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Why Cancer Preventing Sunscreen can Increase the Prevalence of Skin Cancer
The FDA is poised to approve new regulations on sunscreen. One of the questions being asked in the public debate is why skin cancer prevalence has been increasing even as the use of sunscreen as also simultaneously been increasing. In this brief note I show why a sunscreen that substantially REDUCES the degree to which a given hour of sun exposure will lead to skin cancer can none the less INCREASE the prevalence of skin cancer if the sunscreen reduces the discomfort caused by sitting in the sun even more.
Suppose each person has a certain degree of tolerance for sunburn and the discomfort associated with sitting in the hot sun while the skin bakes. For simplicity, let's say this tolerance is for one unit of solar discomfort.
Further, suppose that a sunscreen is effective at reducing solar discomfort, allowing the individual to sit in the sun ten times as long as he or she otherwise would before the solar discomfort level is reached. Thus, instead of ten minutes, an individual wearing this sunscreen can now sit in the sun for 100 minutes.
Suppose further that this sunscreen allows the individual to substantially reduce the incidence of cancer, reducing by five times the degree to which sitting in the sun for a given number of minutes increases the risk of cancer. Before sunscreen, the cancer risk from ten minutes in the sun was one cancer risk unit. After sunscreen the risk from ten minutes in the sun is .20 cancer units.
A quick and naive look at this problem would lead one to think that making the sunscreen available will reduce the incidence of cancer. The sunscreen is in fact known to be effective at preventing skin cancer! This is, however, totally wrong.
Now do the math.
Before sunscreen, the individual will sit in the sun for 10 minutes, incurring one cancer risk unit.
After sunscreen, the individual will sit in the sun for 100 minutes incurring 10 * .2 = 2 cancer units.
The RISK OF CANCER DOUBLES as the result of making available a sunscreen that reduces the cancer risk by five times!
The key to this result is that the sunscreen reduces discomfort more effectively than it reduces cancer risk. Thus, the FDA decision to regulate UVA and UVB exposure may help... but the key issue remains whether the sunscreens are more effective at reducing discomfort or at reducing cancer. If they reduce discomfort more effectively than they reduce cancer, they will probably continue to contribute to the remarkable rise in the incidence of skin cancer we have experienced in this country and around the world.
Disclaimer: I am not a dermitologist, nor do I have significant scholarly knowledge about skin cancer.
Comment by steve, 6/20/2011:
I've never heard that sunscreen increasing the comfort level of sitting in the sun! You just made that whole angle up. There is no greater sitting in the sun with suncreen than without it. Sunscreen simpy lessens the rate of absorption by reflecting and absorbing/dissipating rays. I personally always felt less comfortable using sunscreen/block because it felt like it wasn't leting my skin breath and therefore I felt more uncomfortable. So I think your logic is suspect, at the least.
Response to this comment by Alan, 8/14/2011:
Comment by Annie, 5/16/2012:
okey - doke. never heard of what ur sayin doc, but okey-doke
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