OZONE: PROTECTOR AND POLLUTANT

Ozone, a form of oxygen with the molecular formula 0₃, is a natural component of the earth's stratosphere. It acts as a shield for the earth's inhabitants by filtering out harmful ultraviolet (UV) light from the sun. Ozone absorbs UV light in a decomposition reaction that converts 0₃ to ordinary oxygen molecules and oxygen atoms:

0₃                 0₂             +              0

                                    ozone                       oxygen molecule             oxygen atom

The generated oxygen atoms react with oxygen in a reverse process to form more ozone:

0     +     0₂     ®     0₃

If these opposing processes are not disturbed, the amount of ozone in the stratosphere remains fairly constant. Scientific evidence indicates that some materials used in industrial societies have influenced the natural processes.

A class of compounds called chlorofluorocarbons or CFCs (with formulas such as CF₂Cl₂) has been widely used for years as refrigerants in air conditioners, degreasing agents in the aircraft and electronics industries, and as propellants for various aerosol spray products. CFCs are very unreactive when they are shielded from UV light. However, the gradual circulation of the earth's atmosphere eventually carries CFCs released on the surface up into the stratosphere. Once there, CFCs are changed by UV light into a form that greatly accelerates the decomposition reaction of ozone but does not speed the process by which ozone forms. As a result, the amount of ozone present at any time is decreased. A dramatic thinning of the ozone layer (popularly called a "hole") has been observed over Antarctica each winter for a number of years and has been increasing since 1979. A similar but less severe thinning (a smaller hole) has also been observed in the Arctic region.

The U.S. National Research Council estimates that each 1 % depletion in the amount of ozone in the ozone layer will cause a 5% increase in the number of cases of human skin cancer. World leaders have taken this potential health risk very seriously and are in the process of agreeing to an eventual ban on the production and use of ozone-depleting substances including CFCs. This was a major topic of discusion at the Earth Summit in Rio de Janeiro in 1992.

Interestingly, although ozone in the stratosphere serves to protect life, ozone generated in the lower atmosphere as a component of smog presents health risks and causes the deterioration of materials such as the rubber of automobile tires. At high levels, the health risks include pulmonary edema (fluid buildup in the lungs) and even death for susceptible individuals.