BACKGROUND

Air pollution is the contamination of air by smoke and harmful gases. These airborne pollutants can have serious negative effects on both indoor and outdoor air quality.

According to the Environmental Protection Agency (EPA), air pollution can have a big impact on indoor air quality, which may affect a person's health. The World Health Organization (WHO) estimates that 2.4 million people worldwide die annually, due to the effects of air pollution. In America alone, hundreds of thousands of people die every year from heart and lung damage caused by toxic air pollutants. In 1998, a study published in the journal Environmental Research estimated that $14-55 billion is spent annually on the human health costs related to outdoor air pollution.

There are many sources of toxic, or hazardous, air pollution. Six air pollutants actually represent the biggest health threats to the public. Known as "criteria pollutants," these are: lead (Pb), particulate matter (PM), nitrogen oxides (NOx), sulfur dioxide (SO2) gas, ground-level ozone, and carbon monoxide (CO). Researchers have estimated that the damage caused by these pollutants may range from $75-280 billion annually.

Toxic air pollutants are hard to completely avoid, as they are released by common, everyday sources. For instance, gasoline, dry cleaning fumes, paint and paint remover, and common metals all produce potentially harmful air pollutants. Asbestos and mold can release indoor air pollutants that have negative health effects as well.

Not all air pollutants are equally harmful. Many pollutants can be tolerated by the body, with little chance of affecting overall health. For instance, while aerosol sprays can be used occasionally, their long-term use may damage the environment. Products containing CO, such as paint or paint removers, can also be tolerated in limited amounts. These products should be properly stored and require sufficient ventilation. It is important to note that a person's risk for developing health problems depends on their level of exposure to and concentration of pollutants.

Toxic air pollutants can affect the quality of soil and water, and therefore, nearby plants, fish, and other wildlife. When humans consume contaminated sources, they are more susceptible to dangerous health conditions. Air pollutants target the body's cells interfering with normal body functions. This may result in such health concerns as: eye, nose, and throat irritation, headaches, allergies and asthma, nausea and vomiting, dizziness, fatigue, kidney damage, vision problems, cancer, damaged organs, and neurological, respiratory, immune, reproductive, developmental, and cardiovascular problems.

Many experts believe that air pollution contributes to global warming by trapping greenhouse gases, including carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFCs), in the atmosphere. Greenhouse gases contribute to global warming by allowing sunlight to reach the planets' surface directly and trapping this energy in the atmosphere thus heating up the planet's temperature.

Early history of air pollution: Toxic air pollution is not a new development. In 1273, King Edward I of England banned imported coal. This was considered the first recorded instance of a law against air pollution. During medieval times, burning coal was England's main heat source. This led to excessive amounts of thick, black smoke emitted from chimneys. Proclamations were passed limiting coal-burning. But as the levels of coal-related air pollution continued to affect the nation's towns, these laws proved ineffective.

The industrial revolutions in Europe and the United States were both thought to have increased toxic air pollutant levels. This was likely due to the large number of factories emitting toxic air pollutants.

Events in US air pollution: Toxic air pollutants have been a problem in the United States for more than a century. During the early 1900s, Chicago became the first American city to pass an ordinance to reduce factory smoke. Los Angeles (LA) has been particularly affected by environmental air pollution. In 1943, the city was gripped by severe air pollution events, known as "gas attacks." Visibility was dramatically reduced, and residents suffered from eye and respiratory discomfort, nausea, and vomiting. These attacks occurred because LA is located in a basin surrounded by mountains. Auto exhausts and local refinery emissions accumulate over the city and cannot escape over the mountains. In 1945, LA established its air pollution control program, the Bureau of Smoke Control. Still, the area's air pollution problems continued. On one single day in 1954, smog resulted in 2,000 auto accidents.

From October 26-31, 1948, a six-day temperature increase caused severe air pollution in Donora, Pennsylvania. Twenty residents died, 600 became ill, and 1,400 required medical attention. A similar event occurred in New York City in 1968, killing 168 people. In 1978, the most acidic rainfall ever, 5,000 times more acidic than normal, was recorded in Wheeling, West Virginia. In terms of acidity, the lower the number, the worse the acid rain's effects. Normal rainfall measures a level of five or six in acidity. However, the acid rain in Wheeling measured 2.2, which is considered very harmful to living organisms.

