Sources, Distribution and Toxicity of Polyaromatic Hydrocarbons (PAHs) in Particulate Matter

Polycyclic aromatic hydrocarbons (PAHs) are a group of organic compounds consisting of two or more fused aromatic rings.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.PAHs can be present in both particulate and gaseous phases, depending upon their volatility.Light molecular weight PAHs (LMW PAHs) that have two or three aromatic rings are emitted in the gaseous phase, while high molecular weight PAHs (HMW PAHs), with five or more rings, are emitted in the particulate phase.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.Human beings are exposed to PAH mixtures in gaseous or particulate phases in ambient air.Long-term exposure to high concentrations of PAHs is associated with adverse health problems.Since some PAHs are considered carcinogens, inhalation of PAHs in particulates is a potentially serious health risk linked to an excess risk of lung cancer.Thus, studies on PAHs in particulate matter (PM), such as PM10 and PM2.5 in ambient air, have become attention greater focus of research in recent years. Physical and chemical characteristics of PAHsPAHs are a group of several hundred individual organic compounds which contain two or more aromatics rings and generally occur as complex mixtures rather than single compounds.PAHs are classified by their melting and boiling point, vapor pressure, and water solubility, depending on their structure.Table 1 shows physical and chemical characteristics of 16 priority PAHs, listed by the US EPA.Most PAHs, especially as molecular weight increases, are soluble in non-polar organic solvents and are barely soluble in polar water.Most PAHs are persistent organic pollutants (POPs) in the environment.Many of them are chemically inert.However, PAHs can be photochemically decomposed under strong 5 www.intechopen.com Air Pollution 100ultraviolet 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. Sources and Emission of PAHsPAHs are mainly derived from anthropogenic activities related to pyrolysis and incomplete combustion of organic matter.Emission sources of PAHs affect their characterization and distribution, as well as their toxicity.In this book, the major sources of PAH emissions may be divided into four classes: stationary sources (including domestic and industrial sources), mobile emissions, agriculture activities, and natural sources. Stationary sources Domestic sourcesHeating and cooking are dominant domestic sources of PAHs.The burning and pyrolysis of coal, oil, gas, garbage, wood, or other organic substances are the main domestic sources.Domestic sources are important contributors to the total emissions of PAHs in the environment.Differences in climate patterns and domestic heating systems produce large geographic variations in domestic emissions.PAH emissions from these sources may be a major health concern because of their prevalence in indoor environments (Ravindra et al., 2006).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 fuels, such as wood, for daily cooking.Main indoor PAH sources are cooking and heating emissions and infiltration from outdoors.PAHs 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 in residential non-smoking air.Toxicity of PAH mixtures from indoor sources is lower than mixtures which contain large amounts of HMW PAHs.Cigarette smoke is also a dominant source 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.www.intechopen.com