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Update on Sunscreens

August 01, 2007 By Helen Rose

By Mark B. Levin, M.D.
The Pediatric Group, P.A., Princeton

This is the 63^rd article written by the Pediatric Group, P.A. for Princeton Online

A recent medical publication assembled a panel of experts to reassess sun protection products. They came up with some findings that are useful and some that are surprising. Prior to making any recommendations a complete understanding of what radiation from the sun can cause and how sun protection products work is critical.

The sun bathes the earth in many different types of radiation. The warmth we feel from the sun is caused by infrared (IR) radiation. Prolonged exposure to IR can cause the skin to redden. People with a light complexion are more susceptible to this effect than those with a darker complexion. Although no one wants excessive redness on sun exposure, IR is not the type of radiation that causes skin cancer.

The sun also emits ultraviolet (UV) radiation. UV is a bad actor. UV type A is categorized in 2 types (1 & 2). 95% of the radiation responsible for photoaging and phototoxicity is caused by UVA. UVA is prevalent all through the year and is not blocked by clouds or plain glass windows. UVA type 2 can cause some skin redness.

UV type B, which causes redness and sunburn, is particularly prevalent in the spring and summer at midday. In laboratory experiments, volunteers have been subjected to UVB to see how long it takes for their skin to redden before and after application of sunscreen products. If the product prolongs the time to redness by 3 hours, it is awarded a sun protection factor (SPF) of 3. If it prolongs the time by 15 hours, it gets an SPF of 15. This, of course, dependant on the protective product not wearing off because of deterioration, friction or perspiration.

Both UVA and UVB are responsible for skin DNA damage that predisposes a person to a certain type of skin cancer called squamous cell carcinoma. Basal cell carcinoma and malignant melanoma are not thought to be caused by UV exposure. Squamous cell carcinoma can become metastatic and difficult to treat. Hence the importance of skin protection from all types of UV radiation. Unfortunately we can no longer rely on our atmosphere to protect us from UV radiation since the protective element, ozone, is being rapidly depleted due to pollution.

Sun protective products are sunscreens and sunblocks. Sunscreens work by binding to the skin and absorbing the UV in their chemical bonds. They must be applied early enough before sun exposure to bind to the skin. Unfortunately it is their very mode of action - absorption of UV energy - that causes them to deteriorate and lose their effectiveness. In fact, the only sunscreen chemical (avobenzone) that effectively absorbs UVA loses 60% of its effectiveness after 60 minutes of sun exposure! All of the other chemicals found in combination with avobenzone in various brands of sunscreen products absorb UVB to varying degrees.

Sunblocks remain on the surface of the skin to reflect the UV radiation. The only two sunblock agents are zinc oxide and titanium dioxide. Since they do not need time to bind to the skin, they are very effective in blocking all types of UV immediately upon application. However, because they remain on the surface, they are subject to removal by perspiration and friction from towels, clothing or usual daily activities.

As a result of the short duration of action of sunscreens because of deterioration and of sunblocks because of removal from the skin surface, these products, in order to be effective in preventing skin cancer and sunburn, must be reapplied every two hours regardless of their assigned SPF and even if they are labeled as "very water resistant". Also note that artificial sunless tanning agents offer NO sun protection whatsoever.

There are two last points to be made. If sun protection preparations are used in conjunction with insect repellents, the repellent should be applied first, then the sun protective preparation can be applied second. Repellents containing DEET should be washed off when returning indoors. Also, UV is as destructive to the retina of the eye as it is to the skin. Ophthalmologists now recommend that everyone from infancy wear sunglasses that block UVA and UVB. Inexpensive child-sized UV-blocking plastic sunglasses are available at most variety stores.

Dr. Mark B. Levin

Dr. Levin was a member of the staff at The Pediatric Group starting in 1977. He was an attending Pediatrician at the Medical Center at Princeton, Chairman, Department of Pediatrics, Medical Center at Princeton, 1984 to 1986, 1989 to 1992, and past President, Medical and Dental Staff, Medical Center at Princeton, 1987 to 1988. Dr. Levin served on numerous Departmental and hospital committees. He published original articles both while at Upstate Medical Center in Syracuse and at The Pediatric Group.