Mechanisms of sunscreen failure

Vienna, Austria - For more than two decades, dermatologists have been urging people to apply sunscreen when spending time out of doors. But, it seems that when momentum picks up for the public to actually follow that recommendation a new study comes out contending that sunscreens actually contribute to causing cancer rather than helping reduce the risk of developing malignancies resulting from sun exposure.

At the 10^th World Congress on Cancers of the Skin, John Hawk, M.D., F.R.C.P., professor of dermatological photobiology at St. John's Institute of Dermatology at St. Thomas Hospital in London, explained some of the logic that supports each position, and why some researchers question the effectiveness of sunscreens in avoiding skin cancer.

Sunscreen's job He says ultraviolet radiation (UVR) exposure of skin is the main cause of sunburning, immunosuppression, aging and cancer - and the job of sunscreens is to prevent those problems.

"We know they avoid sunburn, because their efficacy is measured and their sun protection factor (SPF) determined by assessing this. The question arises, however, about whether they also provide protection against cancer and aging," Dr. Hawk explains.

He notes that while some studies suggest sunscreens are not effective against cancer, other epidemiological studies suggest they are, and he asks which view is correct.

Looking at the process logically, Dr. Hawk points out that UVR exposure causes the sunburning of skin and immunosuppression with an apparent result of acute UVR-induced DNA damage to keratinocytes and immunocompetent cells.

If damage goes unrepaired and persists, significant mutation of the DNA can occur to the basal layer stem cells.

But Dr. Hawk says, "You have an inbuilt system that repairs the sun damage for you and which works over a couple of days. The DNA repairs itself by copying the complementary DNA strand - so most of the damage is repaired and, therefore, you don't have mutations. But if you get such bad damage that both strands are damaged and you can't copy the other one, there will be a persisting mutation - particularly if you have quite severe sun damage.

"A lot of DNA is just sitting there doing nothing and many mutations may not matter, but you may get a mutation that really matters and you need a number of those, one after another, to lead to cancer.

"If you keep getting randomly important damage over the years, such that you get one mutation followed by another, followed by another important one, then you end up with a tumor in due course. Several random mutations seem to be needed to lead to cancer. We probably get huge amounts of DNA damage which are not repaired and which do not matter," Dr. Hawk explains.

He adds that photoaging and cancer are probably a cumulate effect of mutations at important DNA sites while immunosuppression, which occurs concurrently with that damage, contributes to the progression of cancer.

"If sunscreen reduces cutaneous DNA damage - which we know it does - that should reduce cutaneous immunosuppression and cell mutation resulting in a reduction of photoaging and sun-induced carcinogenesis," he says.

Dr. Hawk also points out that sunscreen studies done over the past decade do show they reduce malignant lesions, so, he says, the question remains why some epidemiological studies suggest the opposite.

The answer to that question may lie in the person using the sunscreen - and how they use it - rather than in the sunscreen itself.

Application problems People who are spending time out of doors and applying sunscreen generally seem to do so at a concentration of 0.5 mg/cm² rather than at the recommended 2 mg/cm². With that little sunscreen being used, the actual sun protection factor is reduced to a half or a third of what people expect it to be.