Outdoor Air Pollution

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Figure 1 Pollutants emitted by fossil power plants and cars are responsible for 2/3 of all pollution. The rest is from industries.
Figure 1 Pollutants emitted by fossil power plants and cars are responsible for 2/3 of all pollution. The rest is from industries.

The major sources of outdoor air pollution are transportation, stationary sources, and industrial processes. Motor vehicles account for about a third of all carbon dioxide, 98% of all carbon monoxide, and about half of all nitrogen oxides and hydrocarbons emissions (Figure 1) (1). Stationary sources such as large diesel engines, coal power plants, and refineries are major contributors to sulfur emission and particulates. Industrial pollution is mostly a result of manufacturing a huge number of new and non-biodegradable chemical compounds produced from petroleum products.

Table 1. Total US Emission of Carbon Dioxide and Criteria Air Pollutants, 2002
CO2 CO NOx HC Particulate
Tons*  % Tons*  % Tons*  % Tons*  % Tons*  %
Transportation 1875 32 86.6 77.3 11.5 54.3 7.2 43.7 1.0 3.5
Stationary Sources 2250 39 4.4 4.0 8.3 39.3 1.0 6.1 2.5 8.7
Industrial 1665 29 2.7 2.4 0.9 4.0 7.0 42.1 1.1 3.8
Others 0 0 18.3 16.3 0.4 11.0 1.3 8.1 24.3 84.0
Total 5790 100 112 100 21.1 100 16.5 100 28.9

The sources of pollution are many, and the detailed discussion of their harmful effects is outside the scope of this book. Air pollutants are divided into two categories: The so-called criteria air pollutants and hazardous (toxic) air pollutants. The criteria air pollutants are those common in mobile and stationary sources (tailpipe and smokestack emissions) which are thought to cause heart and respiratory problems and may cause cancer. They are particulates, sulfur dioxide, carbon monoxide, nitrogen oxide, ground-level ozone (smog) and lead. Carbon dioxide is not usually considered a pollutant, but because of its increased global background and its effect on global warming, it is sometimes treated as such. However, carbon dioxide emission is not regulated by the federal government. The hazardous air pollutants are less common air pollutants, but they are carcinogenic or may cause damage to the immune or neurological systems. These include volatile organic compounds (unburned hydrocarbons), ammonia, and a wide range of chemicals used as solvents, sterilizers, and components in industrial processes. Table 1 shows the total US emission of criteria air pollutants for various sectors in 1999. In addition to these pollutants, radio nuclides emitted from nuclear sources are also of great concern and will be discussed separately in nuclear radiation.

Depending on the source of the pollution, air pollutants can be considered as primary or secondary.

Primary pollutants are those coming directly from the source and may be natural or anthropogenic (human caused). Natural sources include dust, pollen, sea salt nuclei, volcanic ash, smoke, and particulates from forest burning. The primary anthropogenic source of pollutants results from fossil fuel combustion and includes: carbon monoxide, the oxides of nitrogen (mainly NO and NO2), the oxides of sulfur (mainly SO2), unburned hydrocarbon, and particulates (mainly soot, ash, and metal traces). Because almost all air pollutants come from combustion sources, the amount of emission correlates closely with the amount of fossil fuel consumption (3).

Volatile Organic Compounds (VOC) (a) are the carbon containing compounds derived from living things. Fossil fuels are the main source of VOCs released to the atmosphere and include gasoline, industrial chemicals such as benzene, solvents such as toluene and xylene, and many other chemicals used as dry cleaning solvents.

Carbon monoxide forms when there is not enough oxygen or time for the complete oxidation of carbon to carbon dioxide. When inhaled, it diffuses through the lungs into the blood stream where it reacts with the blood hemoglobin displacing the oxygen. Consequently, the body will be deprived of oxygen resulting in headaches, fatigue, impaired judgment, and in extreme cases, suffocation and death. The overwhelming majority of all carbon monoxide emissions are from cars. Other major contributors include stationary power plants and solid waste disposal.

