AIR POLLUTION

The pernicious effects of air pollution were first documented long ago. As early as 61 c.e., Seneca, a Roman philosopher and noted essayist, wrote, "As soon as I had gotten out of the heavy air of Rome, and from the stink of the chimneys thereof, which being stirred, poured forth whatever pestilential vapors and soot they had enclosed in them, I felt an alteration to my disposition" (Miller and Miller, 1993). With technology being as simple as it was in Rome, however, there was not much that could be done about the problem.

Thirteen hundred years later, controls on the use of coal in London were passed, marking the recorded start of air pollution control. But such controls were not enough to prevent the buildup of pollutants as by-products of industrialization; air pollution was common to all industrialized nations by 1925. Air pollution is still a significant problem in urban centers worldwide. In the United States, pollutant emissions and air pollution concentrations have been falling, for the most part, since the 1970s.

The major constituents of unpolluted air (not including water) at ground level are nitrogen (78.08%) and oxygen (20.95%). The next most abundant constituents are argon (at 0.934%) and carbon dioxide (about 0.034%), followed by the other noble gases: neon (0.002%), helium (0.0005%), krypton (0.0001%), and xenon (0.000009%). A variety of chemicals known as trace gases (some quite toxic) also are found in unpolluted air, but at very low concentrations.

When certain substances in the air rise to the level at which they can harm plant or animal life, damage property, or simply degrade visibility in an area more than it would be in the absence of human action, those substances are considered to be pollutants. Such pollutants enter the atmosphere via both natural processes and human actions.

The pollutants most strongly damaging to human, animal, and sometimes plant health include ozone, fine particulate matter, lead, nitrogen oxides (NO_x), sulfur oxides (SO_x), and carbon monoxide. Many other chemicals found in polluted air can cause lesser health impacts (such as eye irritation). VOC compounds comprise the bulk of such chemicals. Formaldehyde is one commonly mentioned pollutant of this sort, as is PAN (peroxyacyl nitrate). Such chemicals are common components of photochemical smog, a term that refers to the complex mixture of chemicals that forms when certain airborne chemicals given off by plant and human activity react with sunlight to produce a brownish mixture of thousands of different chemical species.

Finally, there are also pollutants that do not cause direct health impacts but that may have the potential to cause harm indirectly, through their actions on the overall ecology, or as they function as precursor chemicals that lead to the production of other harmful chemicals. The major indirect-action pollutants include volatile organic carbon (VOC) compounds that act as precursors to more harmful species; chemicals called halocarbons; and chemicals called greenhouse gases.

ENERGY USE AND POLLUTION

Energy use is the predominant source of global air pollution, though other human actions produce significant amounts of pollution as well. The burning of biomass for agriculture, such as the burning of crop stubble is one such nonenergy source, as is the use of controlled-burn management of forest fires. Natural events such as forest fires as well as volcanic out-gassing and eruption also contribute to air pollution. Finally, even living things produce considerable quantities of emissions considered "pollutants." Animals and insects produce a large share of the world's methane emissions, while plants emit significant quantities of volatile organic carbon compounds—enough, in some areas to produce elevated ozone levels with no other pollutant emissions at all.

Ozone

Ozone, or O₃, a chemical consisting of three atoms of oxygen, is a colorless, odorless gas produced by a variety of chemical reactions involving hydrocarbons and nitrogen oxides in the presence of sunlight. Often there is confusion about ozone's effects because it is found at two different levels of the atmosphere, where it has two very different effects. At ground level, ozone is known to be a respiratory irritant, implicated in causing decreased lung function, respiratory problems, acute lung inflammation, and impairment of the lungs' defense mechanisms. Outdoor workers, elderly people with pre-existing lung diseases, and active children who spend significant amounts of time in areas with elevated ozone levels are thought to be particularly at risk.

Low-altitude ozone forms when sunlight reacts with "precursor" chemicals of both human and nonhuman origin. These precursors include volatile organic carbon compounds, carbon monoxide, and nitrogen oxides. Volatile organic carbon compounds are created both naturally (by plants) and through human activity such as fuel use, biomass burning, and other industrial activities such as painting and coating. Nitrogen oxides are generated by stationary sources of energy use such as power plants and factories, and mobile sources such as cars, trucks, motorcycles, bulldozers, and snowmobiles. Because of the diversity of ozone precursors, sources vary by region.

