Problems with the Ozone Layer

Beginning chemistry students are familiar with ozone. Ozone is an allotrope of oxygen. That is, it consists of nothing but oxygen atoms. But it contains three of them bonded to each other (03) rather than the two found in normal oxygen (02).

Until recently, chemistry students studied ozone primarily as an interesting chemical substance or as an industrially useful substance. Few people thought very much about the role of ozone in their everyday lives.

Scientists have long known that ozone is important to humans. The region of the atmosphere that extends from 10 to 50 kilometers (6 to 30 miles) contains a relatively high concentration of ozone molecules. These ozone molecules absorb some of the ultraviolet radiation that reaches the earth's atmosphere in sunlight.

Ultraviolet radiation is a component of sunlight that causes sunburns, suntans, and skin cancer. In the absence of ozone, the earth's surface would receive a much higher concentration of ultraviolet radiation. This radiation would not only increase the rate of various forms of skin cancer, but might also kill some simpler forms of life.

Scientists have learned that a variety of human activities can cause changes in the ozone layer of the earth's atmosphere. The most serious threat at the present time seems to be the use of certain synthetic organic chemicals known as chlorofluorocarbons (CFCs).

Chlorofluorocarbons are organic compounds consisting of carbon, hydrogen, chlorine, and/or fluorine. Chemically they are related to such familiar compounds as carbon tetrachloride, chloroform, and DDT. CFCs were discovered in 1928 and have since found many important industrial and commercial applications. They are popular because of their chemical and physical properties: they are chemically stable, they do not burn or corrode metals: they hold heat and cold well; and they pose no direct health threat to humans.

Some of the applications of CFCs have been their use as propellants in aerosol sprays, as refrigerant fluids, as solvents, as raw materials in foams of many kinds, and as insulating materials on electric cables. In 1990, nearly a billion kilograms (2 billion pounds) of CFCs were produced; a third of that total was in the United States alone.

The problem that CFCs pose to the ozone layer is that when they are exposed to solar radiation in the atmosphere, they become less stable. They react with ozone molecules and convert them to ordinary oxygen.

CFC + 03 ==> 02

A complicating feature of this reaction is that a CFC molecule is not actually used up in the reaction. It survives to convert another ozone molecule to oxygen, and then another, and another. Scientists believe that a single CFC molecule can convert thousands or tens of thousands of ozone molecules to oxygen.

The conversion of ozone to oxygen is troublesome because oxygen does not have the capacity to absorb ultraviolet radiation, as does ozone. Thus, as ozone is destroyed, the atmosphere becomes more transparent to ultraviolet radiation, increasing the risk of skin cancer among humans on earth. According to some scientists, a 2 percent increase in ultraviolet radiation may result in a 2- to 5-percent increase in basal-skin cancer (a form treated easily and successfully) and an 8- to 20-percent increase in squamose-cell skin cancer (a more dangerous form of the disease). Given current skin cancer rates, these percentages would translate into 10,000 to 30,000 new cases of skin cancer each year in the United States alone.

Governments have begun to respond to the threat that CFCs pose to the ozone layer. For example, the United States government banned the use of CFCs in aerosol sprays in 1978. By the late 1980's, concern had begun to spread and grow rapidly. A conference held in Montreal in 1987 produced an agreement among 23 nations to reduce by 50 percent the use of CFCs by the year 2000.

Less than three years later, increasing concern about ozone damage prompted a second Montreal conference. This time 92 nations agreed to completely eliminate most CFC use by the year 2000. Efforts to move the target date forward to 1997 were blocked by the United States delegation to the conference.

created by: Debbie Anholt, anholt@lclark.edu
updated: May 9, 2000