Nitrogen dioxide is emitted from a reaction between oxygen and nitrogen in the air at combustion temperatures. The main sources of nitrogen dioxide are transportation and stationary power plants, but some are also released from fertilizers and other chemical plants. The most important health effect associated with nitrogen dioxide is the reduction of the body’s resistance to infection. Nitrogen dioxide also limits plant growth, causes corrosion in metals, and is a major cause of global warming, acid deposition, and photochemical smog. In the presence of sunlight, nitrogen dioxide reacts with hydrocarbons to form ozone – a major lung irritant and contributor to bronchitis and asthma.

Sulfur oxides (SO2) are generated by the combustion of sulfur-rich fuels in stationary power stations, refineries, and industrial processing plants. In the atmosphere, it can be further oxidized to form SO3 and eventually reacts with water to form sulfuric acid. Some of the sulfuric acid will subsequently react with trace elements emitted alongside other pollutants to form submicron size sulfate particles – a major cause of acid fog, acid mist, and acid rain. The major impacts are reduced visibility and damage to vegetation, materials, and human health.

Particulates include soot, lead, dust, ocean salt, and metal debris. The main sources of particulates are power plants and diesel cars, although smaller sources such as wood stoves, agricultural burning, and dust also contribute. Particulates can vary in size from a fraction of a micron to many hundreds of microns. Larger particles are responsible for reduced visibility and will eventually settle, whereas smaller particles become airborne and may be inhaled, diffuse, and be adsorbed and deposited on the interiors of the lungs, potentially contributing to lung damage and cancer. Particles larger than 10 microns are entirely blocked by the nose. Those below 10 microns (PM-10) pass through the upper respiratory system. Smaller particles in the range of 0.5-5 microns (PM-2.5) can travel as far as the bronchioles before they are deposited onto the inner walls. Depending on their physical and chemical structure, particles can be toxic (trace of metals such as lead, cadmium, mercury, etc.), or they can interfere with normal breathing (dust). Some particles, such as ocean salt and fine soot, can act as nuclei sites for water condensation and fog formation.

Lead is a poisonous metal emitted from old automobile exhausts, paints, storage batteries, and pipes. The lead problem became particularly significant when it was routinely added to gasoline to reduce knocking and to allow for the construction of higher compression engines in the 1970s. Due to its adverse health effects, lead was eventually removed from gasoline. Depending on its concentration, lead can damage the lungs and may lead to convulsions, brain damage, and even death.

Secondary pollutants result from the further interaction of the primary pollutants with the atmosphere. Ozone is considered the main secondary pollutant. Ground-level ozone is produced by the combination of pollutants from smoke stacks, cars, paints, and solvents in the presence of sunlight.

Ozone is a pale blue gas with a pungent smell that allows it to be detected even in very low concentration. In fact, the word ozone is derived from Greek ozein meaning “to smell.” Major health problems associated with elevated levels of ozone in the atmosphere are coughing, headaches, nausea, and chest pains. Ozone also causes shortness of breath, asthma, chronic bronchitis, and fatigue. Asthma epidemics, especially in children in industrialized countries, can be directly attributed to ozone.



(1) US Environmental Protection Agency, National Emission Inventory Air Pollutant Emission Trends website (http://www.epa.gov/ttn/chief/trends).

(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

(3) US Environmental Protection Agency, National Air Quality and Emissions Trends Report, Annual 2000.

Additional Comments

(a) The EPA does not list VOCs as criteria pollutants, but because of their impact on the formation of smog they are included here.

Further Reading

Gore, A., An Inconvenient Truth, Penguin Books, 2007.

Roleff, T., Pollution: Opposing viewpoints, Greenhaven Press, 2000.

Walsh, P. J., Dudney, C. S., Copenhave, E. D., Indoor Air Quality, CRC Press, 1984.

Environmental Science and Technology, published by the American Chemical Society.

External Links

Environmental Protection Agency (http://www.epa.gov).

Occupational Safety and Health Administration (OSHA) (http://www.osha.gov).

Intergovernmental Panel on Climate Control (IPCC), (http://www.ipcc..ch).

United Nations Environment Programme (http://www.unep.org).

World Health Organization (WHO) (http://www.who.ch).