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Dr. James M. Spencer, a dermatologist in St. Petersburg, Fla., who specializes in skin cancer, said that he hopes the updated standards will clarify how much protection sunscreens provide, the dose needed to achieve significant protection, and the frequency with which a sunscreen should be reapplied.
The F.D.A. in 1978 first proposed a system of labeling products with an S.P.F. or Sun Protection Factor, which measures how effective the product is in preventing burn caused by the sun�s ultraviolet B rays. UVB radiation can also be a factor in skin cancer.

Dr. Spencer said that an S.P.F. 15 product screens about 94 percent of UVB rays while an S.P.F. 30 product screens 97 percent. Manufacturers determine the S.P.F. by dividing how many minutes it takes lab volunteers to burn wearing a thick layer of the product by the minutes they take to burn without the product.

But people rarely get the level of S.P.F. listed because labels do not explain how much to use, said Dr. Vincent A. DeLeo, chairman of dermatology at St. Luke�s-Roosevelt Hospital Center in Manhattan.

�Sunscreen is tested at 2 milligrams per square centimeter of skin, which means you should be using two ounces each time to cover your whole body,� Dr. DeLeo said. �But for most people an eight-ounce bottle lasts the whole summer.�

People who apply S.P.F. 30 too sparingly, for example, may end up with only S.P.F. 3 to S.P.F. 10, according to the Web site of the British Columbia Centre for Disease Control, www.bccdc.org/downloads/pdf/rps/reports/RIN15.pdf, which has comprehensive guidelines.

�The S.P.F. is a terrible system to guide consumers,� Dr. Spencer said. �Nobody is using sunscreen the way it is measured in a lab.� He said he hopes that the new standards will call for S.P.F. to be replaced with a system defining sun protection as high, medium or low.

Until then, Dr. Spencer said that people should use about a shot glass of sunscreen for the body and a teaspoon for the face to best achieve the S.P.F. protection listed on labels. It should be reapplied every few hours and immediately after swimming or sweating.

Dermatologists said that the agency is also likely to introduce a rating system for the sun�s ultraviolet A rays, which can contribute to cancer and skin aging. Many products already contain UVA screening agents, but under the current rules there is no rating for them.

Manufacturers are catching on that some consumers seek UVA protection. In print advertisements this month, Neutrogena and Banana Boat have been battling for UVA supremacy, including graphs in which each shows their product offering the highest coverage.

But Dr. David M. Pariser, the president-elect of the American Academy of Dermatology, said that without a standardized UVA rating system, consumers can�t be sure how much a sunscreen provides.

�Right now, we don�t know whether doubling the percentage of a UVA sunscreen ingredient doubles UVA protection or not,� Dr. Pariser said. �That is part of the muddled system we hope will be cleared up.�

Until then, Dr. Pariser said to choose sunscreens that contain ingredients known to filter UVA. These include Mexoryl SX, avobenzone, titanium dioxide and zinc oxide. He also recommended a database at www.cosmeticsdatabase.com/special/sunscreens/summary.php created by the Environmental Working Group that lists products with UVA protection.

Some doctors, along with Mr. Blumenthal of Connecticut, predicted that the new sunscreen rules would prohibit outsized marketing terms.

� �All-day protection� is just plain false since sunscreen has to be frequently reapplied,� Mr. Blumenthal said. �And �waterproof,� which may be O.K. for an adult taking a quick dip in the pool but not for kids who are in and out of the water all day, is just plain deceptive.�

Dr. Green in Australia said the best way to prevent skin cancer is to stay out of the sun during peak hours and wear sun-protective clothing. But Dr. Halpern said you can�t keep Americans wrapped up.

�There is only a small subset of American society that is willing to wear long-sleeved shirts and wide-brimmed � defined as four inches wide � hats on a sunny day at the beach,� he said. �Until we can get that behavior, the next best thing is sunscreen. Put on two coats, so you won�t miss any spots.�

Precautions

Protection from the sun�s harmful rays requires more than slapping on a coating of cream. There are multiple precautions to take, according to interviews with doctors.

� Avoid outdoor activities during peak sun hours of 10 a.m. to 4 p.m.

� Wear protective clothing, sunglasses and wide-brimmed hats.

� If you are prone to burn, use a sunscreen with S.P.F. 30 or higher.

� Apply about a teaspoon of sunscreen to your face and a shot glass of it to your body.

� Make sure your sunscreen contains at least one ingredient known to filter UVA rays, such as zinc oxide, titanium dioxide, avobenzone or Mexoryl SX.

� Reapply sunscreen frequently, and immediately after swimming.