By Tom Kelly
Two out of three Australians will develop skin cancer over their lifetime. This is the highest incidence of skin cancer in the world, and it is increasing. Recent figures from the Australian Institute of Health and Welfare show a 30% increase in deaths resulting from malignant melanoma between 1985 and 1992. Latest figures indicate that 1 in 40 Australians will be directly affected by melanoma by age 74.
This year skin cancer will kill about 1000 Australians. 150,000 new cases will be treated. Melanoma will account for 5% of these skin cancer cases and around 80% of the deaths. According to figures from the NSW Cancer Council, skin cancer costs the Australian community up to $400 million annually.
Skin cancer is caused by exposure to the ultraviolet component of the sun's rays. Development of skin cancer involves two phases — induction and promotion. Induction, the initial genetic damage to skin cells that can lead to skin cancer is caused by the component of the ultraviolet spectrum known as UVB, which also causes sunburn.
Promotion, the second phase, involves the suppression of the immune system as a result of further UV exposure. The suppressed immune system is less able to defend the body against skin cancers as they arise. Both UVB and another component of the UV spectrum, UVA, act to suppress the immune system.
The relationship between the thinning of the ozone layer and the incidence of skin cancer is complex. The ozone layer selectively absorbs a particular part of the UV spectrum. It absorbs all UVC rays, a large part of UVB and very little of UVA.
The primary impact of the thinning of the ozone layer on skin cancer in humans results from increased penetration of UVB, which will increase the propensity of sunlight to cause sunburn and to induce and promote skin cancer. Because the ozone layer filters little UVA anyway, depletion of the ozone layer will have little impact on the role of UVA in promoting skin cancers.
According to Colin Roy of the Australian Radiation Laboratories, for each 1% loss of ozone from the stratosphere, there is a 1.5% increase in UVB reaching the earth's surface. This is likely to lead to a 3 to 4.5% increase in non-melanoma skin cancers.
Because the relationship between melanoma and UV radiation is more complex, Roy declined to put a figure on the impact of ozone depletion on melanoma incidence.
Skin cancers can develop a decade or more after the initial damage, so we haven't begun to see the full consequences of the current level of degradation of the ozone layer in skin cancer figures. When we consider that three-quarters of ozone damaging substances have been released into the atmosphere in the last 25 years, and that the bulk of these can be expected to continue destroying ozone for 50 years or more, it becomes clear that significant further ozone depletion is already inevitable.
Even if human beings stop producing these substances tomorrow, we and future generations will be living with the consequences of past mistakes for the best part of the next century, with the worst yet to come.