I organic air pollutants I 1 Volatile Organic Compounds (vocs)

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Chapter I



Maria José SANZ, José Vicente CHORDÁ

Air pollution started when tribesmen learned to use fire, and filled the air inside their living quarters with the products of incomplete combustion. But the biggest step forward was the Industrial Revolution, thus the predominant air pollution in the 19th century was smoke and ash from the burning of coal or oil in the boiler furnaces of stationary power plants, locomotives, vessels, home heating fireplaces and furnaces. During the 20th century, after recognition of the problem by countries like Great Britain and the United States, there were great changes in the technology of both the production of air pollution and its control, but not significant changes in the limiting legislation, regulations and understanding of the problem. But, as cities and industry grew in size, the problem increased in severity. Since the 30s the other air pollution problems as well as solutions up to some extent emerged. In the following sections are described the main air pollutants and problems identified today. Due to their different nature, the pollutants are separated in organic and inorganic compounds.


I.1.1.1. Volatile Organic Compounds (VOCs)

Volatile Organic Compounds are substances that contain carbon that evaporates easily. VOCs can be from natural or anthropogenic sources. Anthropogenic VOCs are present in exhaust fumes, cigarette smoke, synthetic materials and household chemicals, and include benzene, formaldehyde and polynuclear aromatic hydrocarbons (PAH).

VOCs are involved in the formation of ground level ozone and in the depletion of the ozone layer. They also contribute to the greenhouse effect, photochemical oxidants produced from the use of VOCs are greenhouse gases. Only the most important VOCs are included below.

Aromatic hydrocarbons.

The most abundant aromatic hydrocarbons in urban atmospheres are benzene, toluene and xylenes, and trimethylbenzenes.

Benzene; the chemical formula for benzene is C6H6, and it has a molecular weight of 78.11 g/mol. (US EPA, 2002). Benzene occurs as a volatile, colorless, highly flammable liquid, it has a sweet odor with an odor threshold of 1.5 ppm (5 mg/m3).

It is a VOC that is a minor constituent of petrol. The main sources of benzene in the atmosphere in Europe are the distribution and combustion of petrol. Of these, combustion by petrol vehicles is the single biggest source (70% of total emissions).

Benzene is present in petrol but not diesel. Hydrocarbons including benzene are emitted during refueling, by evaporating from fuel tanks and exhausts and as unburnt hydrocarbons in exhausts. Benzene is also present in cigarette smoke and in some glues and cleaning products. Acute (short-term) inhalation exposure of humans to benzene may cause drowsiness, dizziness, headaches, as well as eye, skin, and respiratory tract irritation, and, at high levels, unconsciousness.  Chronic (long-term) inhalation exposure has caused various disorders in the blood, including reduced numbers of red blood cells and aplastic anemia, in occupational settings.  Reproductive effects have been reported for women exposed by inhalation to high levels, and adverse effects on the developing fetus have been observed in animal tests.  Increased incidences of leukemia (cancer of the tissues that form white blood cells) have been observed in humans occupationally exposed to benzene.

Xylene, m-, o-, and p-Xylene are the three isomers of xylene; commercial or mixed xylene usually contains about 40-65% m-xylene and up to 20% each of o- and p-xylene and ethylbenzene. Mixed xylenes are colorless liquids that are practically insoluble in water and have a sweet odor. The odor threshold for m-xylene is 1.1 ppm. The chemical formula for mixed xylenes is C8H10 (http://www.epa.gov/ttn/atw/hlthef/).

Xylenes are released into the atmosphere as fugitive emissions from industrial sources, from auto exhaust, and through volatilization from their use as solvents.  Acute (short-term) inhalation exposure to mixed xylenes in humans results in irritation of the eyes, nose, and throat, gastrointestinal effects, eye irritation, and neurological effects.  Chronic (long-term) inhalation exposure of humans to mixed xylenes results primarily in central nervous system (CNS) effects, such as headache, dizziness, fatigue, tremors, and incoordination; respiratory, cardiovascular, and kidney effects have also been reported. 

Toluene, the chemical formula is C6H5CH3, and its molecular weight is 92.15 g/mol. Toluene occurs as a colorless, flammable, refractive liquid, that is slightly soluble in water. Toluene has a sweet, pungent odor, with an odor threshold of 2.9 parts per million (ppm).

Toluene’s major use is to be added to gasoline to improve octane ratings, it is also used to produce benzene and as a solvent in paints, coatings, adhesives, inks, and cleaning agents. It has a minor importance in the production of polymers used to make nylon, plastic soda bottles, polyurethanes and for pharmaceuticals, dyes, cosmetic nail products, and the synthesis of organic chemicals.

Exposure to toluene may occur from breathing ambient or indoor air.  The central nervous system (CNS) is the primary target organ for toluene toxicity in both humans and animals for acute (short-term) and chronic (long-term) exposures. CNS dysfunction and narcosis have been frequently observed in humans acutely exposed to toluene by inhalation; symptoms include fatigue, sleepiness, headaches, and nausea.  CNS depression has been reported to occur in chronic abusers exposed to high levels of toluene. Chronic inhalation exposure of humans to toluene also causes irritation of the upper respiratory tract and eyes, sore throat, dizziness, and headache.  Human studies have reported developmental effects, such as CNS dysfunction, attention deficits, and minor craniofacial and limb anomalies, in the children of pregnant women exposed to toluene or mixed solvents by inhalation. Reproductive effects, including an association between exposure to toluene and an increased incidence of spontaneous abortions, have also been noted.  However, these studies are not conclusive due to many confounding variables. (http://www.epa.gov/ttn/atw/hlthef/toluene.html)

Carbonyl Compounds

The carbonyl compounds of tropospheric interest are formaldehyde, acetaldehyde, acetone, 2- butanone, 1,3-butadiene among others.

