The net effect of these reactions is catalysis of the destruction of several thousand molecules of O3 for each Cl atom produced. Because of their widespread use and persistency, the two CFCs of most concern in ozone destruction are CFC-11 and CFC-12, CFCl3, and CF2Cl2, respectively. Even in the intense ultraviolet radiation of the stratosphere the most persistent chlorofluorcarbons have lifetimes of the order of 100 years.