Air and Water Pollution

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Air and Water Pollution

Air pollution is a general term that covers a broad range of contaminants in the atmosphere. Pollution can occur from natural causes or from human activities. Discussions about the effects of air pollution have focused mainly on human health but attention is being directed to environmental quality and amenity as well. Air pollutants are found as gases or particles, and on a restricted scale they can be trapped inside buildings as indoor air pollutants. Urban air pollution has long been an important concern for civic administrators, but increasingly, air pollution has become an international problem.

The most characteristic sources of air pollution have always been combustion processes. Here the most obvious pollutant is smoke. However, the widespread use of fossil fuels has made sulfur and nitrogen oxides pollutants of great concern. With increasing use of petroleum-based fuels, a range of organic compounds have become widespread in the atmosphere.

In urban areas, air pollution has been a matter of concern since historical times. Indeed, there were complaints about smoke in ancient Rome. The use of coal throughout the centuries has caused cities to be very smoky places. Along with smoke, large concentrations of sulfur dioxide were produced. It was this mixture of smoke and sulfur dioxide that typified the foggy streets of Victorian London, paced by such figures as Sherlock Holmes and Jack the Ripper, whose images remain linked with smoke and fog. Such situations are far less common in the cities of North America and Europe today. However, until recently, they have been evident in other cities, such as Ankara, Turkey, and Shanghai, China, that rely heavily on coal.

Coal is still burned in large quantities to produce electricity or to refine metals, but these processes are frequently undertaken outside cities. Within urban areas, fuel use has shifted toward liquid and gaseous hydrocarbons (petroleum and natural gas). These fuels typically have a lower concentration of sulfur, so the presence of sulfur dioxide has declined in many urban areas. However, the widespread use of liquid fuels in automobiles has meant increased production of carbon monoxide, nitrogen oxides, and volatile organic compounds.

Primary pollutants such as sulfur dioxide or smoke are the direct emission products of the combustion process. Today, many of the key pollutants in the urban atmospheres are secondary pollutants, produced by processes initiated through photochemical reactions. The Los Angeles, California-type, photochemical smog is now characteristic of urban atmospheres dominated by secondary pollutants.

Although the automobile is the main source of air pollution in contemporary cities, there are other equally significant sources. Stationary sources are still important and the oil-burning furnaces that have replaced the older coal-burning ones are still responsible for a range of gaseous emissions and fly ash. Incineration is also an important source of complex combustion products, especially where this incineration burns a wide range of refuse. These emissions can include chlorinated hydrocarbons such as dioxin. When plastics, which often contain chlorine, are incinerated, hydrochloric acid is found in the waste gas stream. Metals, especially since they are volatile at high temperatures, can migrate to smaller, respirable particles. The accumulation of toxic metals, such as cadmium, on fly ash gives rise to concern over harmful effects from incinerator emissions. In specialized incinerators designed to destroy toxic compounds such as polychlorinated biphenyls (PCBs), many questions have been raised about the completeness of this destruction process. Even under optimum conditions when the furnace operation has been properly maintained, great care needs to be taken to control leaks and losses during transfer operations (fugitive emissions).

The enormous range of compounds used in modern manufacturing processes has also meant that there is an ever-widening range of emissions from both the industrial processes and the combustion of their wastes. Although the amounts of these toxic compounds are often rather small, they add to the complex range of compounds found in the urban atmosphere. Again, it is not only the deliberate loss of effluents through discharge from pipes and chimneys that needs attention. Fugitive emissions of volatile substances that leak from valves and seals often warrant careful control.

Air pollution control procedures are increasingly an important part of civic administration, although their goals are far from easy to achieve. It is also noticeable that although many urban concentrations of primary pollutants, for example, smoke and sulfur dioxide, are on the decline in developed countries, this is not always true in developing countries. Here the desire for rapid industrial growth has often lowered urban air quality. Secondary air pollutants are generally proving a more difficult problem to eliminate than primary pollutants like smoke.


Urban air pollutants have a wide range of effects, with health problems being the most enduring concern. In the classical polluted atmospheres filled with smoke and sulfur dioxide, a range of bronchial diseases was enhanced. While respiratory diseases are still the principal problem, the issues are somewhat more subtle in atmospheres where the air pollutants are not so obvious. In photo-chemical smog, eye irritation from a secondary pollutant, peroxyacetyl nitrate (PAN), is one of the most characteristic direct effects of the smog. High concentrations of carbon monoxide in cities where automobiles operate at high density mean that the human heart has to work harder to make up for the oxygen displaced from the blood's hemoglobin by carbon monoxide. This extra stress appears to reveal itself through increased incidence of complaints among people with heart problems. There is a widespread belief that contemporary air pollutants are involved in the increases in asthma, but the links between asthma and air pollution are probably rather complex and related to a whole range of factors. Lead, from automotive exhausts, is thought by many to be a factor in lowering the IQs of urban children.

Air pollution also affects materials in the urban environment. Soiling has long been regarded as a problem, originally the result of the smoke from wood or coal fires, but now increasingly the result of fine black soot from diesel exhausts. The acid gases, particularly sulfur dioxide, increase the rate of destruction of building materials. This is most noticeable with calcareous stones, which are the predominant building material of many important historic structures. Metals also suffer from atmospheric acidity. In today's photochemical smog, natural rubbers crack and deteriorate rapidly.

Health problems relating to indoor air pollution are extremely ancient. Anthracosis, or black lung disease, has been found in mummified lung tissue. Recent decades have witnessed a shift from the predominance of concern about outdoor air pollution into a widening interest in indoor air quality.

The production of energy from combustion and the release of solvents is so large in the contemporary world that it causes air pollution problems of regional and global nature. Acid rain is now widely observed throughout the world. The sheer quantity of carbon dioxide emitted in combustion processes is increasing the concentration of carbon dioxide in the atmosphere and enhancing the greenhouse effect.

Solvents, such as carbon tetrachloride and the aerosol propellants chlorofluorocarbons (CFCs) are now detectable all over the globe and responsible for problems such as ozone layer depletion.

At the other end of the scale, we need to remember that gases leak indoors from the polluted outdoor environment, but more often the serious pollutants arise from processes that take place indoors. Here there has been particular concern with regards to the generation of nitrogen oxides by sources such as gas stoves. Similarly, formaldehyde from insulating foams causes illnesses and adds to concerns about our exposure to a substance that may induce cancer in the long run. In the last decade it has become clear that radon leaks from the ground can expose some members of the public to high levels of this radioactive gas within their own homes. Cancers may also result from the emanation of solvents from consumer products—glues, paints, and mineral fibers (asbestos). More generally these compounds and a range of biological materials—animal hair, skin, pollen spores, and dusts—can cause allergic reactions in some people. At one end of the spectrum these simply cause annoyance, but in extreme cases, such as found with the bacterium Legionella, a large number of deaths can occur.

There are also important issues surrounding the effects of indoor air pollutants on materials. Many industries, especially the electronics industry, must take great care over the purity of indoor air where a speck of dust can destroy a microchip or low concentrations of air pollutants change the composition of surface films in component design. Museums must care for objects over long periods of time, so precautions must be taken to protect delicate dyes from the effects of photochemical smog, paper and books from sulfur dioxide, and metals from sulfide gases.


Air quality is determined with respect to the total air pollution in a given area as it interacts with meteorological conditions such as humidity, temperature, and wind to produce an overall atmospheric condition. Poor air quality can manifest itself aesthetically (as a displeasing odor, for example), and can also result in harm to plants, animals, and people, and even damage to objects.

As early as 1881, cities such as Chicago, Illinois, and Cincinnati, Ohio, passed laws to control some types of pollution, but it was not until several air pollution catastrophes occurred in the twentieth century that governments began to give more attention to air-quality problems. For instance, in 1930, smog trapped in the Meuse River Valley in Belgium caused 60 deaths. Similarly, in 1948, smog was blamed for 20 deaths in Donora, Pennsylvania. Most dramatically, in 1952, a sulfur-laden fog enshrouded London for five days and caused as many as 4,000 deaths over two weeks.

