SPF refers to the ability of a sunscreen to block ultraviolet B (UVB) rays which cause sunburns but does not address UVA rays which are more closely linked to deeper skin damage. Both UVA and UVB contribute to the risk of skin cancer.

American Academy of Dermatology spokesman James M. Spencer, MD says, while it is logical for someone to think that an SPF of 30 is twice as good as an SPF of 15, and so on, “that is not how it works.” An SPF 15 product blocks about 94 percent of UVB rays, an SPF 30 product blocks 97 percent of UVB rays, and an SPF 45 product blocks about 98 percent of rays. “After that, it just gets silly,” he says.

Spencer recommends SPF 30 products to his patients because few people apply sunscreens as heavily or as often as they should. One skin care expert points out that, because most people use far less sunscreen than is recommended, high SPF products may better protect against long-term skin damage and exposure-related skin cancers. “Higher SPFs used over a lifetime may translate to healthier skin in later life,” the statement reads. “While the difference in the percentage of ultraviolet radiation blocked between an SPF 55 and SPF 100+ may be slightly less than 1 percent, applying an SPF 100 may lead to much less cumulative sun damage over a lifetime.”