In 1962, Rachel Carson's Silent Spring was published. Many experts consider it a cornerstone of the environmental movement. This best-selling book discussed the harmful effects that pollution and pesticides have on the environment, especially for birds, As a result of the book, the pesticide dichlorodiphenyltrichloroethane (DDT) was banned in 1972.

On April 22, 1970, the first Earth Day was held nationwide in the United States, bringing attention and recognition to the environmental movement. Earth Day, created by Wisconsin Senator Gaylord Nelson, was set aside as a learning tool for helping the environment.

A 1982 study conducted by the National Center for Health Statistics showed that 4% of all U.S. schoolchildren had high levels of lead in their blood. In addition, the study found that an estimated 675,000 were considered at risk for lead poisoning-related conditions, such as kidney and brain damage and anemia. Anemia occurs when there is a lack of iron or the protein hemoglobin in the blood. The condition results in decreased oxygen delivered throughout the body.

By 1985, studies showed that due to the effects of acid rain, living organisms were dying in an estimated 50,000 American and Canadian streams. A National Academy of Sciences study found that burning fossil fuels, such as coal and gasoline, was linked to acid rain. Burning fossil fuels has also been linked to the deaths of plants and fish in American and Canadian wildlife, as well as the polluting of lake ecosystems.

First passed in 1963, the U.S. Clean Air Act was reauthorized in 1990. The new initiatives in this law make up one of the nation's most far-reaching efforts to reduce air pollution and conserve energy. The goal was to improve air quality by 2005. While it was expected that some of the provisions would take a while to be implemented, there are positive signs that the act is working. According to the EPA, every time a source of air pollution is reduced or eliminated, that is evidence of the Clean Air Act in action. For example, when factories and other industrial facilities have pollution control equipment installed, the level of toxic air pollutants falls. The Clean Air Act also calls for the agency to continually monitor the nation's air quality. The EPA measures the levels of six harmful "criteria pollutants," both for their levels in the air and those emitted from polluting sources. These pollutants (lead particulate matter, nitrogen oxides, sulfur dioxide gas, ground-level ozone, and carbon monoxide) are monitored annually. Their toxic levels are then posted on the EPA's Web site for the public to review. The Clean Air Act also helped to bring about advances to cars, trucks, buses, and other vehicles. Technological boons, such as refined, unleaded gasoline and cleaner cars, have proven very successful at helping to reduce air pollution. Through actions like these, the EPA and other U.S. government organizations are making efforts to eliminate toxic air pollution. However, the EPA and other agencies are aware that there is still much work to be done. Among the new initiatives are: stricter timelines and guidelines for meeting air quality standards, tougher motor vehicle emission standards, requirements for producing alternative (clean-burning) fuels and vehicles, new standards for toxic pollutants, and stronger enforcement authority for the EPA. They also call for efforts to reduce acid rain and hazardous chemical compounds known as CFCs. It is expected that the guidelines would take about 29 years to fully implement.

The efforts to continue the progress of the Clean Air Act have continued. In May 2007, President Bush issued an executive order to reduce motor vehicles' greenhouse gas emissions. In addition, the president passed a bill calling for a 20% reduction in gasoline consumption over the following 10 years. This proposal involved two steps. First, it set a mandatory fuel standard. By 2017, 35 billion gallons of renewable and other alternative fuels have to be in use. Second, it continues to increase fuel efficiency standards for cars and trucks.

Events in international air pollution: In 1952, an unusual five-day temperature increase resulted in a dangerous blend of fog and coal-related air pollutants, known as the "Great London Smog." This resulted in the deaths of more than 4,000 people. 1956 saw a similar incident, claiming 1,000 lives. In 1962, another 700 died due to air pollution. To combat air pollution, London designated smokeless zones in urban areas. The city also constructed tall chimneys to disperse pollutants higher into the air.

The United Kingdom passed stronger industrial controls in the first half of the 20th Century, culminating in the passing of the first Clean Air Act in 1956. This initiative called for areas in which smokeless fuels had to be burned and relocated power plants to rural settings. In 1968, this act was updated to encourage the use of tall chimneys to disperse polluted air. Urban smog was reduced, but pollution continued to be a growing problem. Smog is a brown, murky haze made up of ozone, smoke, nitrogen, and hydrocarbons that are exposed to sunlight, high temperatures, and humidity. Smog may cause serious respiratory problems and impaired visibility.

