Columnist Tara Parker-Pope answers readers' questions.
April 26, 2005; Page D7

Q: I have heard that sunscreen SPF 70 is no better than SPF 30. The argument is that after SPF 30, the higher you go really doesn't matter. Is this true?

-- M.S.

A: The issue of how high the SPF or sun-protection factor remains a subject of ongoing debate in the sun-care industry. Here is what we know. The SPF is based on how long your skin can stay in the sun without burning compared with unprotected skin. So someone whose skin starts to redden after five minutes on the beach would get 75 minutes of skin protection from an SPF 15 sunscreen and 150 minutes of protection from an SPF 30. A sunscreen with an SPF of 30 blocks 97% of the sun's burning rays, compared with 93% blocked by sunscreens with an SPF of 15. Although a higher SPF sunscreen, if used properly, should allow you to stay in the sun that much longer without burning, it isn't clear if high SPF sunscreens block any more than 97% of burning rays.

But one big problem is those numbers are based on sunscreen use under laboratory conditions. In the real world, people use about one-fourth the amount of sunscreen they should, so they are probably getting a lot less protection than the SPF number promises. One worry is that super-high SPF sunscreens will give users a false sense of protection prompting them to spend more time in the sun. But since most people are using far too little sunscreen anyway, some advocates think the higher the number the better.

The far bigger issue is that consumers need to look beyond the SPF number. Sunburn is prevented by blocking UVB rays, the medium wavelength ultraviolet rays that turn the skin red. But the SPF number doesn't tell you whether a product also blocks harmful UVA rays, which are the long wavelength rays given off by the sun, tanning beds and sunlamps. They enter the skin more deeply than UVB rays and may pose the biggest threat for sun damage and skin-cancer risk. Pick a sunblock with an SPF of at least 30, but then look further to see if the product is a broad spectrum blocker, meaning it blocks both UVB and UVA rays. Many doctors think the most effective blocker is Parsol 1789 or avobenzone. Another blocker, Mexoryl, may be even more effective at blocking the broadest spectrum of rays. While it is available in Europe and Australia, it has yet to be approved in the U.S.

When you apply sunscreen, slather it on and reapply it often. Remember your ear lobes and the soles of your feet -- a common place doctors find skin cancer.

Q: I am unable to wear sunscreen. After a couple of hours, my skin turns red and inflamed, itching unbearably. The only thing I have found so far is chemical-free sunblock, made with titanium dioxide. Unfortunately, it has a low SPF, looks like clown make-up, and can stain clothing. Are there any alternatives available?

-- A.B.

A: A true allergy to sunscreen is uncommon, but only an allergist can determine if that is the case. The most common reason for a skin reaction after applying sunscreen is irritant contact dermatitis, in which your skin is reacting to a fragrance or preservative in the sunscreen, according to Ellen S. Marmur, chief of the division of dermatologic surgery at Mount Sinai Hospital in New York. The solution is to search for hypoallergenic, fragrance-free and paba-free alternatives such as Oil of Olay's Complete Defense line for sensitive skin or Neutrogena's Sensitive Skin sunblock. Both products have an SPF of 30, are lightweight and don't leave you looking like a clown.

Another potential cause of your problem may be a phototoxic or photoallergic reaction -- in which exposure to sunlight triggers the skin reaction. Certain medications -- such as birth-control pills, antibiotics, allergy drugs, pain relievers and high-blood-pressure medicines -- can trigger phototoxic reactions. And chemicals and fragrances in lotions can trigger a photoallergic reaction. An allergist can perform a photo patch test to determine your skin's sensitivity to sunlight.

Given that sunscreens can wash off anyway, one solution may be to start wearing sun-protective clothing. Solumbra, at www.solumbra.com, offers an FDA-approved line of shirts, hats and items that offer SPF 30 protection.