What Is Air Pollution?

Air pollution includes solids, liquids, and gases that are in the troposphere in unwanted concentrations. Some pollution is natural-the result of volcanic eruptions, forest fires, decomposition of organic matter, and even salt spray. But most pollution is a by-product of human activities such as car emissions, energy production, chemical reactions, incineration, aerial application of pesticides, land clearing, and so on.

When Did Air Pollution Become a Problem?

Human beings have always lived with some form of air pollution, stemming from either natural causes or simple combustion (burning) activities. Throughout most of our history, as hunters, then simple metal workers, and later industrialists, natural forces have worked to minimize air pollution effects. Small populations also kept the amount of air pollution low. However, increasing industrialization and growing urban populations over the last two centuries have made significant changes in the Earth's basic atmospheric composition. Natural processes can no longer effectively clean the air-we are overloading the atmospheric system.

How Has Air Pollution Changed?

Industrialization has increased the combustion of fossil fuels (coal, oil, and natural gas), placing more carbon dioxide in the atmosphere. Trees and vegetation are being burned (deforestation), adding more carbon dioxide to the atmosphere. Increases in the number of automobiles, all burning carbon based fuels, have even further increased carbon dioxide levels. This increase in carbon dioxide above the 0.035% level has enhanced the "greenhouse effect," which is linked to with global warming. Along with increases in carbon dioxide, we have also increased the types and concentrations of other air pollutants in our atmosphere. Chemical and physical processes, including incineration and vaporization, release chemicals directly into the air. These chemicals react, with the aid of sunlight, to create new pollutants. Throughout all the major industrial areas, crises of smog and acid deposition are posing new global problems. New chemicals threaten the ozone layer, and many scientists argue that these may cause changes in the Earth's climate.

Where Is Air Pollution a Problem?

Because of air circulation and the natural dispersion of pollutants, air pollution is a problem everywhere. However, due to wind patterns and geographic configurations, some areas of the world are more sensitive to air pollution than others. For example, cities near coasts or in valleys may experience temperature inversions. This occurs when the normally warmer air near the surface and the cooler air above are reversed, with the warm air above and the cooler air below. The result is that the air no longer moves. Urban pollutants become trapped under the layer of warm air. If temperature inversion persists for a long period of time, the concentration of pollutants rises and populations are at risk.

What Are These Dangerous Pollutants?

Pollutants may be either gases or particulates, finely suspended liquids or solids. Particulates are small, often less than a micron (1/1000 of a millimeter) in size. They include viruses, bacteria, pollens, minerals, and industrial chemicals. The first recognized particulate was probably smoke, which contains both liquids and solids.

The major gaseous pollutants include sulfur dioxide, carbon monoxide, ozone (in the troposphere, ozone is a pollutant), nitrogen oxides, and lead. Historically, this group of gases was identified when the first environmental air analyses were completed under the Clean Air Act of 1970. Later, volatile organic chemicals were added. Some are primary pollutants in that they are toxic as formed; others are secondary pollutants, formed in the air by the action of sunlight on primary pollutants. Smog is an example of a secondary pollutant.

How Do We Identify These Pollutants?

We can use our senses of sight, hearing, smell, touch, and taste to detect pollutants, but many pollutants are invisible or have no smell. When we need more help, we either amplify our senses-for example, by turning to electronic detectors-or we look for impacts. Air pollutants affect our vegetation (plants and trees), our materials (stone, concrete, metals, and fabrics), and the clarity of our air. Bioindicators, - plants sensitive to specific types of pollutants - including lichens, mosses, white pines, and others, are often used to reveal the presence of one or more air pollutants.

What Are the Sources of Air Pollutants?

We can anticipate impacts on our health, vegetation, materials, and air clarity by noting the presence of well known pollutant sources. For example, we know that agricultural tilling, construction, mining and quarrying, road building and paving, and wind erosion will loosen dust into our troposphere. These are called fugitive emissions (not from specific point sources) and in 1990 were estimated at 40.8 million metric tons in the United States. And as already mentioned, industrialization and the accompanying increases in the combustion of fossil fuels, removal and incineration of trees and vegetation, and use of automobiles all contribute to pollution. Human beings, however, are not the only sources of air pollution, even though we cause severe impacts in specific locations. Lightning strikes may cause major forest fires, which in turn release smoke particles. Strong winds carry toxic-laden dusts. Decaying plant matter releases hydrogen sulfide. Oceans cover us with salt spray, and erupting volcanoes emit sulfur dioxide and particulates that can be carried around the world.

What Does All This Mean?

There are actually hundreds of air pollutants in our troposphere today. To control air pollution, human beings are regulating the releases of pollutants into the atmosphere, monitoring air pollution sources, and studying the pollutants and their impacts. You can help with this process!