In 1992, the Earth Summit was held in Rio de Janeiro, Brazil. It was considered the world's most comprehensive international environmental conference at the time. Representatives from 188 countries attended, and two treaties were signed. One treaty, the United Nations Framework Convention on Climate Change (UNFCCC or FCCC) addressed global warming and recommended curbing greenhouse gas emissions. The other, the Convention on Biological Diversity, focused on protecting endangered plant and wildlife species.

The United States decided not to sign the Biological Diversity Treaty for various reasons. First, the United States objected to the treaty's language in regard to the patenting of products made from natural biological resources, such as plant chemicals. It was thought that this decision would favor Western countries to receive technological patents, rather than developing countries. Next, the United States objected to a section of the treaty that dealt with funding for conservation projects in developing countries. It was believed that this did not provide enough financial control of contributed funds to care workers and other donors of aid. Finally, the United States believed that American countries would be hurt by the safety rules designed to control the export of genetic engineering products.

While toxic air pollution can affect every human and animal in every part of the planet, developed, urbanized cities are both the sources and the victims of many pollutants. However, hazardous pollutants have been shown to be transported by wind and rain fall, through bodies of water, and even through the ground. Even remote areas and natural settings, like national parks, have been affected by air pollution. Several local, national, and international organizations help regulate air pollution. Individual countries have established their own standards for air pollution. In the United States, the Environmental Protection Agency (EPA) and other organizations work to reduce and eliminate pollutants. Although the United Nations (UN) does monitor particularly harmful situations, environmental protection is not its main goal.

Any advanced or developing region produces higher levels of air pollutants. But certain parts of the world have been found to be responsible for increased air pollution. China, in particular, has experienced explosive growth and development. This means more factories are releasing pollutants. Research shows that about 14,000 new vehicles are being driven on China's roads each day; it's estimated that by 2020, the country will have 130 million vehicles being driven. The nation has taken steps to reduce its pollution, but it remains to be seen if these efforts are successful.

TECHNIQUE

Outdoor air pollution: Toxic and hazardous air pollutants are made up of a blend of gases and solid particles suspended in the air. The leading cause of toxic air pollution is compounds (also known as oxides) of carbon, sulfur, and nitrogen. Other leading sources of air pollutants are vehicle exhausts, factory/industrial emissions (such as mercury or carbon monoxide), dust, pollen, and mold spores. Burning trash and backyard fire pits also contribute to environmental air pollution.

The most common outdoor air pollutants include: aerosols, carbon monoxide (CO), chlorofluorocarbons (CFCs), hazardous air pollutants (HAPs), hydrochlorofluorocarbons (HCFCs), lead (Pb), mercury (Hg), methane, nitrogen oxides (NOx), ground-level ozone, ozone substitutes, particulate matter (PM), radon, sulfur dioxide (SO2), and volatile organic compounds (VOCs).

Indoor air pollution: Research shows that the majority of people are indoors more than 90% of the time. Therefore, the indoor air quality in homes, offices, and schools may represent a bigger health threat than outdoor air pollution. Indoor air pollution is caused by gases or particles released inside. Poor ventilation makes the contamination worse because there is not enough outdoor air to dilute or remove the pollutants. High temperatures and humidity can also increase the concentration.

Among the most common indoor air pollutants are: aerosol sprays, mold, fireplace smoke, central heating, cooling and humidification systems, and the fumes released by furnaces and space heaters. Many furnaces and space heaters use the process of combustion (involving burning fuels) to produce heat. Their fumes contain such toxic air pollutants as carbon monoxide (CO), nitrogen dioxide (NOx), and particulate matter (PM). Combustion sources, such as wood- or coal-burning, gas/kerosene, or oil stoves, also release air pollutants. Outdoor air pollutants such as pesticides, vehicle exhaust, and factory/industrial emissions are leading sources of both outdoor and indoor pollution.

