When is a little sunshine too much?

Nearly everyone knows that if they use sunscreen they can avoid being sunburned. The sun gives light that we can see, but it also gives off lots of invisible radiation. Some of this invisible radiation, such as ultraviolet radiation (UV), can be harmful. Sunscreen protects us from this radiation that we can't see. But we are protected from ultraviolet radiation in another way. Scientists have discovered that there is a layer of the upper atmosphere that contains ozone gas. This ozone layer screens out most of the harmful ultraviolet rays. In the 1970s scientists predicted the ozone layer could be damaged by human activity. If very much of the ozone is destroyed, we may see a great increase in cases of skin cancer and other health problems. Some predictions warn that for every 1% drop in the amount of ozone in the stratosphere, there will be a 5-6% increase in skin cancer rates. The additional UV radiation reaching the earth's surface could also have harmful effects on the plants and animals we depend on for food. Since the ozone layer is so important in protecting us, we need to know more about how it protects us and how to keep it from disappearing.

How solving one problem created another!

Refrigerators require a gas that can be liquefied by compression. This gas is called a refrigerant. The cooling in refrigerators occurs when the liquefied refrigerant expands to again form a gas. For refrigerators to be useful, the heat absorbed when the liquid vaporizes must be great enough to lower the temperature in a frozen food compartment to about 0¡C. Only a few common gases can be liquefied at the required pressures and also have a sufficiently large heat change. Ammonia and sulfur dioxide were used in early refrigerators, but they are toxic. If the refrigerator developed a gas leak, the event created a serious hazard to anyone nearby. In the 1930s, a class of compounds called chlorofluorocarbons (CFCs for short) were developed to replace the dangerous refrigerants. CFCs didn't react with anything, and they presented no known human health hazards.

dichlorodifluoromethane (CFC-12)	trichlorofluoromethane (CFC-11)

When CFCs were first developed, they were thought of as miracle molecules, because they were practically nontoxic. Thomas Midgley, one of the scientists who developed CFCs, would often inhale large amounts of CFCs at public demonstrations to show how safe they were.

Electron density maps for dichlorodifluoromethane (left) and trichlorofluoromethane (right). The color red shows regions of highest electron density and the color blue shows regions of the least electron density.

Of course, at the time very little was known about the earth's upper atmosphere. No one really knew what strange chemistry went on in the stratosphere. So CFCs became the standard refrigerant used in industrialized nations. Untold amounts of CFCs were used over the next several decades, the world unaware of what was going on far above the ground, until the scientists Sherwood Rowland and Mario Molina made some disturbing discoveries.

Next: How Do We Know Ozone Is Missing?

Nobel Poster in Chemistry 1995 — an online presentation, adapted from the Nobel Foundation's poster dealing with the prize-winning science of Paul Crutzen, Mario Molina and F. Sherwood Rowland, from the Nobel e-Museum.