Disasters such as these prompted governments in a number of industrial countries to initiate programs to protect air quality. The year of the London tragedy, the United States passed the Air Pollution Control Act granting funds to assist the states in controlling airborne pollutants. In 1963, the Clean Air Act, which began to place authority for air quality into the hands of the federal government, was established. Today the Clean Air Act, with its 1970 and 1990 amendments, remains the principal air quality law in the United States.

The act established a National Ambient Air Quality Standard under which federal, state, and local monitoring stations at thousands of locations, together with temporary stations set up by the Environmental Protection Agency (EPA) and other federal agencies, directly measure pollutant concentrations in the air and compare those concentrations with national standards for six major pollutants: ozone, carbon monoxide, nitrogen oxides, lead, particulates, and sulfur dioxide. When the air we breathe contains amounts of these pollutants in excess of EPA standards, it is deemed unhealthy, and regulatory action is taken to reduce the pollution levels.

A December 1998 EPA report indicates that while air quality continues to improve, approximately 107 million Americans in 1997 lived in areas that did not meet the ambient air quality standards for at least one of the six major pollutants noted above. In general, though, improvements in air quality have been significant: carbon monoxide concentrations have decreased 38%; lead concentrations have decreased by 67%; nitrogen dioxide concentrations are down by 14%; ozone (smog) concentrations have been reduced by 19%; particulate matter concentrations decreased 26%; and sulfur dioxide concentrations decreased 39%. At the same time that air pollution has been decreasing significantly (1970–97), gross domestic product increased 114%, U.S. population increased 31%, and vehicle miles traveled increased 127%.

In addition, urban and industrial areas maintain an air pollution index. This scale, a composite of several pollutant levels recorded from a particular monitoring site or sites, yields an overall air quality value. Public warnings are given if the index exceeds certain values; in severe instances residents might be asked to stay indoors and factories might even be closed down.

While such air quality emergencies seem increasingly rare in the United States, developing countries, as well as Eastern European nations, continue to suffer poor air quality, especially in urban areas such as Bangkok, Thailand and Mexico City, Mexico. In Mexico City, for example, seven out of ten newborns have higher lead levels in their blood than the World Health Organization (WHO) considers acceptable. At present, many Third World countries place national economic development ahead of pollution control—and in many countries with rapid industrialization, high population growth, or increasing per capita income, the best efforts of governments to maintain air quality are outstripped by rapid proliferation of automobiles, escalating factory emissions, and runaway urbanization.

For all the progress the United States has made in reducing ambient air pollution, indoor air pollution may pose even greater risks than all of the pollutants we breathe outdoors. The Radon Gas and Indoor Air Quality Act of 1986 directed the EPA to research and implement a public information and technical assistance program on indoor air quality. From this program has come monitoring equipment to measure an individual's "total exposure" to pollutants both in indoor and outdoor air. Studies done using this equipment have shown indoor exposures to toxic air pollutants far exceed outdoor exposures for the simple reason that most people spend 90% of their time in office buildings, homes, and other enclosed spaces. Moreover, nationwide energy conservation efforts following the oil crisis of the 1970s led to building designs that trap pollutants indoors, thereby exacerbating the problem.


The need to control air pollution was recognized in the earliest cities. In the Mediterranean at the time of Christ, laws were developed to place objectionable sources of odor and smoke downwind or outside city walls. The adoption of fossil fuels in thirteenth-century England focused particular concern on the effect of coal smoke on health, with a number of attempts at regulation with regard to fuel type, chimney heights, and time of use. Given the complexity of the air pollution problem it is not surprising that these early attempts at control met with only limited success.

The nineteenth century was typified by a growing interest in urban public health. This developed against a background of continuing industrialization, which saw smoke abatement clauses incorporated into the growing body of sanitary legislation in both Europe and North America. However, a lack of both technology and political will doomed these early efforts to failure, except in the most blatantly destructive situations (for example, industrial settings such as those around alkali works in England).

The rise of environmental awareness in the current century has reminded us that air pollution ought not to be seen as a necessary product of industrialization. This has redirected responsibility for air pollution towards those who create it. The notion of "making the polluter pay" is seen as a central feature of air pollution control. The century has also seen the development of a range of broad air pollution control strategies, among them:

  1. Air quality management strategies that set ambient air quality standards so that emissions from various sources can be monitored and controlled.
  2. Emission standards strategy that sets limits for the amount of a pollutant that can be emitted from a given source. These may be set to meet air quality standards, but the strategy is optimally seen as one of adopting best available technology not entailing excessive costs (BATNEEC).
  3. Economic strategies that involve charging the party responsible for the pollution. If the level of charge is set correctly, some polluters will find it more economical to install air pollution control equipment than continue to pollute. Other methods utilize a system of tradable pollution rights.
  4. Cost-benefit analysis, which attempts to balance economic benefits with environmental costs. This is an appealing strategy but difficult to implement because of its controversial and imprecise nature.

In general, air pollution strategies have either been air quality or emission based. In the United Kingdom, emission strategy is frequently used; for example, the Alkali and Works Act of 1863 specifies permissible emissions of hydrochloric acid. By contrast, the United States has aimed to achieve air quality standards, as evidenced by the Clean Air Act. One criticism of using air quality strategy has been that while it improves air in poor areas it leads to degradation in areas with high air quality. Although the emission standards approach is relatively simple, it is criticized for failing to make explicit judgments about air quality and assumes that good practice will lead to an acceptable atmosphere.

Until the mid-twentieth century, legislation was primarily directed towards industrial sources, but the passage of the United Kingdom Clean Air Act (1956), which followed the disastrous smog of December 1952, directed attention towards domestic sources of smoke. While this particular act may have reinforced the improvements already under way, rather than initiating improvements, it has served as a catalyst for much subsequent legislative thinking. Its mode of operation was to initiate a change in fuel, perhaps one of the oldest methods of control. The other well-tried aspects were the creation of smokeless zones and an emphasis on tall chimneys to disperse the pollutants.

As simplistic as such passive control measures seem, they remain at the heart of much contemporary thinking. Changes from coal and oil to the less-polluting gas or electricity have contributed to the reduction in smoke and sulfur dioxide concentrations in cities all around the world. Industrial zoning has often kept power and large manufacturing plants away from centers of human population, and "superstacks," chimneys of enormous height, are now quite common. Successive changes in automotive fuels—lead-free gasoline, low-volatility gas, methanol, or even the interest in the electric automobile—are further indications of continued use of these methods of control.

There are more active forms of air pollution control that seek to clean up the exhaust gases. The earliest of these were smoke and grit arresters that came into increasing use in large electrical stations during the twentieth century. Notable here were the cyclone collectors that removed large particles by driving the exhaust through a tight spiral that threw the grit outward where it could be collected. Finer particles could be removed by electrostatic precipitation. These methods were an important part of the development of the modern pulverized fuel power station. However, they failed to address the problem of gaseous emissions. Here it has been necessary to look at burning fuel in ways that reduce the production of nitrogen oxides. Control of sulfur dioxide emissions from large industrial plants can be achieved by desulfurization of the flue gases. This can be quite successful by passing the gas through towers of solid absorbers or spraying solutions through the exhaust gas stream. However, these are not necessarily cheap options.

The catalytic converter is also an important element of active attempts to control air pollutants. Although these can considerably reduce emissions, they have to be offset against the increasing use of the automobile. There is much talk of the development of zero pollution vehicles that do not emit any pollutants.

Legislation and control methods are often associated with monitoring networks that assess the effectiveness of the strategies and inform the general public about air quality where they live. A balanced approach to the control of air pollution in the future may have to look far more broadly than simply at technological controls. It will become necessary to examine the way we structure our lives in order to find more effective solutions to air pollution.


The air pollution index is a value derived from an air quality scale that uses the measured or predicted concentrations of several criteria pollutants and other air-quality indicators, such as coefficient of haze (COH) or visibility. The best-known index of air pollution is the pollutant standard index (PSI).

