But UCLA's Reisner said dermatologists should not dismiss use of higher-numbered products because it is often difficult to determine whether a sunblock that is designed to be invisible once it is on the skin has been applied thickly enough to work.
"If you don't put the sunscreen on thick enough, you don't get as much protection," Reisner said. "It is basically a chemical trapping of the energy of the sun by the molecules of the sunscreen. If you have fewer of those molecules, it doesn't work as well.
"I think there may be some advantage theoretically in using a higher SPF to make up for the differences in effectiveness as a result of different thicknesses. It seems to me there's nothing to be lost by using a 30 and there might be something to be gained, even though, for the average person, 15 is plenty and any more than that is probably gilding the lily."
Most sunblock products rely on a small number of comparatively standardized basic ingredients, the oldest of which is PABA--or para-aminobenzoic acid, a derivative of chemistry naturally found in the body. A chemical called oxybenzone is also widely used. Molecules in the blocks bind to the skin and prevent the passage of UVB rays.
For parents of very young children, however, the chemistry of sunblocks creates special problems. Most skin specialists believe that most commercial sunblocks are probably not toxic to babies, but definitive scientific studies necessary to resolve the question have not been completed.
Today, said Arundell and other specialists, many dermatologists discourage parents from using sunscreens on their babies except for what are called physical sunblocks, or zinc-oxide based products that form an opaque covering on the outside of the skin itself. Babies should probably be sheltered from direct sun as much as possible, experts said, though Reisner said he is confident enough of the lack of toxicity of commercial sunscreens that he would not hesitate to use them on infants.
SUNBLOCKS MADE SIMPLE Sunblock is opaque and blocks all sun; sunscreen chemically filters some ultraviolet rays but allows some to pass through to the skin for tanning. In practice, the terms are used interchangeably. The sun protective factor, or SPF; a multiple indicating the total time a person wearing the product can safely remain in the sun compared with not wearing sun protection. A person using 15 could remain in sun 7 1/2 hours and experience the same burning as 30 minutes with no protection--or 15 times the unprotected exposure. Waterproof indicates the sunscreen should offer protection for at least 80 minutes while swimming. A water-resistent product should last a minimum of 40 minutes in water. All sunscreens lose effectiveness in perspiration and must be reapplied. Most common active ingredients include para-aminobenzoic acid , or PABA, and oxybenzone. Both bind to skin, chemically filter sun's rays. A true sunblock uses zinc oxide as a physical barrier against sun rays.