Some indoor air pollutants can be eliminated, but many pollutants are released continuously. Major modifications to a home or even relocation may be necessary to completely remove them. Building materials and furnishings, such as some floor tiling, carpets, or furniture or cabinetry made from pressed wood, may contain asbestos and may release hazardous air pollutants. Today, asbestos has been banned or partially banned in many countries. In 1989, the Environmental Protection Agency (EPA) announced an asbestos ban. However, this ban was overturned in 1990 by a U.S. Circuit Court of Appeals. As such, the United States has a partial ban on asbestos. Today, asbestos has been found in many areas and is actually still in use. According to studies, an estimated 30 million U.S. homes and schools may contain asbestos. This partial U.S. ban is actually considered very ineffective according to some, as the EPA has stated that there is no safe level of asbestos exposure.

THEORY/EVIDENCE

General: Air pollution has been shown to affect people in different ways. Air pollutants are usually inhaled when breathing or absorbed through the skin. Toxic pollutants can contaminate fish living in contaminated waters, fruits and vegetables grown in contaminated soil, and may contaminate meat, eggs, and milk from animals consuming toxic plants.

Within the body, hazardous air pollutants can accumulate in tissue for long periods of time. Pollutants affect different organs and body parts. For example, asbestos remains in the lungs. Over time, the levels of toxicity can increase or decrease. Toxic air pollutants can interfere with normal body functions, especially within the body's cells. This may result in such health problems as birth defects, damaged organs, and cancer.

Electricity generation is considered a leading source for toxic air pollution. Studies show that generating electricity gives off very high levels of four of the nation's major pollutants: carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxides (NOx), and mercury (Hg). Among the environmental concerns associated with these pollutants are smog, acid rain, mercury contamination, respiratory illness, and global warming. With global warming, greenhouse gases allow sunlight to reach the planets' surface directly. They then trap this energy within the atmosphere, heating up the planet's temperature.

According to the World Health Organization (WHO), global warming may result in food shortages; the spread of diseases; and dramatic climate changes, including higher temperatures and increased hurricanes, flooding, and droughts. Global warming is thought to result in changing climates and warmer temperatures. Medical experts believe these changes will contribute to the rise in deadly diseases and a direct loss of life. Warmer temperatures have been linked with increases in air and water pollution as well. These rising temperatures may seriously reduce or kill off many plants and animals, as well as crops. This may potentially lead to food shortages worldwide.

A study published in the Annual Review of Public Health found that the number of vehicles on the road has increased over the last 50 years and will likely continue to do so, especially in developing countries. Vehicle emissions are a main source of urban air pollution and are the world's fastest-growing source of greenhouse gases.

Acid rain: Airborne pollution can lead to acid rain. This can cause extensive damage to waterways, trees, plants and crops, and structures and vehicles and increase the acidity in soil.

Aerosols: Aerosols are substances stored under pressure that are released as particles suspended in the air. "Propellants" are aerosols released from pressurized containers. "Solvents" are aerosols typically used for cleaning that dissolve into liquids or solids. Besides being flammable, aerosols have been found to contribute to the depletion of ozone. The ozone layer, which surrounds the earth about 10-15 miles above the surface, protects living organisms from the sun's harmful ultraviolet (UV) rays. The reduction of ozone is a leading factor in trapping greenhouse gases. These gases allow sunlight to reach the planets' surface directly. They then keep this energy within the atmosphere, heating up the planet's temperature. Most industrial aerosols are manufactured with organic solvents.

Allergens: Insect matter (especially from cockroaches), animal matter (or "dander"), and dust mites are major sources of indoor air pollution.

Asbestos: The asbestos in insulation has been classified as a very harmful pollutant. Asbestos is made up of tiny fibers that if inhaled, may scar the lungs.

Carbon monoxide (CO): CO is a colorless, odorless gas produced when materials containing carbon are burned. Without ventilation, carbon monoxide emissions can build up. The fumes emitted by CO can be very harmful; CO is the nation's leading cause of accidental poisoning deaths.

Chlorofluorocarbons (CFCs): Compounds of chlorine, fluorine, and carbon (CFCs) are usually employed as solvents or foam-blowing agents (for fighting fires). They are also employed as refrigerants, which provide the cooling ability of refrigerators and air conditioners. In the layer of the atmosphere known as the troposphere, CFCs are very stable. But when they move to the layer known as the stratosphere, they are broken down by strong ultraviolet (UV) light. CFCs can then deplete the ozone layer, allowing stronger concentrations of sunlight to heat up the planet's surface. In 1995, the United States banned the production of CFC-11, the first and most popular CFC.

