Sunday, July 15, 2007
Sunburns, Sunscreens, and Skin Cancer
Summertime and the living's easy. Easy enough, for instance, to fall asleep in the sun and wake up medium rare. As our parents taught us, before hitting the water we should slap on the sunscreen, lest we become extra crispy, or worse, we get skin cancer.
We put on sunscreen to block out one particularly vicious element of sunlight: ultraviolet (UV) rays. These are waves of light that are a higher frequency than visible violet rays, and are known for their ability to damage the DNA of our cells, for instance creating a bond between two Thymidines in our DNA known as a Thymidine dimer or introducing free radicals that can then damage the cell. When too much damage is dealt to a cell's DNA, it triggers a type of abort sequence and that cell undergoes programmed cell death, or apoptosis. When UV light hits your skin, it can injure the cells that populate the epidermis, or outermost layer, of the skin: commonly, keratinocytes and melanocytes. By then undergoing cell death, the epidermis becomes inflammed and reddened in the phenomenon known as sunburn, or solar erythema. If too many cells die the epidermis may seperate from the dermis underneath it, resulting in the blistering so common to second degree burns.
Actually, in some ways the sunburn is a good sign: it means that the cells that have been damaged by the sun are dying. On the other hand, if damage is not "caught" and the skin cells don't die by apoptosis, certain DNA injuries may cause malignant changes. The most common skin cancers (95%) are basal cell and squamous cell carcinomas, which stem from keratinocytes. Melanocyte (pigment cell) cancers, or melanomas, only make up 5% of skin cancers but are responsible for 75% of the mortality associated with skin cancer. This can be a brutal illness, often metastasizing to the liver, lungs, bone, and brain. The interesting thing is that there appears to be an association between the type of UV light and the type of cancer. UVB light, which has long been associated with sunburns, creates Thymidine dimers, which are more easily seen by the cell and usually cause cell death, but may also cause cancer. This was the light normally blocked by most sunscreens. There is also UVA light, which is a longer wavelength and does not nomally cause burns. However, it may cause sub-lethal mutations in your skin cells, particularly melanocytes, which can mean cancerous change. So while your sunscreen blocks UVB, it may let UVA through with deleterious results.
The bottom line: have fun in the sun but be safe. Make sure you use a sunblock that works against both UVA and UVB light. And, as an article on sunscreen from The New York Times last week discusses, make sure that you use it correctly. A shotglass of sunscreen for the body and a teaspoon for the face, reapplying every few hours. Don't let a sunburn (or worse) get in the way of your summer fun.