Ultraviolet (UV) radiation can promote skin cancer in by damaging DNA in skin cells and by weakening the body’s natural defenses against cancer cells. UVA radiation penetrates deeper, tanning and aging skin; UVB radiation causes sunburn.
The sunscreens we tested have an SPF between 30 and 75+. (Note that the Food and Drug Administration doesn't have sufficient data to show that products with an SPF above 50 offer additional protection.) Most labels claim “broad spectrum” protection against both UVA and UVB rays.
All things being equal, if you sunburn after 20 minutes without sunscreen, an SPF 30 product would protect you for about 10 hours—20 minutes x 30. But in reality, the sun's intensity, your geographic location, your skin type, and other factors come into play, so the SPF is a relative measure of protection.
Also keep in mind that selecting a sunscreen with the highest SPF may not necessarily offer the protection you think, according to Sophie J. Balk, M.D., an attending physician at Children’s Hospital at Montefiore in the Bronx, New York. “People think if they use an SPF 30 instead of 15, they are getting twice the protection, she says, "but the difference is actually much less.”
In addition, “Nobody’s getting the SPF they think because most people do not apply enough sunscreen," she says. "An adult would need to use about one ounce per sitting to get the full SPF.”
Remember to check sunscreen ingredients when shopping. Oxybenzone may interfere with hormones in the body, and nanoscale zinc and titanium oxides have been linked to potential reproductive and developmental effects. In skin, retinyl palmitate converts readily to retinoids, which have been associated with a risk of birth defects in people using acne medication containing the substance. Pregnant women may want to avoid products with retinyl palmitate, noted in the sunscreen Ratings.