Story last updated at 7:34 a.m. Monday, June 21, 2004
Charleston.net
United Feature Syndicate

Q: With summer approaching, please review the best sunscreens. Which ones will provide protection against UVA and UVB rays?

A: It's quite simple to choose a sunscreen if you follow a few recommendations and accept some uncertainty.

First, you may not realize it, but there's a difference between sunblocks and sunscreens. True to their name, sunblocks block the sun from reaching your skin.

Sunscreens, however, absorb rather than reflect ultraviolet (UV) radiation. At one time, the main ingredient in most sunscreens was para-aminobenzoic acid, or PABA. But PABA can cause allergic reactions. Most sunscreens now contain benzophenones, which protect against UVA; salicylates, which protect against UVB; or cinnamates, which may protect against both. You'll see these listed as active ingredients on sunscreen labels. You'll also see ingredients such as oxybenzone, octyl salicylate and octyl methoxycinnamate.

The sun's energy is carried on three different wavelengths of UV radiation. The Earth's atmosphere absorbs the shortest of these, ultraviolet C (UVC), long before it can reach the skin. Intermediate-wavelength ultraviolet B (UVB) carries the energy that causes sunburn. It's also the major factor in basal cell carcinoma and probably in squamous cell carcinoma, the two most common types of skin cancer.

Long-wavelength ultraviolet A (UVA) is 1,000 times less effective in producing sunburn than UVB. Actually, it is more plentiful and penetrates more deeply into the skin. UVA used to be considered relatively harmless. But researchers now believe it's as important as UVB in causing skin cancer, including melanoma, the rarest but most serious form of the disease. UVA also speeds skin aging by damaging collagen, a protein found in skin.

Federal regulations require manufacturers to identify the SPF ratings of their products. The SPF rating is based on a calculation comparing how long a small patch of skin takes to redden with and without sunscreen. If skin reddens in 10 minutes without sunscreen, it will redden in 150 minutes when protected by an SPF-15 sunscreen. And likewise, in 300 minutes with an SPF-30 sunscreen. In essence, SPF measures how much UVB is getting to the skin.

The standard advice is to get a sunscreen or sunblock with an SPF rating of at least 15. People who have fair skin, many freckles or moles, a family history of skin cancer, skin that burns easily or a prescription drug that makes their skin light-sensitive should go for an SPF of 30. But keep in mind that SPF is based on the use of a set amount of sunscreen, 2 milligrams per square centimeter of skin. That works out to a bit more than an ounce for a 5-foot-8 person per application. But most people use a half to a quarter of that amount. At those levels, an SPF-15 sunscreen has an SPF of just 3 to 7. Consider this: to get the full SPF value out of sunscreen, a family of four should be using an entire 4-ounce bottle during a day at the beach, even more if they are spending a lot of time in water.

Any product with an ingredient that absorbs or blocks UVA can be billed as offering "broad-spectrum protection" or "UVA-radiation protection." But the FDA and the sunscreen/sunblock makers haven't agreed upon a standard test or rating scale for UVA protection. Such a scale would be comparable to the SPF. High-SPF sunscreens do protect skin against part of the UVA spectrum. However, they don't usually provide any protection against longer UVA wavelengths.

Research suggests that few sunscreens fully deliver on their broad-spectrum claims. One study evaluated 59 products offering UVA protection. Most (93 percent) did shield against the UVA wavelengths nearest to UVB. But only 10 percent protected against the longer UVA wavelengths. The most effective products contained titanium dioxide, zinc oxide or avobenzone.

Don't go by just the SPF. Look for a brand that protects against a broad-spectrum of ultraviolet radiation. The UVA protection might not be 100 percent, but it is better than nothing. You also need to make sure that a broad-spectrum sunscreen contains ingredients that are proven UVA protectors, zinc oxide, titanium dioxide or avobenzone. And there are enough question marks about avobenzone that a preference for zinc or titanium is warranted. Some research suggests that avobenzone might degrade when exposed to UVA, the very thing it is supposed to protect against.

When shopping for protection against the sun, know that avobenzone is sometimes marketed as Parsol 1789. Also, many companies market products that give UVA protection as sunblock, rather than sunscreen. Look for sunblocks and you shouldn't have any problem finding avobenzone, titanium dioxide or zinc oxide. Brands that contain these chemicals include Neutrogena Sensitive Sunblock Lotion, Banana Boat Quick-Dry Sport and Hawaiian Tropic Ozone Sunblock.

If you would like to e-mail questions to the Harvard Medical School Adviser, you can submit questions to the Harvard Medical School Adviser at www.health.harvard.edu/adviser. Unfortunately, personal responses are not possible. For more consumer health information from Harvard Medical School, please visit www.health.harvard.edu.