The PSI has a scale that spans from 0 to 500. The index represents the highest value of several subindices; there is a subindex for each pollutant, or in some cases, for a product of pollutant concentrations and a product of pollutant concentrations and COH. If a pollutant is not monitored, its subindex is not used in deriving the PSI.

The subindex of each pollutant or pollutant product is derived from a PSI nomogram that matches concentrations with subindex values. The highest subindex value becomes the PSI. The PSI has five health-related categories: good (0–50); moderate (50–100); unhealthy (100–200); very unhealthy (200–300) hazardous (300–500).

CLEAN AIR ACT (1963, 1970, 1990)

The 1970 Clean Air Act and major amendments to the act in 1977 and 1990 serve as the backbone of efforts to control air pollution in the United States. This law established one of the most complex regulatory programs in the country. Efforts to control air pollution in the United States date back to 1881, when Chicago and Cincinnati passed laws to control smoke and soot from factories in the cities. Other municipalities followed suit and the momentum continued to build. In 1952, Oregon became the first state to adopt a significant program to control air pollution, and three years later, the federal government became involved for the first time, when the Air Pollution Control Act was passed. This law granted funds to assist the states in their air pollution control activities.

In 1963, the first Clean Air Act was passed. This act provided permanent federal aid for research, support for the development of state pollution control agencies, and federal involvement in cross-boundary air pollution cases. An amendment to the act in 1965 directed the Department of Health, Education, and Welfare (HEW) to establish federal emission standards for motor vehicles. (At that time, HEW administered air pollution laws. The EPA was not created until 1970.) This represented a significant move by the federal government from a supportive to an active role in setting air-pollution policy. The 1967 Air Quality Act provided additional funding to the states, required the states to establish Air Quality Control Regions, and directed HEW to obtain and make available information on the health effects of air pollutants and to identify pollution control techniques. All of these components of the law were designed to assist the states, but they further demonstrated increasing federal involvement in the issue.

The Clean Air Act of 1970 marked a dramatic change in air pollution policy in the United States. Following the passage of this law, the federal government, not the states, would be the focal point for air pollution policy. This act established the framework that continues to be the foundation for air pollution control policy. The impetus for this change was the belief that the current state-based approach was not working. Public sentiment was growing so significantly that environmental issues demanded the attention of high-ranking officials. In fact, the leading policy entrepreneurs on the issue were President Richard Nixon and Senator Edmund Muskie of Maine.

These men and other leaders devised a plan with four key components. First, National Ambient Air Quality Standards (NAAQS) were established for six major pollutants: carbon monoxide, lead (in 1977), nitrogen dioxide, ground-level ozone (a key component of smog), particulate matter, and sulfur dioxide. For each of these pollutants, sometimes referred to as criteria pollutants, primary and secondary standards were set. The primary standards were designed to protect human health; the secondary standards were based on protecting crops, forests, and buildings if the primary standards were not capable of doing so. The act stipulated that these standards must apply to the entire country and be established by the EPA, based on the best available scientific information. Relatedly, the EPA was to establish standards for less common toxic air pollutants.

Second, New Source Performance Standards (NSPS) would be established by the EPA. These standards would determine how much air pollution would be allowed by new plants in the various industrial sectors. The standards are to be based on the best affordable technology available for the control of pollutants at sources such as power plants, steel factories, and chemical plants.

Third, mobile source emission standards were established to control automobile emissions. These standards were specified in the statute (rather than left to the EPA), and schedules for meeting these standards were also written into the law. It was thought that such an approach was crucial in having success with the powerful auto industry. The pollutants regulated were carbon monoxide, hydrocarbons, and nitrogen oxides, with goals of reducing the first two pollutants by 90% by 1975, and nitrogen oxides by 82% by 1975.

The final component of the air quality protection framework involved the implementation of the above procedures. Each state would be encouraged to devise a state implementation plan (SIP), which would indicate how the state would achieve the national standards. This gave each state some flexibility while still maintaining national standards. These plans had to be approved by the EPA; if a state did not have an approved SIP, the EPA would administer the Clean Air Act in that state. However, since the federal government is in charge of establishing pollution standards for new mobile and stationary sources, even the states with an SIP have limited flexibility. The main focal point for the states was the control of existing stationary sources, and if necessary, mobile sources. The states had to set limits in their SIPs that allowed them to achieve the NAAQS by a statutorily determined deadline (originally 1975, but subsequently delayed). One problem with this approach was the construction of tall smokestacks, which helped move pollution out of a particular airshed but did not reduce overall pollution levels. The states are also charged with monitoring and enforcing the Clean Air Act.

The 1977 amendments to the Clean Air Act dealt with three main issues: nonattainment, auto emissions, and the prevention of air quality deterioration in areas where the air was already relatively clean. The first two issues were resolved

National Ambient Air Quality Standards

National Ambient Air Quality Standards (NAAQS) have been established by the U.S. Environmental Protection Agency for the following six criteria air pollutants:

PollutantAveraging TimePrimary StandardSecondary Standard
CO8 Hours9 ppmNone
1 Hour35 ppmNone
Lead (Pb)Calendar Quarter1.5 μg/m3Same as Primary
NO2Annual0.053 ppmSame as Primary
O31 Hour0.12 ppmSame as Primary
PM10Annual50 μg/m3Same as Primary
24 Hours150 μg/m3Same as Primary
SO2Annual0.03 ppmNone
24 Hours0.14 ppmNone
3 HoursNone0.5 ppm
The TSP NAAQS is no longer applicable.
It was superseded by the PM10 NAAQS on 07/01/87.
The old TSP NAAQS is provided for information only.
TSPAnnual75 μg/m360 μg/m3
24 Hours260 μg/m3150 μg/m3

The NAAQS are the allowable ambient (outdoor) concentrations that must be maintained in order to protect public health and welfare. Limits have been set for carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), and particulate matter (PM10). EPA is currently reviewing the adequacy of the ozone and PM10 standards.

(Courtesy of U.S. government publication.)

primarily by delaying deadlines and increasing penalties. Largely in response to a court decision in favor of environmentalists (Sierra Club v. Ruckelshaus, 1972), the 1977 amendments included a program for the prevention of significant deterioration (PSD) of air that was already clean. This program would prevent polluting the air up to the national levels in areas where the air was cleaner than the standards. In Class I areas, areas with near pristine air quality, no new significant air pollution would be allowed. Class I areas are airsheds over larger national parks and wilderness areas. In Class II areas a moderate degree of air quality deterioration would be allowed. And finally, in Class III areas, air deterioration up to the national secondary standards would be allowed. Most of the country that had area cleaner than the NAAQS was classified as Class II. Related to the prevention of significant deterioration is a provision to protect and enhance visibility in national parks and wilderness areas even if the air pollution is not a threat to human health. The impetus of this section of the bill was the growing visibility problem in parks, especially in the Southwest.

Throughout the 1980s efforts to further amend the Clean Air Act were stymied. President Ronald Reagan was opposed to any strengthening of the Act, which he argued would hurt the economy. In Congress, the controversy over acid rain between members from the Midwest and the Northeast further contributed to the stalemate. Gridlock on the issue broke with the election of George Bush, who supported amendments to the Act, and the rise of Senator George Mitchell of Maine to Senate Majority Leader. Over the next two years, the issues were hammered out between environmentalists and industry and between different regions of the country. Major players in Congress were Representatives John Dingell of Michigan and Henry Waxman of California and Senators Robert Byrd of West Virginia and Mitchell.

