short-term exposure to PAHs also has been reported to cause impaired lung function in asthmatics and
Chronic or long-term health effects
For workers exposed to mixtures of PAHs and other work place chemicals, a series of health problems (an increased risk of skin, lung, bladder, and gastrointestinal cancers) have been reported (Bach et al., 2003, Boffetta et al., 1997, Diggs et al., 2011, Olsson et al., 2010). Long-term exposure to low levels of some PAHs (e.g., pyrene and BaP) has been identified as the cause of cancer in laboratory animals (Diggs et al., 2012). Animal studies have also shown adverse reproductive and
Biomarkers of exposure to PAH
Several methods have been developed to assess internal levels of PAHs after exposure from the environment and workplaces. In many studies, 1-Hydroxypyrene, a metabolite of pyrene, has been widely used as urinary biomarker of PAH exposure (Jongeneelen, 2001, Mcclean et al., 2004, McClean et al., 2012, Sobus et al., 2009, Stroomberg et al., 2003). Most importantly, pyrene is present in all PAH mixtures at relatively high concentrations (2–10% of the total PAH load). In certain environments, the
Regulation
PAH compounds are typically constituents of complex mixtures. Some PAHs are potent carcinogens, which may interact with a number of other compounds. U.S. government agencies have established standards that are relevant to PAH exposures in the workplace and the environment. There is a standard relating to PAH in the workplace as well as in drinking water. The National Institute for Occupational Safety and Health (NIOSH) has recommended that the workplace exposure limit for PAHs should be set at
Conclusion
To learn more about the significance of PAHs in the environment, enormous efforts have been devoted to quantifying the level of emissions, assessing ambient concentrations, characterizing speciation, and determining temporal/spatial trends. Currently, there is broad agreement on the main emission sources but harmonization of emission estimation and reporting is still at an early stage of development.
Data from a number of occupational health studies suggest that there is an association between
Acknowledgment
This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No. 2009-0093848).
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