Air Pollution

views updated

Air Pollution

Introduction

Air pollution refers to the change in the composition of the atmosphere by gases or other material released from Earth’s surface. The pollutants can originate naturally. A well-known example is ash that can be propelled upward during an eruption of a volcano. More commonly, however, air pollution is the result of human activities. Examples include the smoke emitted from the chimneys of factories and wood-burning homes as well as vehicle exhaust.

The released pollutants enter an atmosphere that is only about 250 miles (400 km) thick. Compared to the vastness of the surrounding universe, Earth’s atmosphere is extremely thin.

Air pollution has occurred ever since the formation of the atmosphere. Human-mediated air pollution dates back to prehistoric times, when fire was harnessed as a source of warmth and for food preparation. However, air pollution increased markedly during the Industrial Revolution in the mid-nineteenth century, and accelerated during the latter half of the twentieth century.

A legacy of the twentieth century is the use of compounds such as chlorofluorocarbons (CFCs). Their entry into the atmosphere has caused the depletion of a constituent called ozone, which in turn has led to an increasing amount of potentially harmful ultraviolet radiation reaching Earth’s surface. Other pollutants such as carbon dioxide (CO₂) are fueling global warming—the increasing warming of the atmosphere that has been occurring since the late nineteenth century and which has accelerated since the mid-twentieth century. In its 2007 report, the Intergovernmental Panel on Climate Change (IPCC) stated that air pollution of human origin is at least partially responsible for global warming.

As well, since the pollutants are in the air we breathe, air pollution can be unhealthy. Respiratory diseases can be aggravated or even caused by breathing polluted air.

Historical Background and Scientific Foundations

Atmospheric pollution has been part of Earth’s history since the atmosphere formed. Early in the planet’s history, outgassing from volcanoes contributed ash and gases to the atmosphere. This form of air pollution continues. For example, Kilauea Volcano—the most recently active volcano on the island of Hawaii, which has been active since 1983—spews out about 2,000 tons of sulfur dioxide every day during eruptions. In another and more spectacular example, a series of huge eruptions of the Krakatoa Volcano on August 27, 1883, sent millions of tons of ash 50 miles (80 km) into the atmosphere. As the ash dispersed throughout the atmosphere, the resulting haze partially blocked the passage of sunlight. Indeed, the average global temperature during 1884 was more than one degree Celsius below normal.

As visually spectacular as volcanic eruptions are, their contribution to air pollution is miniscule compared to the air pollution resulting from human activities, which have occurred for hundreds of thousands of years, ever since the harnessing of fire for cooking and warmth. In these ancient times, human-mediated air pollution was relatively insignificant compared to centuries later. With time and increasingly sophisticated human activities, air pollution began to increase. A historical record of atmospheric change can be found in ice cores—long plugs of compacted snow that are retrieved from the centuries-old accumulation of snow that occurs in glaciers in far northern and southern regions. The analysis of ice cores recovered from the Greenland ice sheet reveals elevated amounts of lead, mercury, and nickel beginning about 5,000 years ago. This corresponds to the beginning of mining and smelting of metal ores in Europe. The smelting byproducts released to the atmosphere dispersed and eventually settled in far-flung regions including Greenland.

Air pollution began to increase near the end of the eighteenth century and early nineteenth century, during the period that is known as the Industrial Revolution. At that time, technological advances including the introduction of the steam engine made it feasible to establish factories and manufacturing plants in or near major cities. The cheap labor pool and ready access to land and water transportation spurred the growth of factories. Their use of coal as fuel increased the amount of concentration of air pollution.

Over the intervening two centuries, the composition of the atmosphere has been affected due to air pollution. Atmospheric data collected since the Industrial Revolution has demonstrated a change in the content of what are termed trace gases. Furthermore, entirely new compounds have appeared and their content has increased. The best example is CFCs, a product of twentieth-century technology.

Air pollutants are classified as being primary or secondary compounds. Primary pollutants are directly given off by a particular process or activity. Emissions from a

WORDS TO KNOW

ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

ANTHROPOGENIC SOURCES: Sources that are due to human activity. An example of anthropogenic air pollution is the burning of wood for fuel.

GREENHOUSE GAS: A gas that accumulates in the atmosphere and absorbs infrared radiation, contributing to the greenhouse effect.

PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.

factory smokestack, the tailpipe of an idling car, or the ash propelled out of an erupting volcano are examples of primary pollutants. Secondary pollutants are not given off directly, but rather form in the air when other compounds interact with one another.