Major amendments to the Clean Air Act were finally passed in the fall of 1990. These amendments addressed four major topics: (1) acid rain, (2) toxic air pollutants, (3) nonattainment areas, and (4) ozone layer depletion. To address acid rain, a 10-million-ton reduction in annual sulfur dioxide emissions (a 40% reduction based on 1980 levels) and a 2-million-ton annual reduction in nitrogen oxides by the year 2000 was required. Most of this reduction came from old utility power plants. The law also creates marketable pollution allowances, so that a utility that reduces emissions more than required can sell those pollution rights to another source. Economists argue that such an approach should become more widespread for all pollution control, to

increase efficiency. Due to the failure of the toxic air pollutant provisions of the 1970 Clean Air Act, new, more stringent provisions were adopted requiring regulations for all major sources of 189 varieties of toxic air pollution within 10 years. Areas of the country still in nonattainment for criteria pollutants will be given from three to 20 years to meet these standards. These areas are also required to impose tighter controls to meet these standards. To help these areas and other parts of the country, the act requires stiffer motor vehicle emissions standards and cleaner gasoline. Finally, three chemical families that contribute to the destruction of the stratospheric ozone layer (chlorofluorocarbons [CFCs], hydrochlorofluorocarbons [HCFCs], and methyl chloroform) are to be phased out of production and use.

The Clean Air Act has met with mixed success. The national average pollutant levels for the criteria pollutants have decreased. Nevertheless, many localities have not achieved these standards and are in perpetual nonattainment. Not surprisingly, major urban areas are those most frequently in nonattainment. The pollutant for which standards are most often exceeded is ozone, or smog. The greatest successes have come with lead, which has been reduced by 96% (largely due to the phasing-out of leaded gasoline), and particulates, which were reduced by over 60%. Additionally, despite numerous delays, the carbon monoxide, hydrocarbon, and nitrogen oxides pollution from new cars has decreased by 96%, 96%, and 76% over the period from 1967 to 1990. A final point of caution concerning evaluating the Clean Air Act: due to the tremendous complexity of air quality, we cannot conclude that all changes in pollutant levels are due to the law. These changes may be due to shifts in the economy at large, changes in weather patterns, or other such variables.


The Clean Air Act defines an Air Quality Control Region as a contiguous area where air quality, and thus air pollution, is relatively uniform. In those cases where topography is a factor in air movement, AQCRs often correspond with airsheds. AQCRs may consist of two or more cities, counties, or other governmental entities, and each region is required to adopt consistent pollution control measures across the political jurisdictions involved. AQCRs may even cross state lines and, in these instances, the states must cooperate in developing pollution control strategies. Each AQCR is treated as a unit for the purposes of pollution reduction and achieving National Ambient Air Quality Standards. As of 1993, most AQCRs had achieved national air quality standards; however the remaining AQCRs where standards had not been achieved were a significant group, where a large percentage of the United States population dwelled. AQCRs involving major metro areas like Los Angeles, New York, Houston, Denver, and Philadelphia were not achieving air quality standards because of smog, motor vehicle emissions, and other pollutants.


Ozone (O3) is a toxic, colorless gas (but can be blue when in high concentration) with a characteristic acrid odor. A variant of normal oxygen, it has three oxygen atoms per molecule rather than the usual two. Ozone strongly absorbs ultraviolet radiation at wavelengths of 220–290 nanometers (nm) with peak absorption at 260.4 nm. Ozone will also absorb infrared radiation at wavelengths in the range 9–10 μm. Ozone occurs naturally in the ozonosphere (ozone layer), which surrounds Earth, protecting living organisms at Earth's surface from ultraviolet radiation. The ozonosphere is located in the stratosphere at 6–30 mi above Earth's surface, with the highest concentration at 7.5–12 mi. The concentration of ozone in the ozonosphere is 1 molecule per 100,000 molecules, or if the gas were at standard temperature and pressure, the ozone layer would be 0.12 in thick. However, the ozone layer absorbs over 90% of incident ultraviolet radiation.

Ozone in the stratosphere results from a chemical equilibrium between oxygen, ozone, and ultraviolet radiation. Ultraviolet radiation is absorbed by oxygen and produces ozone. Simultaneously, ozone absorbs ultraviolet radiation and decomposes to oxygen and other products. Ozone layer depletion occurs as a result of complex reactions in the atmosphere between organic compounds that react with ozone faster than the ozone is replenished. Compounds of most concern include the byproducts of ultraviolet degradation of chlorofluorocarbons (CFCs), chlorine, and fluorine.

Ozone is also a secondary air pollutant at Earth's surface as a result of complex chemical reactions between sunshine and primary pollutants, such as hydrocarbons and oxides of nitrogen. Ozone can also be generated in the presence of oxygen from equipment that gives off intense light, electrical sparks, or creates intense static electricity, such as photocopiers and laser printers. Human olfactory senses are very sensitive to ozone, being able to detect ozone odor at concentrations of 0.02–0.05 parts per million. Toxic symptoms for humans from exposure to ozone include headaches and drying of the throat and respiratory tracts. Ozone is highly toxic to many plant species and destroys or degrades many building materials, such as paint, rubber, and some plastics. The total losses in the United States each year due to ozone damage to crops, livestock, buildings, natural systems, and human health is estimated to be in the tens of billions of dollars. The threshold limit value (TLV) for air quality standards is 0.1 ppm, or 0.2 mg O3 per m3 of air.

Industrial uses of ozone include chemical manufacturing and air, water, and waste treatment. Industrial quantities of ozone are typically generated from air or pure oxygen by means of silent corona discharge. Ozone is used in water treatment as a disinfectant to kill pathogenic microorganisms or for oxidation of organic and inorganic compounds. Combinations of ozone and hydrogen peroxide or ultraviolet radiation in water can generate powerful oxidants useful in breaking down complex synthetic organic compounds. In wastewater treatment, ozone can be used to disinfect effluents, or decrease their color and odor. In some industrial applications, ozone can be used to enhance biodegradation of complex organic molecules. Industrial cooling tower treatment with ozone prevents transmission of airborne pathogenic organisms and can reduce odor.


The ozone layer in Earth's upper atmosphere helps make life on the planet possible by shielding it from 95–99% of the Sun's potentially deadly ultraviolet radiation. This radiation is harmful and sometimes lethal to wildlife, crops, and vegetation, and can cause fatal skin cancer, cataracts, and immune system damage in humans.

Destroying the ozone shield

Ozone, a form of oxygen consisting of three atoms of oxygen instead of two, is considered an air pollutant when found at ground levels and is a major component of smog. It is formed by the reaction of various air pollutants in the presence of sunlight. Ozone is also used commercially as a bleaching agent and to purify municipal water supplies. Since ozone is toxic, the gas is harmful to health when generated near Earth's surface. Because of its high rate of breakdown, such ozone never reaches the upper atmosphere.

But the ozone that shields Earth from the Sun's radiation is found in the stratosphere, a layer of the upper atmosphere found 9–30 mi above ground. This ozone layer is maintained as follows: the action of ultraviolet light breaks O2 molecules into atoms of elemental oxygen (O). The elemental oxygen then attaches to other O2 molecules to form O3. When it absorbs ultraviolet radiation that would otherwise reach Earth, ozone is, in turn, broken down into O2 + O. The elemental oxygen generated then finds another O2 molecule to become O3 once again.

In 1974, chemists F. Sherwood Rowland and Mario J. Molina realized that chlorine from chlorofluorocarbon (CFC) molecules was capable of breaking down ozone in the stratosphere. In time, evidence began to accumulate that the ozone layer was indeed being broken apart by these industrial chemicals, and to a lesser extent by nitrogen oxide emissions from jet airplanes as well as hydrogen chloride emissions from large volcanic eruptions.

When released into the environment, CFCs slowly rise into the upper atmosphere, where they are broken apart by solar radiation. This releases chlorine atoms that act as catalysts, breaking up molecules of ozone by stripping away one of their oxygen atoms. The chlorine atoms, unaltered by the reaction, are each capable of destroying ozone molecules repeatedly. Without a sufficient quantity of ozone to block its way, ultraviolet radiation from the Sun passes through the upper atmosphere and reaches Earth's surface.

When damage to the ozone layer first became apparent in 1974, propellants in aerosol spray cans were a major source of CFC emissions, and CFC aerosols were banned in the United States in 1978. Production of CFC-12 (also known by R-12 or the trade name Freon), used in cooling and refrigeration, ended in 1995, although use is allowed until supplies are depleted. However, CFCs have since remained in widespread use in thermal insulation, as cleaning solvents, and as foaming agents in plastics, resulting in continued and accelerating depletion of stratospheric ozone.