Today, CFCs have been almost entirely banned, although some are occasionally used for air conditioning and refrigeration systems. But even these are gradually being replaced by more environmentally friendly alternatives, known as hydrochlorofluorocarbons (HCFCs). HCFCs are synthetic compounds designed to replace CFCs. They are comprised of hydrogen, chlorine, fluorine, and carbon. HCFCs have been found to offer 98% less risk to the ozone layer than CFCs.

Although they perform the same functions as CFCs, HCFCs are more expensive to produce. They are also major greenhouse gas sources. As such, many countries have agreed to outlaw HCFCs by the year 2040. CFCs were long used in inhalers for medical conditions, such as asthma, a long-term, inflammatory lung disease; however, they are now being phased out for this use.

Criteria pollutants: With the passing of the Clean Air Act in 1963, the Environmental Protection Agency (EPA) was charged with monitoring the build-up of six common air pollutants: particle pollution (PM), ground-level ozone, carbon monoxide (CO), sulfur oxides (SO2), nitrogen oxides (NOx), and lead (Pb). These pollutants are also referred to as "criteria pollutants." Studies illustrate that these six pollutants may cause serious harm to human and animal health and to the environment. Criteria pollutants can also cause major damage to structures and property.

The EPA measures the levels of these pollutants in two ways: by tracking outside air concentrations at selected American monitoring sites and by tracking the estimated emissions of the total tons of airborne pollutants annually. To accomplish this, the EPA estimates the amount of outdoor air pollutants released nationwide. These estimates are based on the actual monitored readings of the levels of pollutants released by factories, vehicles, and other sources. If there are no actual readings, the EPA estimates these levels. They utilize such factors as the levels of industrial activity, technological developments, fuel consumption, vehicle miles traveled, and other activities that may cause air pollution.

Hazardous air pollutants (HAPs): The U.S. government has identified 188 toxic air pollutants shown to affect human health and the environment. HAPs include both chemical compounds (such as benzene and methylene chloride, both used in industrial practices), and minerals (such as cadmium and mercury). These pollutants are released by a variety of sources. For instance, the harmful chemical compound benzene is emitted from gasoline fumes. Another compound, perchloroethlyene, can be released from dry cleaning facilities. These particular compounds have been found to cause a variety of health effects, including: drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, unconsciousness, and eventually death.

Hydrochlorofluorocarbons (HCFCs): HCFCs are man-made compounds that were designed to replace banned CFCs. They are comprised of hydrogen, chlorine, fluorine, and carbon. HCFCs perform the same functions as CFCs but are more expensive to produce. HCFCs have been found to offer 98% less risk to the ozone layer. However, HCFCs have been found to produce large amounts of greenhouse gases. As such, many countries have agreed to outlaw HCFCs by the year 2040. A variety of healthier alternatives have been considered, such as using carbon dioxide (CO2) for heating water and ammonia for refrigeration. However, it will take time to locate and remove all of the equipment and systems using HCFCs. These modifications may incur excessive costs, which is another reason for the banning deadline being set 30 years in the future.

Lead (Pb): A naturally occurring metal, lead has been found to be a major source of toxic air pollution. Lead is typically released by motor vehicles, power plants, utility plants (such as electricity-generating and sewage treatment centers), waste incinerators (in which organic materials are burned), contaminated soil and dust, and plumbing materials (and therefore drinking water). Lead has long been used in paint and other home-based products. Many people may be at risk for lead poisoning, especially children six years old and younger. Once it enters the body, lead can travel all over, and it accumulates in bones. Depending on exposure, lead may lead to serious health effects.

The EPA's efforts to remove lead from gasoline resulted in a 95% decrease of leaded gasoline levels from 1980-1999. In addition, air-based lead levels decreased by 94% during these years and have remained stable.

However, the high levels of lead released into the atmosphere have caused a lot of damage that still lingers today. The automotive industry has been found to produce the majority of lead pollution. Research shows that in North America alone, this industry releases over 300 million pounds of lead. Among the sources of this lead are manufacturing, mining, the recycling and disposing of lead-containing automotive components (such as batteries), and especially normal vehicle use.