A variety of primary pollutants are associated with human activities (anthropogenic sources). Sulfur oxides are compounds made up of a sulfur atom and varying numbers of oxygen atoms. Sulfur dioxide (SO₂) is given off by the burning of coal and oil. Carbon monoxide (CO) is gas that is given off when natural gas, wood, or coal are inefficiently burned, as for example when wood that is wet or contains resin is used as fuel in home wood stoves, and from automobile exhaust. Particularly in the home, this type of primary pollution is dangerous since the colorless and odorless gas can restrict the amount of oxygen the body receives. Carbon dioxide (CO₂) is also emitted in exhaust. It is one of several compounds that are known as greenhouse gases, since they restrict the reflection of sunlight back out of the atmosphere. Nitrogen oxides are compounds made up of a nitrogen atom and varying numbers of oxygen atoms. Nitrogen dioxide (NO₂), which is emitted by the burning of material at high temperature, is responsible for the brown-colored haze above cities such as Los Angeles, California, and Beijing, China. Smoke, dust, and volatile organic compounds are other primary pollutants.

Examples of secondary pollutants are particles that form when gases in the air interact with another pollutant that is a mixture of smoke and SO₂ (better known as smog), peroxyacetyl nitrate (which is formed by the interaction of nitrogen oxides and volatile organic compounds), and ground-level ozone that forms near Earth’s surface. Ground-level ozone is a particular problem in some major cities including Los Angeles and Mexico City.

Ground-level ozone irritates the respiratory tract and eyes. In cities where it forms frequently, ground-level ozone can be dangerous, causing chest tightness,

IN CONTEXT: TYPES OF AIR POLLUTION

Air pollution is of special concern to the human population. The seven types of air pollution considered the greatest threat to human health in the United States, and the first regulated by the 1970 U.S. Clean Air Act, include sulfur dioxide, particulates (dust, smoke, etc.), carbon monoxide, volatile organic compounds, nitrogen oxides, ozone, and lead. In 1990, another 189 volatile chemical compounds from more than 250 sources were added to the list of regulated air pollutants in the United States.

Air contaminants are divided into two broad categories: primary pollutants are those released directly into the air. Some examples include dust, smoke, and a variety of toxic chemicals such as lead, mercury, vinyl chloride, and carbon monoxide. In contrast, secondary pollutants are created or modified into a deleterious form after being released into the air.

coughing, and wheezing, especially in those with respiratory and heart problems. Paradoxically, higher in the atmosphere, the problem with ozone is its depletion, which is allowing more harmful ultraviolet sunlight to reach Earth’s surface.

Other air pollutants include heavy metals and both chemical and biological toxins. Some air pollutants are able to persist in the atmosphere for a long time. Known as persistent organic pollutants, compounds such as dioxin and furan may be transported long distances and so have the potential to affect people far from their point of release. Additionally, they can cause adverse health effects at low concentrations.

Impacts and Issues

The adverse health effects of air pollution are well-known. According to the World Health Organization (WHO), nearly 2.5 million people die each year from deaths that are the direct result of air pollution. These include pollution-aggravated bronchitis, asthma, emphysema, lung disease, heart disease, and allergic reactions.

A graphic example of the health threat posed by air pollution occurred in December 1952, when the weeklong formation of smog over London, England, was attributed to the immediate death of more than 4,000 people and the respiratory- and heart-related deaths over the next few months of 8,000 more.

Human industrial activity is overwhelmingly the greatest contributor to atmospheric pollution. According to the U.S. Environmental Protection Agency, in the United States alone more than six billion pounds of toxic compounds are released to the atmosphere every year.

High resolution satellite imaging has documented the influence of industrial development on air quality over developing regions of India and the city of Beijing. Satellite monitoring of China as part of the European Space Agency’s Dragon Programme has revealed that Beijing’s growth into an industrially robust mega-city of over ten million people and millions of cars and other vehicles has produced the planet’s highest levels of nitrogen dioxide. The gas, which is a respiratory irritant, is the main reason why the Chinese government is contemplating banning vehicle traffic in the city during the 2008 Olympics.

Air pollution is also the principal reason for the warming of the atmosphere that has been occurring since the Industrial Revolution, and which has accelerated since the mid-twentieth century.

Efforts to reduce air pollution include implementation of air quality regulations in many countries; use of more environmentally friendly energy sources, such as wind power and solar power; and the use of hybrid vehicles. Also, in the United States, in tandem with a move to reduce dependence on non-U.S. oil resources, the George W. Bush administration committed the nation to an increased use of biofuels.

However, the latter strategy is being re-examined in the light of water availability in the present and the coming century. A U.S. Department of Energy document released in February 2008 chronicled the heavy dependency on water and the large energy requirement of U.S. production of biofuels initiatives.

Efforts to reduce air pollution also require cooperative efforts between different governments. For example, in 1991 the United States and Canada signed an air quality agreement to address the trans-border movement of pollutants. Without such efforts, local pollution efforts would be much less effective.