While ozone concentrations vary naturally with sunspots, the seasons, and latitude, these process are well understood and predictable. Records have been established spanning several decades that detail normal ozone levels during these natural cycles. Each natural reduction in ozone levels is followed by a recovery. Recently, however, convincing scientific evidence has shown that the ozone layer shield is being depleted well beyond changes due to natural processes. Thinning of the ozone layer has been caused by a variety of ozone destroying chemicals released by human activities. More than half the damage has been caused by chlorofluorocarbons. For over 50 years, chlorofluorocarbons,(CFCs), were thought of as miracle substances. They are stable, nonflammable, low in toxicity, and inexpensive to produce. Over time, CFCs found uses as refrigerants, solvents, foam blowing agents, and in smaller applications. CFCs and other compounds have atmospheric life times long enough to allow them to be transported by winds into the stratosphere. Because they release chlorine and bromine when they breakdown, they damage the protective ozone layer. In the early 1970's, researchers began to investigate the effects of various chemicals on the ozone layer, particularly CFCs, which contain chlorine. Potential impacts of other chlorine sources were examined also. Chlorine from pools, for example, do not reach the stratosphere. Those chlorine compounds readily combine with water and repeated measurements show that they rain out of the troposphere very quickly. In contrast, CFCs are so stable that they do not dissolve in rain. There are no natural processes that will remove the CFCs from the lower atmosphere. Over time, the CFCs are driven into the stratosphere by winds. It typically takes seven to ten years for CFC molecules to rise through the lower atmosphere and reach the stratosphere. The CFCs are stable that only exposure to strong UV radiation can break them down.