Most lead pollution today is caused by leaded paint and leaded gasoline, although it is in the process of being removed in most countries. However, there are still potential sources of lead pollution. For instance, lead bullets fired over the years continue to represent a health risk. The recent development of Chinese toys causing lead poisoning shows that efforts must still be made to reduce and eliminate these risks.

Mercury (Hg): Mercury is a naturally occurring toxic metal that has been found to be very harmful to human and animal health. Mercury may also be associated with serious environmental damage. Mercury is primarily released by coal-burning, incinerating solid wastes, chlorine production, and the improper treatment and disposal of mercury products. In its pure, liquid form, mercury, or "quicksilver," is used to produce thermometers, switches, light bulbs, and other industrial products. Liquid mercury is generally not harmful. But mercury vapors can be very dangerous if inhaled. These vapors are released into the air when products break. In the case of mercury thermometers, they have been phased out in the United States. Both the American Academy of Pediatrics and the EPA recommend that alternative thermometers be used in the home, due to the potential health effects. Mercury vapors accumulate in warm or poorly-ventilated indoor spaces, and can contaminate water and soil.

Mercury can also transform into a much more toxic form called methylmercury. This more toxic form builds up in fish, crustaceans, and animals that eat fish. Depending on the exposure, mercury contamination can result in serious health effects. The EPA has been working to limit the nation's mercury levels and requires industries and state and local governments to properly treat and dispose of mercury waste.

On March 15, 2005, the EPA issued the clean air mercury rule. This permanently capped and reduced mercury emissions from coal-fired power plants. As a result, the United States became the first country to regulate utilities' mercury emissions.

Methane (CH4): A vital component of natural gas, methane is released from various natural sources. However, methane is very effective at trapping greenhouse gases and thereby contributes to global warming. Greenhouse gases allow sunlight to reach the planets' surface directly. They then trap this energy within the atmosphere, heating up the planet's temperature. It can remain in the atmosphere for about 9-15 years. Methane sources include coal-mining, landfills, natural gas and petroleum systems, agricultural activities, engine combustion, wastewater treatments, and industrial processes.

To reduce methane emissions, the EPA is working with many companies to implement cost-effective programs and technologies. These include the AgSTAR Program, a voluntary effort jointly sponsored by the EPA, the U.S. Department of Agriculture (USDA), and the U.S. Department of Energy (DOE). This program involves reducing the actual emissions of animals producing methane gas, such as cows. Another program is the Natural Gas STAR Program, a voluntary initiative for U.S. and international oil and gas companies. This program encourages the adoption of cost-effective technologies and practices that have been found to reduce methane emissions, while improving overall efficiency.

Mold: A leading source of indoor and outdoor air pollution, mold can grow year-round. Mold has been found to cause allergic reactions in some people. Efforts to combat mold-related health issues took off in the 1970s. As this was the time of the energy crisis, people were searching for ways to save energy in their homes. The 1973 energy crisis involved an oil embargo that led to sky-rocketing fuel prices. As such, Americans were forced to conserve energy and money. Laying down insulation was thought to be a very effective method for saving on heating and cooling costs. This led to increased levels of indoor humidity and a dramatic increase in mold growth.

Nitrogen oxides (NOx): Nitrogen oxides are highly reactive gases containing nitrogen and oxygen in varying amounts, most of which are colorless and odorless. But one gas, nitrogen dioxide (NO2), mixes with airborne particles, forming a reddish-brown layer over many urban areas. NO2 is a harmful pollutant shown to contribute to ground-level ozone, deteriorating water quality, acid rain, and global warming. Normally, the ozone layer of the atmosphere, located 10-15 miles above the planet's surface, defends against the sun's harmful ultraviolet (UV) rays. But the increases in greenhouse gases have greatly damaged the ozone layer. Ground-level ozone refers to ozone gas that is located in the lower atmosphere, and it is one of the biggest causes of air pollution. Ground-level ozone also combines with sunlight, humidity, high temperatures, smoke, nitrogen, hydrocarbons, and other compounds to form smog, a murky, brown, harmful haze.

Global warming involves the accumulation of toxic air pollutants that allow higher levels of direct sunlight to reach the planet's surface. Then, this sunlight is trapped in the lower atmosphere. As a result, the surface of the planet continually increases in temperature.

