Considering our winters, it’s not surprising we love the summer sun. Unfortunately, too much sun isn’t good for us: the thinning ozone layer is letting in more ultraviolet radiation than it used to, and as a result, skin cancer is on the increase.

That unhappy fact has made sunscreens, concoctions that keep ultraviolet radiation from reaching the skin, all the rage. Just one problem: although they prevent sunburns, we don’t know for sure if they prevent skin cancer.

Ultraviolet radiation, which make up 15 percent of the solar energy that reaches Earth’s surface, packs enough energy to damage our skin’s DNA. Our skin tries to fight this by tanning: producing more of the dark pigment melanin.

There are two kinds of ultraviolet radiation, ultraviolet A (UVA) and ultraviolet B (UVB). UVA penetrates more deeply into the skin, but UVB is more energetic, and hence more damaging. Both play a role in promoting cancer: UVB damages DNA while UVA apparently suppresses the immune system, making it harder for the body to repair that damage.

Sunscreens block ultraviolet radiation by absorbing it or scattering it. The very first UV-absorbing sunscreen developed, para-aminobenzoic acid (PABA), has fallen out of use because it stained clothing and some people were allergic to it. Today, active ingredients such as Padimate O, octyl methoxycinnamate and octyl salicylate absorb mostly UVB radiation, while chemicals such as oxybenzone and Parsol 1789 absorb UVA.

Sunscreens that scatter ultraviolet rays contain substances such as talcum, zinc oxide and titanium dioxide. Originally these kinds of sunscreens were opaque (remember the white paste on the noses of lifeguards?), but today manufacturers use smaller, invisible particles.

All of this sounds wonderful. Ozone layer thinning? Slap on some more sunscreen. But at a conference in February, Marianne Berwick, an epidemiologist at Memorial Sloan-Kettering Cancer Center in New York, surveyed the research to date and reported that right now, there’s no proof that sunscreens prevent cancer at all.

For example, only two of four studies of squamous cell skin cancer (a generally non-lethal form) indicated that sunscreen use protected against it. Two studies of basal cell carcinoma, another non-lethal (and the most common) form of skin cancer actually showed that people who used sunscreen were more likely to get it. And out of 10 studies of melanoma, the most virulent form of skin cancer, five showed people who used sunscreen were more likely to get it, two showed that sunscreen protected against it, and three showed no correlation at all.

One problem with these studies, other scientists point out, is that they mostly involved people in their 50s, who didn’t wear sunscreen until fairly recently. Since most people receive 50 percent of their total exposure to the sun before age 20, that could skew the results.

As well, it may be that people who wear sunscreens simply stay out in the sun longer, thinking they’re protected. Unfortunately, if you’re out all day, even when you’re wearing a sunscreen, enough ultraviolet radiation can sneak through damage DNA. And fair-haired, fair-skinned people, who are six times as likely to develop melanoma than dark-haired, darker-skinned people, are the ones most likely to rely heavily on sunscreens.

Sunscreens themselves may cause some DNA damage. When a chemical sunscreen “absorbs” ultraviolet light, it’s really converting it to some other form, which may also be capable of damaging DNA. For example, PABA is known to increase the formation of a particular DNA defect in human cells which in some people increases the susceptibility to skin cancer. Some current sunscreen ingredients can oxidize DNA; they also produce free radicals, energetic molecules that can break DNA strands.

DNA damage is of little concern on the surface of the skin, since the epidermis is made up of dead cells. Some sunscreens are known to penetrate the skin, however.

Does this mean we shouldn’t wear sunscreen? Nobody is saying that. Even if sunscreens can themselves cause some DNA damage, the sun causes far more.

The message, instead, is not to rely solely on sunscreen. Use common sense. In addition to sunscreen, wear a hat and long pants and a long-sleeved shirt if you’re going to be out in the sun all day. Don’t sunbathe. Enjoy the sun, like all good things, in moderation.

Unfortunately, that’s hard message to sell in a wintry country like Canada–which means that this summer, despite uncertainties about its effectiveness, most of us will continue to baste ourselves in sunscreen and hope for the best.

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