IISection
6.2 ConstItuents of polluted aIR
6.2.1 particulate matter: tHe major toxic componentof urBan air
Particulate matter (PM) is a complex mixture of suspended solid and liquid particles (Brook et al., 2003) classified into primary (particles emitted directly by emission sources) and secondary (particles formed through the atmospheric reaction of gases) particles. The PMs vary greatly in size, composition, concentration, depending on
origin and age. The size distributions are important for health impacts. Particles larger than 10 µm in diameter are deposited almost exclusively in the nose and throat, whereas those smaller than 1 µm reach the lower regions of the lung. The intermedi- ate size range gets deposited between these two extremes of the respiratory tract.
Outdoor (ambient) PM size ranges from approximately 0.001 to 100 µm in aerody- namic diameter. There are three main size categories for PM measured in urban air.
6.2.2 coarSe particleS (pm10)
The particles in this category are larger than 1 µm and extend up to 100 µm. However, for toxicity studies, the most important particles are those that have a diameter of less than10 µm (PM10) because they are respirable whereas the larger particles are not.
PM10 deposit relatively quickly with a lifetime of less than 2 days, and exposure may lead to adverse responses in the lungs triggering an array of problems in the lungs and heart (Brunekreef and Forsberg, 2005; Harrabi et al., 2006). PM10 has also been associated with emergency hospital admission for asthma, bronchitis, and pneumo- nia in older people (Ye et al., 2001). For every 10 µg/m3 increase of PM10, mortality from all causes increases by 0.51% (Samet et al., 2000).
6.2.3 accumulation modeor fine particleS (pm2.5)
They consist of PM with a diameter between 0.1 and 2.5 µm. Airborne particles smaller than 2.5 µm (PM2.5) are usually called fine particles. They account for the majority of the mass of suspended particles and deposit slowly leading to a long atmospheric lifetime of 5–10 days. These particles may penetrate deep inside the airways and are more strongly linked with adverse health effects (USEPA, 1996).
Fine particles are composed mainly of carbonaceous materials (organic and ele- mental), inorganic compounds (sulfate, nitrate, and ammonium), and trace metal compounds (iron, aluminum, nickel, copper, zinc, and lead). There are potentially thousands of different compounds adsorbed on fine particles that may exert harm- ful biological effects. A modest rise in PM2.5 level has been shown to be associated with changes in cardiac function (Mar et al., 2005). The toxicity of fine parti- cles is mediated by generation of oxidative stress and production of free radicals (Furuyama et al., 2006).
6.2.4 nuclei modeor ultrafine particleS
The particles in this category are smaller than 0.1 µm. They are also known as ultra- fine particles (UFPs). They do not last long in the air since they deposit or rapidly form fine parti cles by coagulation. UFPs are present in great numbers in polluted urban air (Jaques and Kim, 2000). They have a carbonaceous core with attached inorganic and organic materials that can cause adverse health effects (Oberdorster, 2000). The UFPs have less mass than coarse particle fractions but they are much greater in number and have a relatively large surface area-to-mass ratio, making them potential carriers of harmful gaseous compounds. UFPs escape alveolar mac- rophage (AM) surveillance in the lungs, which is otherwise very efficient for larger
particles (Hahn et al., 1977). Exposure to high doses of UFPs can cause severe pulmonary inflammation and hemorrhage, high degree of alveolar and interstitial edema, disruption of epithelial and endothelial cell layers, and death (Oberdorster et al., 1992; Peters et al., 1997; Oberdorster, 2000). Even modest exposure to UFPs from vehicular exhausts can lead to cardiovascular problems, pulmonary diseases, and cancer (Vinzents et al., 2005).
6.2.5 So2and nox
Coal burning is a major source of sulfur dioxide (SO2) in air. It is an acidic gas, which combines with water vapor in the atmosphere to produce acid rain. SO2 in ambient air can affect human health (Routledge et al., 2006), particularly in those suffering from asthma and chronic lung diseases and exacerbates respiratory symptoms and impaired breathing in sensitive individuals (Lipfert, 1994). Nitrogen oxides (NOx) are formed during combustion processes at high temperatures from the oxidation of nitrogen in air or fuel. The major types of oxides of nitrogen are nitric oxide (NO) and nitrogen dioxide (NO2). The main source of NO is road traffic, emitted from both petrol and diesel engine motor vehicles. NOx is a precursor of ozone formed in the troposphere. Oxides of nitrogen are immunotoxic and increase the susceptibility to respiratory tract infection. Continued or frequent exposures to high concentra- tions of NOx in breathing air may cause irritation of the lungs and consequent acute respiratory illness (Hasselblad et al., 1992).
