Even though CFCs are extremely stable in the lower atmosphere, some chemists started to wonder about the effects of the compounds if they drifted to higher levels of the atmosphere. In the stratosphere, high-energy radiation can break apart the CFCs and cause an abundance of chlorine atoms at that level. In fact, each additional chlorine atom could destroy about 100,000 ozone molecules! After decades of CFC use, this could mean a substantial decline in the amount of O3 in the stratosphere.

Predictions by chemists in the mid-1970s were that ozone depletion would be slow, and perhaps not detectable for some years to come. Many were surprised by the announcement of British scientists that a detectable decline in stratospheric ozone had been found over Antarctica. By 1984, ozone levels had diminished by as much as 40%, and by the early 1990’s, certain regions were almost completely depleted. This decrease in concentration over Antarctica was termed the ozone hole. Note the relative decrease of ozone in the chart below. The amount of ozone above the poles is called the Dobson Unit (DU). The DU is defined to be 0.01 mm thickness at standard temperature and pressure (0 degrees Celsius and 1 atmosphere of pressure).

An interesting note in the journal Nature reports on another sources of ozone-depleting substance, and it's natural! Apparently methyl chloride, which can be produced by natural or anthropogenic sources, is also destroying stratospheric ozone. For more on this, see the Mystery of the Missing Gases.