The Antarctic ozone hole

The most dramatic evidence of the destruction of the ozone layer has occurred over Antarctica, where a massive "hole" in the ozone layer appears each winter and spring, apparently exacerbated by the area's unique and violent climatological conditions. The destruction of ozone molecules begins during the long, completely dark, and extremely cold Antarctic winter, when swirling winds and ice clouds begin to form in the lower stratosphere. This ice reacts with chlorine compounds in the stratosphere (such as hydrogen chloride and chlorine nitrate) that come from the breakdown of CFCs, creating molecules of chlorine.

When spring returns in August and September, a seasonal vortex—a rotating air mass—causes the ozone to mix with certain chemicals in the presence of sunlight. This helps break down the chlorine molecules into chlorine atoms, which, in turn, react with and break up the molecules of ozone. A single chlorine, bromine, or nitrogen molecule can break up literally thousands of ozone molecules.

During December, the ozone-depleted air can move out of the Antarctic area, as happened in 1987, when levels of ozone over southern Australia and New Zealand sank by 10% over a three-week period, causing as much as a 20% increase in ultraviolet radiation reaching Earth. This may have been responsible for a reported increase in skin cancers and damage to some food crops.

The seasonal hole in the ozone layer over Antarctica has been monitored by scientists at the National Aeronautics and Space Administration's (NASA) Goddard Space Flight Center outside Washington, D.C. NASA's NIMBUS-7 satellite first discovered drastically reduced ozone levels over the Southern Hemisphere in 1985, and measurements are also being conducted with instruments on aircraft and balloons. Some of the data that has been gathered is alarming.

In October 1987, ozone levels within the Antarctic ozone hole were found to be 45% below normal, and similar reductions occurred in October 1989. A 1988 study revealed that since 1969, ozone levels had declined by 2% worldwide, and by as much as 3% or more over highly populated areas of North America, Europe, South America, Australia, and New Zealand.

In September 1992, the NIMBUS-7 satellite found that the depleted ozone area over the southern polar region had grown 15% from the previous year, to a size three times larger than the area of the United States, and was 80% thinner than usual. The ozone hole over Antarctica was measured at approximately 8.9 million mi2, as compared to its usual size of 6.5 million mi2. The contiguous 48 states are, by comparison, about 3 million mi2, and all of North America covers 9.4 million mi2. Researchers attributed the increased thinning not only to industrial chemicals but also to the 1991 volcanic eruptions of Mount Pinatubo in the Philippines and Mount Hudson in Chile, which emitted large amounts of sulfur dioxide into the atmosphere.

Dangers of ultraviolet radiation

The major consequence of the thinning of the ozone layer is the penetration of more solar radiation, especially ultraviolet-B (UV-B) rays, the most dangerous type, which can be extremely damaging to plants, wildlife, and human health. Because UV-B can penetrate the ocean's surface, it is potentially harmful to marine life forms and indeed to the entire chain of life in the seas as well.

UV-B can kill and affect the reproduction of fish, larvae, and other plants and animals, especially those found in shallow waters, including phytoplankton, which forms the basis of the oceanic food chain/web. The National Science Foundation reported in February 1992 that its research ship, on a six week Antarctic cruise, found that the production of phytoplankton decreases at least 6–12% during the period of greatest ozone layer depletion, and that the destructive effects of UV radiation could extend to depths of 90 ft.

A decrease in phytoplankton would affect all other creatures higher on the food chain and dependent on them, including zooplankton, microscopic ocean creatures that feed on phytoplankton and are also an essential part of the ocean food chain. And marine phytoplankton are the main food source for krill, tiny Antarctic shrimp that are the major food source for fish, squid, penguins, seals, whales, and other creatures in the Southern Hemisphere.

Moreover, phytoplankton are responsible for absorbing, through photosynthesis, great amounts of carbon dioxide (CO2) and releasing oxygen. It is not known how a depletion of phytoplankton would affect the planet's supply of life-giving oxygen, but more CO2 in the atmosphere would exacerbate the critical problem of global warming, the socalled greenhouse effect.

There are numerous reports, largely unconfirmed, of animals in the southern polar region being harmed by ultraviolet radiation. Rumors abound in Chile, for example, of pets, livestock, sheep, rabbits, and other wildlife getting cataracts, suffering reproductive irregularities, or even being blinded by solar radiation. Many residents of Chile believe these stories, and wear sunglasses, protective clothing, and sun-blocking lotion in the summer, or even stay indoors much of the day when the sun is out. If the ozone layer's thinning continues to spread, the lifestyles of people across the globe could be similarly disrupted for generations to come.

Particularly frightening have been incidents reported to have taken place in Punta Arenas, Chile's southernmost city, at the tip of Patagonia. After several days of record low levels of ozone were recorded in October 1992, people reported severe burns from short exposure to sunlight. Sheep and cattle became blind, and some starved because they could not find food. Trees wilted and died, and melanoma-type skin cancers seem to have increased dramatically. Similar stories have been reported from other areas of the Southern Hemisphere. And malignant melanoma, once a rare disorder, is now the fastest rising cancer in the world.

Ozone thinning spreads

Indeed, ozone layer depletion is spreading at an alarming rate. In the 1980s, scientists discovered that an ozone hole was also appearing over the Arctic region in the late winter months, and concern was expressed that similar thinning might begin to occur over, and threaten, heavily populated areas of the globe. These fears were confirmed in April 1991, when the Environmental Protection Agency (EPA) announced that satellite measurements had recorded an ominous decrease in atmospheric ozone, amounting to an average of 5% over the mid-latitudes (including the United States), almost double the loss previously thought to be occurring.

The data showed that ozone levels measured in the late fall, winter, and early spring over large areas of the United States, Europe, and the mid-latitudes of the Northern and Southern Hemispheres had dropped by 4–6% over the last decade—twice the amount estimated in earlier years. The greatest area of ozone thinning in the United States was found north of a line stretching from Philadelphia to Denver to Reno, Nevada. One of the most alarming aspects of the new findings was that the ozone depletion was continuing into April and May, a time when people spend more time outside, and crops are beginning to sprout, making both more vulnerable to ultraviolet radiation.

The new findings led the EPA to project that over the next 50 years, thinning of the ozone layer could cause Americans to suffer some 12 million cases of skin cancer, 200,000 of which would be fatal. Several years earlier, the agency had calculated that over the next century, there could be an additional 155 million cases of skin cancers and 3.2 million deaths if the ozone layer continued to thin at the then current rate. Another EPA projection made in the 1980s was that the increase in radiation could cause Americans to suffer 40 million cases of skin cancer and 800,000 deaths in the following 88 years, plus some 12 million eye cataracts.

No one can say how accurate such varying projections will turn out to be, but evidence of ozone layer thinning is well documented. In October 1991, additional data of spreading ozone layer destruction were made public. Dr. Robert Watson, a NASA scientist who co-chairs an 80-member panel of scientists from 80 countries, called the situation "extremely serious," saying that "we now see a significant decrease of ozone both in the Northern and Southern Hemispheres, not only in winter but in spring and summer, the time when people sunbathe, putting them at risk for skin cancer, and the time when we grow crops."

In February 1992, a team of NASA scientists announced that they had found record high levels of ozone-depleting chlorine over the Northern Hemisphere. This could, in turn, lead to an ozone "hole" similar to the one that appears over Antarctica developing over populated areas of the United States, Canada, and England. The areas over which increased levels of chlorine monoxide were found extended as far south as New England, France, Britain, and Scandinavia.