At high altitudes, ozone forms naturally from the interaction of high-energy solar radiation and normal diatomic oxygen, or O₂. High-altitude ozone is not considered a pollutant, but is actually beneficial to life on Earth, as it screens out solar radiation that can damage plants, or cause skin cancer and cataracts in animals. High-altitude ozone can be destroyed through

the action of chemicals called halocarbons, which are commonly used in refrigeration and air conditioning. Ozone levels across the United States fell, on average, by four percent between 1989 and 1998.

Volatile Organic Carbon Compounds

There are many different chemical species in the VOC compounds group, including such commonly known chemicals as formaldehyde and acetone. The common feature that VOCs share is that they are ring-shaped (organic) molecules consisting principally of carbon, hydrogen, oxygen, and nitrogen. VOCs are released into the environment as a result of human activity, but also because of natural biological processes in plants and animals. Table 1 shows the percentage of global contribution to VOC concentrations from all sources.

VOCs react in the presence of sunlight to produce photochemical smog, a mixture of organic chemicals that can irritate the eyes and other mucous membranes. VOCs also constitute a major precursor chemical leading to ozone production. VOC levels across the United States fell, on average, by 20.4 percent between 1989 and 1998.

Particulate Matter

Particulate matter, generated through a range of natural and manmade processes including combustion and physical abrasion, has been implicated in increased mortality for the elderly, as well as those members of the population with damaged respiratory systems. Studies also have linked particulate matter with aggravation of preexisting respiratory and cardiovascular disease, resulting in more frequent and/or serious attacks of asthma in the elderly or in children.

The particulate matter of most concern consists of particles that are most likely to be trapped in tiny air sacs of the lung (alveoli) after inhalation. Studies suggest that such particles are in the microscopic range, from less than 1 micrometer in diameter to about 2.5 microns in average diameter.

Although it has been shown that exposure to certain air pollutants can aggravate preexisting lung ailments such as asthma, no causal link has been identified between exposure to low-level air pollution and asthma. In fact, while asthma levels have been rising, air pollution levels have been declining, suggesting that there is probably a different cause for the increase in asthma rates. While various indoor air pollutants have been suggested as the cause, no definitive cause for the increase in asthma rates has yet been found.

Particulate matter is generated by stationary sources such as power plants and factories and by mobile sources such as cars, trucks, motorcycles, bulldozers, and snowmobiles. Particulate matter levels across the United States fell, on average, by 25 percent between 1989 and 1998.

Carbon Monoxide

Carbon monoxide, or CO, is a highly toxic chemical that chemically binds to hemoglobin, rendering it incapable of carrying oxygen to the tissues of the body. CO is produced by the incomplete combustion of fossil fuels. Carbon monoxide levels across the United States fell, on average, by 39 percent between 1989 and 1998.

Lead

Lead is an element used in many industrial processes and also has been used in fuels and coatings. Tetraethyl lead was added to gasoline to improve performance as a motor fuel, and elemental lead was extensively used in paints and coatings to improve coverage and durability until the 1970s, when phase-out efforts began to reduce lead emissions to the environment.

Long-term exposure to airborne lead was shown to lead to a variety of health problems, primarily neurological. Atmospheric lead concentrations fell dramatically through the 1970s and continue to fall: Atmospheric lead concentrations fell, on average, by 56 percent between 1989 and 1998. The amount of lead found in the bloodstream of children growing up in urban environments also fell dramatically.

Sulfur Oxides

Sulfur oxide emissions enter the atmosphere from a variety of sources, some of human origin, others of natural origin. The main sulfur oxide is sulfur dioxide, or SO₂.

High concentrations of SO₂ can produce temporary breathing difficulties in asthmatic children and in adults who are active outdoors. Sulfur dioxide also can directly damage plants and has been shown to decrease crop yields. In addition, sulfur oxides can be converted to sulfuric acid and lead to acid rain. Acid rain can harm ecosystems by increasing the acidity of soils as well as surface waters such as rivers, lakes, and streams. Sulfur dioxide levels fell, on average, by 39 percent between 1989 and 1998.

Nitrogen Dioxide

Nitrogen dioxide (NO₂) is a reddish-brown gas that is formed through the oxidation of nitrogen oxide (NO). The term "nitrogen oxides," or NO_x is used to encompass NO₂ as well as NO and the other oxides of nitrogen that lead to NO₂ production.

