Are sunscreen chemicals something to worry about?

As the season of bare skin and scorching sun draws near, 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 that 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 beachgoers 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 percent of the samples. The findings suggest that 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 that 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 that a sunburn damages the skin, and Hanson found that degraded UV filters can amplify the effect.

A Swiss study published in 2001 found that oxybenzone and octyl methoxycinnamate could also behave like estrogens, causing changes in uterine weight in rats that were fed very high doses — much higher than a person would absorb using sunscreen every day. But a 2004 study failed to find detectable hormone changes in people using sunscreen, so its hormone-disrupting potential remains unclear.

Still, these studies have led some people to choose physical sunscreens over chemical ones.

In addition, the negatives aren’t limited to chemical sunscreens. The titanium dioxide in many physical sunscreens also degrades in light and can generate free radicals in the skin.

What’s the best way to avoid the negatives? According to Hanson, the key is to use sunscreen properly, so it doesn’t degrade in the sunlight.

“It all comes back to the need to reapply,” she says. Most sunbathers don’t reapply their sunscreen, or they apply it too infrequently. Hanson urges anyone venturing into the sunshine to follow the Skin Cancer Foundation’s guidelines, which call for 1 ounce (about 2 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.

Both the Skin Cancer Foundation and the American Academy of Pediatrics recommend that babies younger than 6 months of age be kept out of the sun and receive only very small amounts of sunscreen when sun exposure can’t be avoided. They say that infants’ skin is more likely to absorb or react to the ingredients in sunscreens.

When shopping for sunscreen, Hanson suggests seeking out brands that contain antioxidants such as vitamin E (often listed as vitamin E acetate) or vitamin C (often listed as sodium ascorbyl phosphate), which are added to many physical and chemical sunscreen formulations to counteract free radical formation.

“Look for sunscreens that have antioxidants highest up on the list of ingredients,” she says, “as that means they have a higher antioxidant content.”

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.