<P> The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UVB exposure . The mechanism by which UVB induces these cancers is well understood--absorption of UVB radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates . These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery . By combining epidemiological data with results of animal studies, scientists have estimated that every one percent decrease in long - term stratospheric ozone would increase the incidence of these cancers by two percent . </P> <P> Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15--20 percent of the cases diagnosed . The relationship between malignant melanoma and ultraviolet exposure is not yet fully understood, but it appears that both UVB and UVA are involved . Because of this uncertainty, it is difficult to estimate the effect of ozone depletion on melanoma incidence . One study showed that a 10 percent increase in UVB radiation was associated with a 19 percent increase in melanomas for men and 16 percent for women . A study of people in Punta Arenas, at the southern tip of Chile, showed a 56 percent increase in melanoma and a 46 percent increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UVB levels . </P> <P> Epidemiological studies suggest an association between ocular cortical cataracts and UVB exposure, using crude approximations of exposure and various cataract assessment techniques . A detailed assessment of ocular exposure to UVB was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity . In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date . Based on these results, ozone depletion is predicted to cause hundreds of thousands of additional cataracts by 2050 . </P> <P> Increased surface UV leads to increased tropospheric ozone . Ground - level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties . The risks are particularly high for young children, the elderly, and those with asthma or other respiratory difficulties . At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts . </P>

Name the common air pollutant which causes depletion of ozone layer