Formaldehyde is a colorless gas with the chemical formula CH2O and the molecular weight 30.03 g/mol. The vapor pressure for formaldehyde is 10 mm Hg at -88 °C, and its log octanol/water partition coefficient (log Kow) is -0.65. Formaldehyde is a colorless gas with a pungent, suffocating odor at room temperature; the odor threshold for formaldehyde is 0.83 ppm. Formaldehyde is readily soluble in water at room temperature.

It is emitted from foam insulation, chipboard, plywood, some fabrics, motor exhausts, bonfires and cigarettes; it is also used mainly to produce resins used in particleboard products and as an intermediate in the synthesis of other chemicals. Exposure to formaldehyde may occur by breathing contaminated indoor air, tobacco smoke, or ambient urban air.  Acute (short-term) and chronic (long-term) inhalation exposure to formaldehyde in humans can result in respiratory symptoms, and eye, nose, and throat irritation.  Limited human studies have reported an association between formaldehyde exposure and lung and nasopharyngeal cancer. Animal inhalation studies have reported an increased incidence of nasal squamous cell cancer.


1,3-butadiene is a colorless gas with a mild gasoline-like odor. The odor threshold for 1,3-butadiene is 1.6 parts per million (ppm). The chemical formula for 1,3-butadiene is C4H6, and the molecular weight is 54.09 g/mol. Like benzene, is a VOC emitted into the atmosphere principally from fuel combustion of petrol and diesel in vehicles. 1,3-butadiene is also an important chemical in certain industrial processes, particularly the manufacture of synthetic rubber. Acute (short-term) exposure to 1,3-butadiene by inhalation in humans results in irritation of the eyes, nasal passages, throat, and lungs.  Epidemiological studies have reported a possible association between 1,3-butadiene exposure and cardiovascular diseases. Epidemiological studies on workers in rubber plants have shown an association between 1,3-butadiene exposure and increased incidence of leukemia. Animal studies have reported tumors at various sites from 1,3-butadiene exposure.


Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a class of very stable organic molecules made up of only carbon and hydrogen. These molecules are flat, with each carbon having three neighboring atoms much like graphite. The structures of a variety of representative PAHs can be seen in Figure I.1.1. PAHs are emitted from coke production, coal burning and motor vehicles. With the introduction of smoke control areas and the decline in the use of coal in domestic heating atmospheric concentrations have fallen. Some PAHs are carcinogenic, but with decline in overall concentrations, adverse effects are only likely in an occupational situation (Murley, 1995).

hese molecules are highly carcinogenic but they are also very common. They are a standard product of combustion from automobiles and airplanes and some (such as benzo[a]pyrene) are present in charcoal broiled hamburgers.

Figure I.1.1. Structures of a variety of representative PAHs. A database of PAHs can be found at: http://chrom.tutms.tut.ac.jp/JINNO/DATABASE/00alphabet.html

I.1.1.2. Other Compounds

Methane (CH4(g)) is the most reduced form of carbon in the air. It is also the simplest and most abundant hydrocarbon and organic gas. Methane is a greenhouse gas that absorbs thermal-IR radiation 25 times more efficiently, molecule for molecule, than CO2(g), but the mixing ratios of carbon dioxide are much larger than are those of methane.

Methane slightly enhances ozone formation in photochemical smog, but, because the incremental ozone produced from methane is small in comparison with ozone produced from other hydrocarbons, methane is a relatively unimportant component of photochemical smog. In the stratosphere, methane has little effect on the ozone layer, but its chemical decomposition provides one of the few sources of stratospheric water vapor. Neither the emission nor ambient concentration of methane is regulated in any country.

Table I.1.1 summarizes the sources and sinks of methane. Methane is produced in anaerobic environments, where methanogenic bacteria consume organic material and excrete methane. Ripe anaerobic environments include rice paddies, landfills, wetlands, and the digestive tracts of cattle, sheep, and termites.

Table I.1.1. Sources and Sinks of Atmospheric Methane



Methanogenic bacteria (lithotrophic autotrophs)

Natural gas leaks during fossil-fuel mining

and transport

Biomass burning

Fossil-fuel combustion

Kinetic reaction

Kinetic reaction

Transfer to soils and ice caps

Methanotrophic bacteria (conventional


Methane is also produced in the ground from the decomposition of fossilized carbon. The resulting natural gas, which contains more than 90 percent methane, often leaks to the air or is harnessed and used for energy. Methane is also produced during biomass burning, fossil-fuel combustion, and atmospheric chemical reactions. Its sinks include chemical reactions, transfer to soils, ice caps, the oceans, and consumption by methanotrophic bacteria. The effective lifetime of methane due to chemical reaction is about 8 to 12 years, which is small in comparison with the lifetimes of other organic gases. Because methane is relatively insoluble, its dissolution rate into ocean water is slow. Approximately 80 percent of the methane in the air today is biogenic in origin; the rest originates from fuel combustion and natural gas leaks.

Methane's average mixing ratio in the troposphere is near 1.8 ppmv, which is an increase from about 0.8 ppmv in the mid-1800s (Ethridge et al., 1992). Its tropospheric mixing ratio has increased steadily due to increased biomass burning, fossil-fuel combustion, fertilizer use, and landfill development. Mixing ratios of methane are relatively constant with height in the troposphere, but decrease in the stratosphere due to chemical loss. At 25 km, methane's mixing ratio is about half that in the troposphere. Methane has no harmful human health effects at typical outdoor or indoor mixing ratios.

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