In some areas of the world, the air quality is so poor that mountains and hills that used to be visible almost every day can hardly be seen anymore. For example, when I first moved to a small town near Sacramento, California, you could see the glorious Sierra Madre mountains every day, even though they were about a hundred miles distant. When I left the area in the 1990s, the mountains were no longer visible except on a few days a year. We have come to take air pollution, like noise pollution, sea, stream, and lake pollution, and other forms of pollution for granted.

Scientists, however, need to quantify pollution of different kinds and determine what pollution is a serious health problem and what pollution is a minor nuisance. To do this, scientists define pollution in two ways:

1. emissions refers to the amount of a pollutant that actually enters the environment

2. exposure refers to the amount of a substance that people are actually exposed to.

A good example of the difference in emissions and exposure is the chemical benzene, a volatile liquid that is used in enormous quantities in industry and is emitted in automobile exhaust also. It is well documented that exposure to benzene can cause leukemia, a form of blood cancer.

Emissions of benzene: Approximately 50 percent of all the benzene released in the air comes from automobiles. However, the concentration of benzene in the air we breathe is very low because the benzene is diluted into an enormous quantity of air in the atmosphere.

Exposure to benzene: Cigarettes emit a relatively tiny amount of benzene, but at least 50 percent of peoples' exposure to benzene comes from cigarette smoke. Even non-smokers get most of their exposure to benzene, not from breathing emissions from automobiles, but from second hand cigarette smoke.

Another example is exposure to chloroform, another volatile solvent that can cause cancer. The major sources of chloroform come from sewage treatment plants where chloroform is used to sterilize sewage. Tons of chloroform are released into the atmosphere every year. However, the major source of exposure to chloroform is from shower and bath water. Most potable water in the United States is treated with small amounts of chlorine to kill harmful microorganisms that might be present in the water. A tiny amount of the chlorine is converted to chloroform and, because we take so many showers and baths, most of our exposure to chloroform comes from these sources. If you are concerned about exposure to chloroform in household water, you can purchase filters that will eliminate it in drinking water and bath water.

Written by Gordon Edlin, Ph.D.