6.2.6 carBon monoxide
CO is a toxic gas emitted into the atmosphere as a result of combustion processes and by oxidation of hydrocarbons and other organic compounds. CO is produced almost entirely from road traffic and remains in the atmosphere for approximately one month before being oxidized to CO2. The largest contributors of CO are petrol- fueled vehicles. CO binds strongly to hemoglobin in red blood corpuscles resulting in the production of carboxyhemoglobin. This impairs the transport of oxygen within the blood and can result in adverse effects on tissues with high oxygen needs such as the cardiovascular and nervous systems. A recent study has shown that chronic exposures to CO may cause adverse birth outcomes such as reduced birth weight and intrauterine growth retardation (Salam et al., 2005).
6.2.7 polycyclic aromatic HydrocarBonS
About 200 different kinds of hydrocarbons are emitted from the combustion of petrol and diesel. Of these, the polycyclic aromatic hydrocarbons (PAHs) are of par- ticular interest due to their carcinogenic (cancer-causing) potential. PAHs are usually adsorbed on the PM and enter the body through inhalation. These compounds are semivolatile in nature. Several PAHs like benzo(a)pyrene [B(a)P] are highly carcino- genic (Hrudkova et al., 2004). Incidence of lung cancer has been reported in persons directly exposed to B(a)P from automobile exhausts and biomass fuel burning during household cooking).
6.2.8 Volatile organic compoundS
Volatile organic compounds (VOCs) consist of various classes of carbon-containing chemicals that are gaseous at room temperature. They are released into the environ- ment from petrol and diesel, especially the former, by evaporation or as combustion products. Some VOCs (e.g., benzene) are hematotoxic, neurotoxic, leukemogenic, and carcinogenic (Wallace, 1984, 1989; Farris et al., 1993).
6.2.9 air toxicSof Biological origin
Biological agents present in polluted air may cause several diseases. There are many sources of these pollutants. Pollens originate from plants; people and animals trans- mit viruses; bacteria are carried by man, animal, soil, and plant debris; and house- hold pets are sources of saliva and animal dander. The protein in urine from rats and mice is a potent allergen. When it dries, it can become airborne. Along with particu- late pollution, numerous airborne bacteria enter the body during respiration. Several of these are pathogenic to humans. For example, Mycobacterium tuberculosis causes tuberculosis while Streptococcus pneumoniae causes bacterial pneumonia. Globally, pneumonia causes 2 million deaths in children (20% of the total number of child deaths) every year and 70% of them occur in Africa and Southeast Asia.
Like bacteria, viral infections have been linked to air pollution. Mumps virus (mumps), Myxovirus influenza (influenza), Poliovirus (poliomyelitis), Rhinovirus causing common cold, Rubella virus (measles), Varicella virus (chicken pox), Variola pox virus causing small pox, Haemophilus influenzae, respiratory syncytial virus (RSV), influenza, parainfluenza, and adenoviruses are some of the viruses that spread through polluted air.
Increased presence of fungi in indoor environment due to poor housing con- ditions, older house age, relative lack of sun exposure, and absence of insulation enhances the risk of respiratory illnesses including oral toxicosis and airway allergy (Howden-Chapman et al., 2005). The problems may turn serious requiring hospital- ization (Khalili et al., 2005). Airborne pollens and other allergens are major caus- ative agents of bronchial hypersensitivity and asthma. Asthma exacerbation is the most common cause of hospital admission in children. Airborne bacteria and virus infections in allergic asthmatics further increase the risk of hospitalization (Murray et al., 2006). Pollen exposure is usually associated with respiratory tract allergy and eosinophil accumulation in the nasopharynx and the airways (Onbasi et al., 2005).