Action to protect the ozone layer

As evidence of the critical threats posed by ozone layer depletion has increased, the world community has begun to take steps to address the problem. In 1987, the United States and 22 other nations signed the Montreal Protocol, agreeing, by the year 2000, to cut CFC production in half, and to phase out two ozone-destroying gases, Halon 1301 and Halon 1211. Halons are human-made bromine compounds used mainly in fire extinguishers, and can destroy ozone at a rate 10 to 40 times more rapidly than CFCs. Fortunately, these restrictions appear to already be having an impact. In 1992, it was found that the rate at which these two Halon gases were accumulating in the atmosphere had fallen significantly since 1987. The rate of increase of levels of Halon 1301 was about 8% per year during 1989–1992, about half of the average annual rate of growth over previous years. Similarly, Halon 1211 was increasing at only 3% annually, much less than the previous growth of 15% a year.

Since the Montreal Protocol, other international treaties have been signed limiting the production and use of ozone-destroying chemicals. When alarming new evidence on the destruction of stratospheric ozone became available in 1988, the world's industrialized nations convened a series of conferences to plan remedial action. In March 1989, the 12-member European Economic Community (EEC) announced plans to end the use of CFCs by the turn of the century, and the United States agreed to join in the ban. A week later, 123 nations met in London to discuss ways to speed the CFC phase-out. The industrial nations agreed to cut their own domestic CFC production in half, while continuing to allow exports of CFCs, in order to accommodate Third World nations.

The large industrial nations, which have created the CFC problem, are now much more willing to take effective action to ban the compounds than are many developing nations, such as India and China. The latter nations resist restrictions on CFCs on the grounds that the chemicals are necessary for their own economic development.

After the meeting in London, leaders and representatives from 24 countries met in an environmental summit at The Hague, Netherlands, and agreed that the United Nations' authority to protect the world's ozone layer should be strengthened.

In May 1989, members of the EEC and 81 other nations that had signed the 1987 Montreal Protocol decided at a meeting in Helsinki, Finland, to try to achieve a total phase-out of CFCs by the year 2000, as well as phase-outs as soon as possible of other ozone-damaging chemicals like carbon tetrachloride, halons, and methyl chloroform. In London in June 1990, most of the Montreal Protocol's signatory nations formally adopted a deadline of the year 2000 for industrial nations to phase out the major ozone-destroying chemicals, with 2010 being the goal for developing countries.

Finally, in November 1992, 87 nations meeting in Copenhagen, Denmark, decided to strengthen the action agreed to under the Montreal Protocol and move up the phase-out deadline from 2000 to January 1, 1996 for CFCs, and to January 1, 1994 for halons. A timetable was also agreed to for eliminating hydrochlorofluorocarbons (HCFCs) by the year 2030. HCFCs are being used as substitutes for CFCs even though they also deplete ozone, albeit on a far lesser scale than CFCs. The conference failed to ban the production of the pesticide methyl bromide, which may account for 15% of ozone depletion by the year 2000, but did freeze production at 1991 levels.

Environmentalists were disappointed that stronger action was not taken to protect the ozone layer. But Environmental Protection Agency (EPA) Administrator William K. Reilly, who headed the United States delegation, estimated that the reductions agreed to could, by the year 2075, prevent a million cases of cancer and 20,000 deaths.

Although the restrictions apply to developed nations, which produce most of the ozone-damaging chemicals, it was also agreed to consider moving up a phase-out of such compounds by developing nations from 2010 to 1995. A month after the Copenhagen conference, the nations of the European Community agreed to push bans on the use of CFCs and carbon tetrachloride to 1995 and to cut CFC emissions by 85% by the end of 1993.

The private sector has also taken action to reduce CFC production. The world's largest manufacturer of the chemicals, DuPont Chemical Company, announced in 1988 that it was working on a variety of substitutes for CFCs, would phase out production of them by 1996, and would partially replace them with HCFCs. Environmentalists charge that DuPont has been moving too slowly to eliminate production of these chemicals.

There are many ways that individuals can help reduce the release of CFCs into the atmosphere, mainly by avoiding products that contain or are made from CFCs, and by recycling CFCs whenever possible. Although CFCs have not generally been used in spray cans in the United States since 1978, they are still used in many consumer and industrial products, such as styrofoam. Other products manufactured using CFCs include solvents and cleaning liquids used on electrical equipment, polystyrene foam products, and fire extinguishers that use halons.

Refrigerants in cars and home air conditioning units that still use CFCs must be poured into closed containers to be cleaned or recycled, or they will evaporate into the atmosphere. Using foam insulation to seal homes also releases CFCs. Many alternatives to foam insulation exist, such as cellulose fiber, gypsum, fiberboard, and fiberglass.

Unfortunately, whatever steps are taken in the next few years, the problem of ozone layer depletion will continue even after the release of ozone-destroying chemicals is limited or halted. It takes six to eight years for some of these compounds to reach the upper atmosphere, and once there, they will destroy ozone for another 20–25 years. Thus, even if all emissions of destructive chemicals were stopped, compounds already released would continue to damage the ozone layer for another quarter century.

Understanding ozone depletion

As detailed collection of data about interactions in the stratosphere progresses, the observational support for the ozone depletion theory continues to grow more compelling. Yet atmospheric scientists are beginning to realize that their understanding of the upper atmosphere is still quite crude. While certain key reactions that maintain and destroy ozone are theoretically and observationally supported, scientists will have to comprehend the interaction of dozens, if not hundreds, of reactions between natural and artificial species of hydrogen, nitrogen, bromine, chlorine, and oxygen before a complete picture of ozone-layer dynamics emerges. The eruption of Mt. Pinatubo, for example, made scientists aware that heterogenous processes—those reactions which require cloud surfaces to take place—may play a far greater role in causing ozone depletion than originally believed. Such reactions had previously been observed taking place only at Earth's poles, where stratospheric clouds form during the long winter darkness, but it is now thought that sulfur aerosols ejected by Pinatubo may be serving as catalysts to speed ozone depletion at nonpolar latitudes.

Ozone-depleting reactions are best understood around the thinly inhabited polar regions, where stable and isolated conditions over the winter allow scientists to understand stratospheric changes most easily. In contrast, at the temperate latitudes where constantly moving air masses undergo no seasonal isolation, it is difficult to determine whether a fluctuation in a given chemical's density is a result of local reactions or atmospheric turbulence. It is hoped that increasingly detailed measurements using a new generation of equipment (such as NASA's Perseus remote-control aircraft) will begin to shed more light on the processes occurring away from the poles. Joe Waters of NASA's Jet Propulsion Laboratory summarizes the urgent task: "We must be able to lay out the catalytic cycles that are destroying ozone at all altitudes all over the globe—from its production region in the tropics to the higher latitudes and the polar regions."


Natural phenomena affecting air quality

The concentration of atmospheric pollutants observed at different locations depends on more than just the quantity of pollutants emitted at the various sources. The atmosphere is the agent that transports and disperses pollutants between sources and receptors. Consequently, the state of the atmosphere helps to determine the concentrations of pollutants observed at receptors. Unlike emissions sources, which can be controlled, the state of the atmosphere is not at present susceptible to human control.

Some skill has been attained, however, in predicting the future state of the atmosphere. Since meteorological conditions that favor high concentrations of pollutants are known, severe air pollution episodes can therefore be forecast.

In general, three parameters are used to describe atmospheric transport and dispersion processes. These are wind speed, wind direction, and atmospheric stability. For emissions at a given source, a higher wind speed provides the pollutants with a greater air volume within which to disperse. This causes ground level pollutant concentrations, other things being equal, to be inversely proportional to wind speed.

Horizontally, the wind direction is the strongest factor affecting pollutant concentrations. For a given wind direction, nearly all the pollutant transport and dispersion will be downwind. Wind direction determines which sector of the area surrounding a source will receive pollutants from that source.

Atmospheric stability directly affects the vertical dispersion of atmospheric pollutants. Unlike wind direction and wind speed, atmospheric stability cannot be measured directly. Atmospheric stability is a measure of air turbulence and may be defined in terms of the vertical atmospheric temperature profile. When the temperature decreases rapidly with height, vertical motions in the atmosphere are enhanced, and the atmosphere is called unstable. An unstable atmosphere, with its enhanced vertical motions, is more effective for dispersing pollutants, and because of the large volume of air available for the spread of pollutants, ground-level concentrations can be relatively low. When the temperature does not decrease rapidly with height, vertical motions are neither enhanced nor repressed and the stability is described as neutral. Under these conditions, pollutants are also allowed to disperse vertically in the atmosphere, although not as rapidly as when it is unstable.

