NEW YORK (Wall Street Journal) — Most people use sunscreen only on bare skin. But dermatologists say more people need to think about protecting the skin that is underneath your clothes.Summer clothes — white T-shirts, breezy gauze tops and lycra bathing suits — do a poor job of protecting skin from the sun’s harmful rays. In fact, a few recent studies have shown that the incidence of skin cancer on body parts usually covered by clothing is on the rise.

Whether clothing offers much protection to skin depends on a number of factors. A white T-shirt, for instance, has a sun protection factor of only about five to seven.

If it gets wet, the SPF is typically less than three. That means that if your unprotected skin normally would start to burn in five minutes, the skin under a white T-shirt would start to burn after just 15 to 35 minutes in the sun. And even if you don’t burn under your shirt, skin damage still occurs.

The colour, fabric construction, fibre content, weave and whether the fabric has been specially treated all determine clothing’s effectiveness in blocking harmful UV rays. Dark colours do a better job; a green cotton T-shirt, for instance, offers about 50 percent more skin protection than a white shirt. However, even a dark cotton shirt has an SPF of only about ten.

While swimsuits offer some protection, lycra can degrade after exposure to chlorine, which creates tiny spaces in the weave that allow dangerous rays to seep through.

One option is to cover the entire body with sunscreen before putting on your swimsuit or summer clothes. But that’s impractical if you’re spending hours swimming, because it’s not easy to reapply lotion under a suit.

A better solution is to wear specially treated protective clothing. These suits and shirts, from companies like Coolibar, Solumbra and Lands’ End, boast of sun protection factors of 30 to 50, but they aren’t cheap. A kid’s swimshirt from Coolibar costs about $35. A toddler swimsuit from Solumbra costs $50.

The clothes protect skin in several ways. Some Coolibar garments, for instance, are made with titanium dioxide fibres — the same ingredient used in white, opaque sunblocks. Instead of lycra, the company’s swimsuits are made with knit polyester, which isn’t degraded by chlorine, says Coolibar founder John Barrow.

Sun-protective clothes typically have short or long sleeves and cover large areas of the body.

While that may not be desirable by the fashion-conscious, dermatologists say small children, people with very fair skin and people at high risk for melanoma should use the clothes. And if you know you’re going to be spending hours in the sun — such as on a fishing boat — it might be worth the investment, says Susan Weinkle, assistant clinical professor of dermatology at University of South Florida.

But unlike sunscreens, which are regulated by the FDA, there is no regulation of sun-protective clothing. Manufacturers follow voluntary testing guidelines and use private labs to determine a fabric’s SPF or the similar UPF (ultraviolet protection factor) rating.

The only way for consumers to determine if a fabric has been tested is to check with the manufacturer. For instance, Solumbra (solumbra.com), founded by a melanoma survivor, and Cabana Life (cabanalife.com), both detail testing information on their websites. Some brands, including Coolibar (coolibar.com), have paid a fee to have their lab findings certified by the Skin Cancer Foundation.

In May, Good Housekeeping magazine sent 15 clothing items that made sun-protection claims to an independent testing laboratory. All but four items passed the test. And two of the items that didn’t pass fell only slightly short, testing to a UPF rating of 20, rather than 30.

Another option is to wash your clothes in Rit Sun Guard, which increases the UPF rating of an item of clothing to about 30 and lasts for 20 washes. The laundry additive has been tested and approved by the Skin Cancer Foundation.