Acid rain is rain that contains airborne pollution. This can cause extensive damage to waterways, trees, plants and crops, and structures and vehicles and may increase the acidity in soil. Acid rain may also have long-term effects on heart and lung health.

NO2 may result in serious respiratory problems and vision impairment. As NO2 can be transported over long distances by wind patterns, maintaining its release requires regional efforts. It is important to note that "NOx" actually refers to a group of chemical compounds, some of which have health-related and other benefits. For instance, the compound nitrous oxide is used by dentists as an anesthetic or to block pain. But as an air pollutant, NOx has been found to be very harmful.

Ozone: In urban areas, the gas called ozone is one of the biggest causes of air pollution. Normally, this layer of the atmosphere, located 10-15 miles above the planet's surface, defends against the sun's harmful ultraviolet (UV) rays. But the increases in greenhouse gases have greatly damaged the ozone layer. However, recent studies show that due to the efforts of the United States and other nations at reducing emissions, ozone damage has been dramatically reduced. A report released by the U.S. government's Climate Change Science Program shows that the output of harmful ozone-depleting substances has been reduced by an estimated 97% since the late 1980s. Despite this improvement, some experts predict that the damage to the ozone layer will not be completely healed until at least 2070. Ground-level ozone can also cause environmental problems. The combination of sunlight, humidity, high temperatures, ozone, smoke, nitrogen, hydrocarbons, and other oxides can form smog, which is a murky, brown, harmful haze.

Ozone substitutes: Because air pollutants may cause serious damage to the ozone layer, various alternatives were created. These products are not replacements for ozone, but rather alternatives for products shown to deplete the ozone layer. These substitutes were formulated to allow a safe, smooth transition to the alternative products, while reducing the overall risks to human health and the environment. Ozone substitutes have been created for the following: aerosols, refrigeration and air conditioning agents, foam-blowing, cleaning solvents, fire suppression and explosion protection, adhesives, and coatings and inks. There are also substitutes for sterilants, which kill microorganisms on medical equipment and devices.

Substitutes for tobacco expansion have been formulated as well. During this process, hydrogen peroxide and an enzyme known as catalase are added to tobacco. These two gases then form oxygen gas. Through the use of negative air pressure, the oxygen gas forms large bubbles in the tobacco leaves' cells. As a result, the leaves puff up, increasing the volume of tobacco in cigarette production.

Particulate matter (particle pollution, PM): Particulate matter air pollutants are a mixture of very small particles and liquid droplets, in many shapes and sizes. The EPA classifies PM into two categories. "Inhalable coarse particles" are larger than 2.5 micrometers and smaller than 10 micrometers in diameter. These particles are found near dusty industrial areas and roadways. "Fine particles" are 2.5 micrometers in diameter and smaller than inhalable coarse particles. These particles are directly emitted from forest fires, smoke, haze, and other sources. They can also form when power plants, industries, and automobiles release gases into the air. PM health risks are related to particle size; the smaller the particles, the greater the chance they're inhaled. Once inhaled, they can cause serious respiratory and cardiovascular problems. PM also affects the environment by making streams and lakes very acidic, depleting soil nutrients, damaging forests and crops, and destroying ecosystem diversity. These small particles are responsible for the reduced visibility in the nation's national parks. PM also stains and damages stone and other materials. PM has been shown to be a component of acid rain as well.

The EPA has established guidelines to reduce the emissions that form PM. The first nationwide air quality standards for PM were established in 1971. In 1997, the EPA changed the requirements to regulate fine particles in regards to their potential health risks. These new guidelines also revised standards for smaller particles (known as PM10).

Power plants: Generating electrical power releases large amounts of air pollutants. Research shows that these facilities release high levels of fine particulate matter, mercury, sulfur dioxide, and nitrogen oxides. These air-based pollutants may be linked to health problems during pregnancy, as well as damage the respiratory, cardiovascular, and central nervous systems. A 2000 study found that the toxic air pollutants released by power plants caused more than 603,000 asthma cases annually. If these facilities followed stricter environmental regulations, it is thought that about 366,000 of these cases could be avoided.

Radon: A naturally occurring, clear, odorless gas, radon has few practical uses. Some people believe that radon may be effective as an integrative medical treatment for such conditions as arthritis and asthma. However, radon therapy is not an approved alternative therapy in the United States.