Ozone. Ozone (O₃) is a gas with molecules made of three atoms of oxygen. This is different from the oxygen we breath (O₂), which has two atoms of oxygen and makes up 21% of the Earth's atmosphere. Ozone is found throughout the Earth's atmosphere in minute quantities (about 0.6 parts per million, on average). Ozone is found in higher concentrations in pollution, and can be a health risk.

Ozone layer. Ozone in the upper atmosphere is important to health. In the stratosphere, the region of the atmosphere about 12 to 45 km above the surface of the Earth, ozone exists in larger amounts. Between 20 and 40 km high, ozone makes up about 6 parts per million of the air. This higher concentration of ozone, called the ozone layer, absorbs much ultraviolet light from the Sun.

The ozone layer exists because it is routinely created from oxygen by solar ultraviolet light. Ultraviolet (UV) light is higher energy light than visible light. The ozone concentrations in the stratosphere represent an equilibrium between creation of ozone and the ozone destruction that results when it absorbs UV. The stratosphere is warmer than the air above and below it as a result of these processes.

Ultraviolet light is harmful to plants and animals. In humans high amounts of UV can cause sunburns, skin cancer, and possibly cataracts. Scientists categorize ultraviolet into UV-A (lower energy), UV-B (medium energy), and UV-C (higher energy). UV-B light is absorbed by ozone so that very little reaches the surface of the Earth. Lower energy UV-A is not absorbed by ozone, however.

Skin cancer, the most serious human health effect of UV light, includes several different types. Most types are non-malignant and treatable, with the main problem being cosmetic (i.e., affecting skin appearance rather than being life-threatening). These types are caused by both UV-A and UV-B. Melanoma, the type of skin cancer which is potentially fatal, is caused apparently only by UV-A--which is not absorbed by ozone.

Chlorofluorocarbons. Chlorofluorocarbons (CFCs) are a group of man-made chemicals. They are hydrocarbons which include chlorine or fluoride atoms. CFCs have many uses, the most important of which is as a refrigerant. (Before the invention of CFCs in 1928 refrigerators had to use chemicals which were toxic or flammable or both. Since such refrigerators were unsafe, few people used them.) Freon is a CFC used in most air conditioners and refrigerators. The physical properties of Freon make it a particularly good refrigerant. Also, Freon is nontoxic and nonflammable, making it safe to use in everyday devices. Other CFCs are used to make Styrofoam, to clean electronic equipment, and in certain types of fire extinguishers. CFCs were formerly used in aerosol spray cans (such as hair spray) until this use was banned in the U.S. in 1978.

Ozone depletion. In the 1970s, some scientists expressed concerns about CFCs causing damage to the ozone layer. The theory they outlined was this:

• CFCs are stable molecules--some types of CFCs can last in the atmosphere for decades before being broken down into other chemicals.
• CFCs are in the atmosphere long enough to mix throughout the atmosphere, including the stratosphere.
• In the stratosphere, solar UV light breaks down CFCs and separates chlorine atoms.
• Chlorine atoms act as a catalyst in chemical reactions where ozone (O₃) is converted to oxygen (O₂).
• If significant amounts of ozone are destroyed, harmful levels of solar UV-B will reach the surface of the Earth.

These concerns were cited by environmentalists who persuaded the U.S. government to restrict CFCs in 1978, and then in 1992 persuaded many nations to agree to completely phase out CFCs over a period of time. Each part of the theory above is supported by observation or computer models. On the other hand, the entire series of events is not demonstrated to be a primary cause of changes in the ozone layer.

The first graph show average annual ozone for the Earth (heavy line), midlatitudes in the Northern Hemisphere (dashed line), the equator (thin line), and Antartica (thin line with markers). The second graph shows global average ozone (heavy line), sunspot numbers (dashed line), and chlorine in the atmosphere from man-made chemicals (line with markers). (Chlorine values are in units of 10 parts per trillion.) In the second graph compare peaks in ozone to peaks in sunspot number (a measure of solar activity). Also note the temporary dip in ozone levels in 1991-1993, corresponding to the aftermath of the eruption of Mt. Pinatubo.

Ozone hole. In the early 1980s, scientists measuring the amount of ozone over Antarctica found the levels much lower than normal in October. They found that the ozone layer over Antarctica lost ozone in late winter and early spring (August to October in the southern hemisphere) until it had 20% less ozone than normal. This ozone was restored during the late spring and summer, but the depletion came back the next winter and spring. This became known as the "ozone hole". It has been observed every year since then, always developing during the winter and spring--sometimes up to a 50% depletion--and then restoring naturally in the summer. A less severe temporary depletion or "hole" has since been found over the North Pole. Environmentalists and some scientists have cited this as evidence that man-made chemicals are thinning the ozone layer.

Scientists have found that this phenomena results from the unique Antarctic polar vortex, a region over Antarctica of isolated atmospheric circulation and extremely low temperatures. In this "vortex", particles in stratospheric clouds allow chlorine to destroy ozone more efficiently than normal. Also, some ozone is displaced toward areas farther north. With sunlight increasing through the spring, solar UV recreates ozone from oxygen (O₂) over Antarctica and the "hole" disappears, until next year. A less intense seasonal depletion and replenishing of the ozone layer has been observed over Antarctica since the 1950s, before CFCs were in widespread use.

Problems with the depletion theory. Environmentalists want CFCs banned because they say they destroy the ozone layer. However, scientifically we also see the following:

• There are significant natural sources of chlorine in the atmosphere. Natural sources account for at least 20% of the chlorine in the ozone layer, and volcanic eruptions can temporarily accelerate ozone depletion.
• Recorded amounts of ozone rise and fall over time, but the amount of CFCs in the atmosphere has continuously increased. On the other hand, ozone levels do correlate with solar activity (lower solar activity means less UV radiation to create ozone).
• Ozone levels vary naturally by large amounts. The amount of ozone over the United States, for example, can increase and decrease by 15% or more many times during a year.
• It is not clear that the amount of UV light reaching the surface of the Earth has increased.

Our understanding of chemical processes in the stratosphere have increased tremendously in the last 25 years, but many questions remain. It is clear that many predictions regarding ozone depletion have been exaggerated. On the other hand, the Antarctica ozone depletion, while only cyclical, is a significant phenomena.

With the phaseout of CFCs, alternative chemicals are being introduced for air conditioners and refrigerators. Several replacement chemicals have been developed, none of which are as efficient as freon. Many of these are toxic, flammable, or corrosive. Refrigerators and air conditioners are more expensive as a result. This will especially affect people in the third world, who need them for health reasons. Even in the United States, the phaseout of CFCs is costing everyone indirectly. Opponents of the CFC ban say that scientifically, the evidence that man is destroying the ozone layer is too weak to justify policy decisions that harm people.