Nitrogen oxides are generated by both human and nonhuman action, but the major sources of NO_x are high-temperature combustion processes such as those occurring in power plants and automobile engines. Natural sources of NO_x include lightning, chemical processes that occur in soil, and the metabolic activities of plants.

Short-term exposure to elevated levels of NO_x have been shown to cause changes in the function of human lungs, while chronic exposures have been linked to increased susceptibility to lung infections and to lasting changes in lung structure and function. Nitrogen oxides also are of concern because of their roles as ozone precursors, and through their contribution to acid rain in the form of nitric acid.

Nitrogen dioxide concentrations have changed little since 1989, and alterations in measuring techniques make it difficult to accurately assess the trend. Data suggest that NO₂ concentrations may have increased by 2 percent between 1989 and 1998.

AIR POLLUTION REGULATION IN THE UNITED STATES

Air pollution in the United States is regulated at federal, state, and local levels. Allowable concentrations of the major air pollutants are set by the U.S. Environmental Protection Agency (EPA) under the auspices of the Clean Air Act. States and localities implement pollution control plans in accordance with the provisions of the Clean Air Act in regions where air pollutant concentrations exceed the federal standards. Some states and localities have air pollution standards of their own, and in the past, such standards have occasionally been more stringent than those of the EPA.

The EPA sets two kinds of national ambient air quality standards. The primary standard is set at a level intended to protect human health with an adequate margin of safety. The secondary standard, usually less stringent, is set based on protecting the public welfare, which can include factors other than health impacts, such as reduced visibility, and damage to crops.

Table 3 shows the current health-related national ambient air quality standards set by the EPA as of 1999. Regions that violate these standards may be classified as "nonattainment" areas by the EPA, and can face sanctions if they do not promulgate pollution control plans that are acceptable to the agency.

Environmental regulations to curtail air pollution have had a major impact on energy producers, manufacturers of energy-using products, and energy consumers. Before the Clean Air Act of 1970, coal-fired steam turbines were the least expensive means available for utilities to generate electricity. Coal still remains the least expensive fossil fuel, yet the high capital costs of installing the technology to comply with ever more stringent environmental regulations have resulted in many utilities reconsidering their options for new electric-power-generating facilties. The combined cycle natural gas turbine is being favored not only because of the much easier compliance with current air quality regulations but also because of the likelihood of stricter future regulations. A typical coal power plant burns more than 70 lb of coal each second, and considering that these plants number in the hundreds, coal-burning emissions are likely to be a major future target of legislators and regulators eager to curtail emissions further.

Transportation is another sector that is a major contributor to air pollution. To improve air quality, particularly in urban areas, regulations require the use of reformulated gasoline and alternative fuels to reduce emissions of nitrogen oxides and carbon monoxide. Reflecting the higher cost of refining, these regulations have added at least 5 to 10 cents to the price of gasoline at the pump.

Besides cleaner fuels, vehicle makers have developed many emission-reducing technologies—both in "cleaner combustion" and in catalytic converter technologies—to comply with ever stricter tailpipe emission standards. The U.S. EPA stringent standards proposed in 1999 for model year 2004 vehicles will result in new vehicles emitting less than 1 percent of the VOC and NO_x emissions of their 1960s counterparts.

Kenneth Green

See also:Acid Rain; Air Quality, Indoor; Atmosphere; Automobile Performance; Climatic Effects; Emission Control, Vehicle; Emission Control, Power Plant; Environmental Economics; Environmental Problems and Energy Use; Gasoline and Additives; Transportation, Evolution of Energy Use and; Turbines, Gas.

BIBLIOGRAPHY

Finlayson-Pitts, B. J., and Pitts, J. N., Jr. (2000). Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications.San Diego: Academic Press.

Klaassen, C. D. (1996) Casarett & Doull's Toxicology: The Basic Science of Poisons, 5th ed. New York: McGraw-Hill.

Miller, F. W., and Miller, R. M. (1993). Environmental Hazards: Air Pollution—A Reference Handbook. Santa Barbara, CA: ABC-CLIO.

U. S. Environmental Protection Agency. (1998). "National Air Quality and Emissions Trends Report, 1998." <http://www.epa.org/>.