When the temperature decreases very little, remains the same, or increases with increasing height, the atmosphere is called stable. Under these conditions, the atmosphere inhibits the upward spread of pollutants. Upward-moving smoke, which rapidly assumes the temperature of the surrounding air, reaches a point where it is colder, and hence denser, than the air above it, so it can rise no further. This suppression of upward motion effectively forms a lid beneath which pollutants can disperse freely. The weaker the temperature decrease with height, the higher the lid is. The extreme case is an inversion, when the temperature increases with height. Often, clouds are topped by a stable or inversion layer, which stops their vertical growth.

The well-mixed layer beneath a stable layer is called the mixing layer. When it extends to the ground its vertical extent is known as the mixing height or the mixing depth. Generally, turbulence is enhanced in the early morning hours as the sun heats the ground and temperature decreases with height, causing unstable conditions. At night, as the earth cools, temperature increases with height causing less turbulence and stable atmospheric conditions.

Wind speed, wind direction, and atmospheric stability will vary greatly with time. For a certain location, some combinations occur more frequently than others.

Where detailed meteorological records have been kept for a year or more, a stability wind rose can be calculated. This wind rose is a set of tables, one for each stability class (ranging from very stable to very unstable), listing the frequency of occurrence of all possible combinations of wind speed and wind direction. Such roses are available for many locations in the United States from the National Climatic Data Center in Asheville, North Carolina. It should be noted that topographical features such as mountains, hills, valleys, bodies of water, buildings, and other terrain features can change airflow patterns, resulting in unexpected pollution effects.

Near a large body of water, local sea breezes influence the spread of pollutants. Early in the morning, when the air is still or the wind is off the land, pollutants can accumulate over their sources or downwind of them. Later in the day, when a local sea breeze develops, a fresh breeze blows in the direction from the water toward land. This breeze brings with it not only the pollutants emitted from the sources at this time of day, but also those accumulated earlier in the day, because they are carried back from water to land. Unexpectedly high pollutant concentrations can occur near the shore when the high pollutant loading blows past. In addition to this effect, which generally occurs close to land, the sea breeze itself can penetrate as far inland as 40 mi or more.

Mountains and valleys have characteristic airflow patterns, too. In the evening, as the earth cools, the coldest air will sink into the lowest part of the valley. This creates a stable inversion layer because lighter, warmer air stays above the valley. In this way, pollutants are trapped in the valleys all night. During the daytime when heating occurs, the air in the valley is warmed and rises, permitting the pollutants to escape. Unfortunately, this heating and upward motion does not always occur. During periods when high pressure settles over a region and the air is stagnant, the atmosphere is stable all day long, and pollutants continue to accumulate in the valley. Some of the worst episodes of air pollution have occurred in mountain chains like the Appalachians, where industries are located in the valleys between adjacent hills.

In cities, buildings form the topography. Where rows of tall buildings front on narrow streets the air flows through the streets as though they were canyons. Since ventilation is determined by building configuration, many distortions in wind, and hence pollution flows, take place in a city. Air flows over a building and into a street downwind of it. The building, because the air cannot flow through it, creates an obstruction in the pattern of the smooth airflow. Downwind of the building, an eddy, or circular movement of air at variance with the main airflow, is formed in its wake. The eddy can trap pollutants emitted by cars in the street, and can cause concentrations of pollutants, for example, carbon monoxide, to be as much as three times higher on the side of the street further downwind than at the site of pollutant origin.

High air pollution potential advisories

High Air Pollution Potential Advisories (HAPPA) are prepared at the National Meteorological Center (NMC) in Suitland, Maryland, by meteorologists of the National Oceanographic and Atmospheric Administration (NOAA), U.S. Department of Commerce.

Advisories are based both on reports received hourly via teletype from National Weather Service stations in the United States and on numerous analyses and forecasts prepared by the NMC. With its electronic computer facilities, the NMC prepares mixing-depth and wind-speed data from all upper-air-observing stations in the contiguous United States (about 70 stations). These data are analyzed, interpreted, and integrated with other meteorological information.

National air pollution potential advisories based on these data are transmitted daily at 12:20 p.m., EST, to Weather Service stations. When meteorological conditions do not warrant issuance of a HAPPA, the teletype message is "none today." When the forecast indicates that an advisory of high air pollution potential should be issued, the message designates the affected areas. The daily message indicates significant changes in the boundaries of advisory areas, including termination of an episode.

Because conditions of atmospheric transport and dispersion typically vary with location and time, the forecasting staff cannot prepare advisories for each city in the United States. For this reason, the NOAA meteorologists limit their forecasts to areas at least as large as 75,000 mi2; roughly the size of Oklahoma, in which stagnation conditions are expected to persist for at least 36 hours. Individual Weather Service stations may modify these generalized forecasts on the basis of local meteorological conditions.

Users of the service should realize that boundaries of the forecast areas of high air pollution potential cannot be delineated exactly. For practical purposes, the lines defining the advisory area should be interpreted as bands roughly 100 mi wide.

To be notified of these advisories, air pollution control or research officials must initiate arrangements with the nearest Weather Service station.


The U.S. Environmental Protection Agency (EPA) has announced that the agency is working with the agricultural community to control water pollution from the nation's largest livestock operations while keeping American agriculture viable. The EPA has joined the Agriculture Department in announcing a final rule that will require all large Concentrated Animal Feeding Operations (CAFOs) to obtain permits that will ensure they protect America's waters from wastewater and manure. The rule will control runoff from agricultural feeding operations, preventing billions of pounds of pollutants from entering America's waters.

The EPA looks forward to continuing to work with USDA and with the agricultural community to ensure that the goal we all share—cleaner, purer water—is being advanced by their efforts. The new rule is unique in that it comes after unprecedented cooperation between EPA and USDA to find a way to help producers meet their own and society's goals for environmental quality and profitability. USDA stands ready to provide assistance in an incentive-based approach combining information and education, research and technology transfer, direct technical assistance, and financial assistance through the Environmental Quality Incentives Program (EQIP) and other farm bill programs.

The December 2002 announcement finalizes a rule that will replace 25-year old technology requirements and permitting regulations that did not address today's environmental needs and did not keep pace with growth in the industry. Effective manure management practices required by this rule will maximize the use of manure as a resource for agriculture while reducing adverse impacts on the environment.

The new rule applies to about 15,500 livestock operations across the country. Under the new rule all large CAFOs will be required to apply for a permit, submit an annual report, and develop and follow a plan for handling manure and wastewater. In addition, the rule moves efforts to protect the environment forward by: placing controls on land application of manure and wastewater, covering all major animal agriculture sectors, and increasing public access to information through CAFO annual reports. The rule also eliminates current permitting exemptions and expands coverage over types of animals in three important ways: the rule eliminates the exemption that excuses CAFOs from applying for permits if they only discharge during large storms; second, the rule eliminates the exemption for operations that raise chickens with dry manure handling systems; and third, the rule extends coverage to immature swine and immature dairy cows.

Currently about 4,500 operations are covered by permits. Because of the new rule, EPA expects that up to 11,000 additional facilities will be required to apply for permits by 2006. This rule will enhance protection of the nation's waters from nutrient over-enrichment and eutrophication, which causes algal blooms, fish kills, and the expansion of the Gulf of Mexico dead zone. The rule will also reduce pathogens in drinking water and improve coastal water quality. The amount of phosphorus released into the environment will be reduced by 56 million pounds, while nitrogen releases will be slashed by more than 100 million pounds. In addition, over two billion pounds of sediments and nearly one million pounds of metals will not be released.

