Fitness N' Health

Posted by awbigboz on June 17, 2011 at 4:55 am · Under Skin care

Jesse Richman, 6/16/2011

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