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This article was published 12/7/2013 (1441 days ago), so information in it may no longer be current.
The season of bare skin and scorching sun is here, and you -- like so many other people -- may find yourself scratching your head over sunscreen.
Yes, skin protection is essential, especially with skin-cancer rates on the rise in many populations around the world. But sunscreens come with often confusing labels and long, unpronounceable lists of chemical and other ingredients. How do you know which are safe to slather on you or your kids?
The first thing to keep in mind is not all sunscreens are created equal, says Mary Sheu, an assistant professor of dermatology at the Johns Hopkins School of Medicine.
"There are physical sunscreens that reflect light -- they're like little mirrors that sit on your skin," she says. Such products, made with zinc oxide or titanium dioxide, sit on your skin and block the sun's UVA and UVB rays. (These are the ones that can cause sunburns, cell damage and skin cancer.)
The minerals are opaque, giving beach-goers that classic white-nose look, though new versions are often tinted or "micronized" (ground into tinier-than-usual particles) so they'll blend into the skin.
Physical sunscreens are the least likely to produce rashes or other allergic reactions, so they're often recommended for people with sensitive skin, Sheu says.
The other kind of protection is chemical sunscreen. Instead of blocking or reflecting the sun's rays, these products absorb UVA and UVB light to keep it from damaging skin, Sheu says. Unlike physical sunscreens, they can be absorbed into the skin -- and that's where the question of safety comes in.
"Even though the data are soft, we do know that a certain amount of the chemical sunscreens are absorbed into the body, and we don't know exactly what their effects are," says Robert Friedman, a clinical professor of dermatology at the New York University School of Medicine and the chief executive of MDSolarSciences, a company based in Norwalk, Conn., that develops sunscreen and skin-care products.
The data Friedman refers to come from a 2008 survey by the Centers for Disease Control and Prevention that examined urine samples from more than 2,500 people selected as a representative sample of the U.S. population. Researchers detected oxybenzone (sometimes called benzophenone-3) -- a key ingredient in many chemical sunscreens -- in 97 per cent of the samples. The findings suggest almost all Americans have absorbed oxybenzone into their bodies, but it doesn't clarify where the compound came from (oxybenzone is also used in cosmetics) or how it may affect health.
Scientists have seen hints of what oxybenzone -- and two other common sunscreen ingredients, octocrylene and octyl methoxycinnamate -- can do in other studies, and the results have raised some concern. In a small 2006 study that tested these UV filters on a skin model made from human cells, chemist Kerry Hanson of the University of California at Riverside found the filters break down in UV light, losing their ability to protect the skin and ultimately generating more free radicals -- molecules that can "steal" electrons from cells, damaging them in the process -- in the skin than if there were no sunscreen applied at all. Free radical formation is one of the ways a sunburn damages the skin, and Hanson found degraded UV filters can amplify the effect.
"Your skin naturally makes free radicals, but it also has natural antioxidants (molecules that protect the body from free radicals) that balance out the free radical load," says Hanson, who has consulted with sunscreen companies interested in improving their products.
"But the three UV filters we tested show more free radical generation than would naturally occur."
This has led some people to choose physical sunscreens over chemical ones -- but they come with their own risks. The titanium dioxide in many physical sunscreens also degrades in light and can generate free radicals in the skin.
"It all comes back to the need to reapply," Hanson says. Most sunbathers don't reapply their sunscreen, or they apply it too infrequently. She urges anyone venturing into the sunshine to follow the Skin Cancer Foundation's guidelines, which call for one ounce (about two tablespoons) of sunscreen for an adult's entire body, reapplied every two to three hours. The reapplications replace light-degraded sunscreen, preventing some of the free radical formation and more consistently protecting the skin, Hanson says.
What about SPF? SPF, or sun protection factor, is a number that denotes how effectively a sunscreen will protect your skin from UVB rays. (It doesn't measure UVA protection.) The higher the number, the longer lasting the protection, no matter what type of sunscreen it is. An SPF of 15, for example, roughly means it will take you 15 times as long to develop a sunburn as it would without wearing sunscreen.
Sheu, Friedman and Hanson agree an SPF of around 30 is the magic number. Sunscreens with higher SPFs can create a false sense of security and lead users to stay in the sun too long.
-- Washington Post-Bloomberg