The new rule will affect large livestock operations including those with hundreds of thousands of hogs, cattle, and poultry. Large CAFOs are defined in the rule as operations raising more than 1,000 cattle, 700 dairy cows, 2,500 swine, 10,000 sheep, 125,000 chickens, 82,000 laying hens, and 55,000 turkeys in confinement. Approximately 500 million tons of manure are generated annually by an estimated 238,000 livestock operations. From 1982 to 1997 these large livestock operations have grown by 51%, with some of the largest facilities having capacities exceeding one million animals. Since 1978 the number of animals per confined-animal operation has increased significantly. The largest per operation increases have been: layers (176%), broilers (148%), swine (134%), turkeys (129%), dairy (93%), and beef cattle (56%).

To help these livestock operations meet the rule's requirements, Congress increased funding for land and water conservation programs in the 2002 Farm Bill by $20.9 billion, bringing total funding for these programs to $51 billion over the next decade. The Environmental Quality Incentives Program (EQIP) was authorized at $200 million in 2002 and will ultimately go up to $1.3 billion in 2007; 60% of those funds must go to livestock operations. New technology is also being perfected to aid farmers in meeting this new rule. States are being given significant flexibility to find geographically appropriate means of implementing the CAFO rule. For example, states retain the authority to determine the type of permit—general or individual—to be issued to a given operation. This enables states to develop permits that take into account the size, location, and environmental risks that may be posed by an operation. States will also have substantial flexibility to tailor nutrient management plans for CAFOs, and may authorize alternative performance standards for existing and new CAFOs that will help promote the use of innovative technologies.


The U.S. Army Corps of Engineers and the U.S. Environmental Protection Agency, in conjunction with the Departments of Agriculture, Commerce, Interior, and Transportation, have strengthened their commitment to achieve the goal of no net loss of our nation's wetlands with the release of a comprehensive action plan and improved guidance to ensure effective, scientifically based restoration of wetlands impacted by development activities. The Corps regulatory guidance and the multi-agency action plan will help advance technical capabilities for wetlands restoration and protection, as well as clarify policies to ensure ecologically sound, predictable, and enforceable wetlands restoration completed as part of Clean Water Act and related programs. Both actions are the result of extensive multi-agency collaboration.

The National Wetlands Mitigation Action Plan lists 17 action items that the agencies will undertake to improve the effectiveness of restoring wetlands that are impacted or lost to activities governed by clean water laws. Completing the actions in the plan will enable the agencies and the public to make better decisions regarding where and how to restore, enhance, and protect wetlands; improve their ability to measure and evaluate the success of mitigation efforts; and expand the public's access to information on these wetland restoration activities.

A revised Regulatory Guidance Letter leads the list of action items in the National Wetlands Mitigation Plan. Crafted with input from the federal agencies that play a role in wetlands protection, the Corps Regulatory Guidance Letter will improve wetlands restoration implemented under the Clean Water Act in support of the Administration's "no net loss of wetlands" goal.

In order to advance the goal of no net loss of wetlands, the guidance letter emphasizes the following:

  • A watershed-wide approach to prospective mitigation efforts for proposed projects impacting wetlands and other waters;
  • The increased use of functional assessment tools; and
  • Improved performance standards.

In addition, the guidance letter emphasizes monitoring, long-term management, and financial assurances to help ensure that restored wetlands actually result in planned environmental gains. The guidance letter also provides greater consistency across the Corps' 38 district offices on issues such as the timing of mitigation activities and the party responsible for mitigation success.

Recent independent evaluations published in 2001 by the National Academy of Sciences (NAS) and the General Accounting Office (GAO) reviewed the effectiveness of wetlands compensatory mitigation for authorized losses of wetlands and other waters under Section 404 of the CWA. In its study the NAS concluded that, despite progress in the last 20 years, the goal of no net loss of wetlands is currently not being met for wetland functions by the compensatory mitigation programs of federal agencies. The action plan and guidance were developed in response to, and are consistent with, the recommendations made in those reports.

"Wetlands" is a collective term for marshes, swamps, bogs, and similar areas that filter and cleanse drinking water supplies, retain flood waters, harbor extensive fish and shellfish populations, and support a diverse array of wildlife. In performing these functions, wetlands provide invaluable ecosystem services. Consequently, their destruction increases flooding and runoff, harms neighboring property, causes stream and river pollution, and results in the loss of valuable habitat.

The agencies are committed to achieving the goal of no net loss of wetlands under the regulatory program and are hopeful of attaining in the near future an increase in the overall function and value of the nation's wetlands. This is especially important in light of the fact that, since the late 1700s, over half the nation's wetlands have been lost to development and other activities. These losses are wide-spread—almost half of all states have lost more than 50% of their historic wetland resources.

The CWA prohibits the discharge of dredged or fill material into regulated wetlands and other waters of the United States unless a permit is issued under Section 404 of the CWA authorizing such a discharge. The Corps makes decisions regarding Section 404 permit requests after it completes a careful environmental review of the impacts of proposed discharges, including the potential adverse effects on wetlands. This permit program is designed to avoid impacts to wetlands where possible and minimize these impacts when they are unavoidable. However, if a permit is issued for a project that will result in a loss of wetlands, compensatory mitigation is necessary to replace those lost wetlands. EPA leads the development of the environmental criteria used to evaluate proposed discharges under the CWA.

In addition to the Corps of Engineers and EPA, the Department of Commerce's National Oceanic and Atmospheric Administration, the Department of Interior, and the Department of Transportation implement programs involving the restoration of wetlands and other aquatic resources. In combination with the Department of Agriculture's Wetlands Reserve and Conservation Reserve Programs, these restoration efforts are expected to take the country from annual net wetlands loss to net wetlands gain.


The U.S. has the safest drinking water in the world: 91% of people served by public water systems now drink water meeting all federal health standards—up from 79% in 1993. Even so, there is a lot we need to do to make sure water is safe for everybody.

No matter where we live, our drinking water originates in a watershed, a land area that drains to a single body of water that may be surface water or groundwater. These watersheds are constantly under siege from multiple threats. As rain washes over roofs, pavement, farms, and grassy areas, and as snow melts and soaks into the ground, it picks up pollution and deposits it into surface water and groundwater.

As our population expands, our need for food, shelter, clothing, electricity, and recreation places more demands on our water supply. As the number of households and businesses increase, so does the amount of natural resources we consume and the amount of waste we produce. These are just a few of the activities that create pollution that can enter our drinking water sources:

  • Over-application and abuse of pesticides and fertilizers—67 million pounds of pesticides annually;
  • Overburdened land fills—230 million tons annually; 5 pounds per person per day;
  • Huge volumes of animal waste—half a million animal factory farms produce 130 times the amount of waste of the human population; and
  • Careless or ignorant activities at home, work, and play—12 million recreational and house boats and 10,000 boat marinas release solvents, gasoline, detergents, and raw sewage directly into waterways.

This pollution is caused by humans, but choices we make in our communities and as individuals can help eliminate it and greatly reduce threats to our drinking water. Four basic protective barriers help keep water safe to drink:

Prevention: Keep contaminants out of the drinking water source to protect the environment and reduce the need for costly treatment.

Risk management: Support your local utilities. Your public water system makes sure pollution that has entered source water is removed before it is distributed to the community. Water utilities treat nearly 34 billion gallons of water daily. The total miles of water pipeline and aqueducts equal approximately one million miles—enough to circle the globe 40 times.

Risk and compliance monitoring: Learn about your drinking water quality. Our communities constantly monitor water quality—at the source, at the treatment plant, in the distribution system that delivers water to our homes, and, in some cases, at the tap. Your local water system can provide you with this information. If you receive water from a private well, make sure it is tested annually.

Individual action: The actions we take as individuals really do add up when it comes to protecting our water.

  • Be informed! Read the annual Consumer Confidence Report provided by your water system.
  • Be involved! Speak up at public hearings on land use and permitting.
  • Be observant! Report any suspicious activities in or around your water supply to local authorities or call 911 immediately. Look for announcements in the local media for activities that could pollute your source water.
  • Don't contaminate! Reduce or eliminate pesticide application. Reduce the amount of trash you create. Recycle used oil. Reduce paved areas. Keep pollutants away from boat marinas and waterways.