Q. Does tanning in a tanning bed cause less damage than natural sunlight?

A. It doesn't matter whether you get it from the sun or from artificial sources such as sun lamps and tanning beds -- ultraviolet (UV) radiation is linked to skin cancers (including basal cell carcinoma, squamous cell carcinoma, and melanoma) and to other sorts of skin damage, particularly premature skin aging (photoaging).

UV radiation is one part of the spectrum of light that reaches the earth from the sun. At the UV end of the spectrum, the wavelengths are too short to be visible to the naked eye. They range in length from 100 to 400 nanometers (nm, or billionths of a meter) and are classified -- from the longest to the shortest -- as UVA (320 to 400 nm), UVB (290 to 315 nm), and UVC (100 to 280 nm). UVA rays, which penetrate deep into the skin, are responsible for tanning. UVB rays damage superficial skin cell layers, causing sunburn. UVC rays, the shortest, are considered harmless, since most UVC light is absorbed by ozone in the upper atmosphere and thus does not reach the earth. Of the UV solar radiation that does reach the earth, up to 95 percent is UVA, and about 5 percent is UVB. For years, scientists believed UVB rays were the most harmful, because sunburn is linked to melanoma, the deadliest form of skin cancer. But since the late 1980s, we've learned that UVA rays also increase skin cancer risk -- and they are the main cause of photoaging.

Tanning beds use fluorescent bulbs that emit mostly UVA, with smaller doses of UVB. The UVA radiation is up to three times more intense than the UVA in natural sunlight, and even the UVB intensity may approach that of bright sunlight.

We first learned about the harmful effects of sunlight from long-term population studies. Our understanding of the risks associated with tanning beds has developed more slowly, because they are a relatively recent phenomenon, first appearing commercially in the United States during the late 1970s. Since the late 1990s, however, mounting evidence has shown a link between tanning bed use and all skin cancers. In 2002, a National Cancer Institute study found that use of an indoor tanning device was associated with a 50 percent increase in the risk of basal cell carcinoma and a more than 100 percent increase in the risk of squamous cell carcinoma.

In 2007, the International Agency for Research on Cancer analyzed 19 studies on indoor tanning and the risk for melanoma. It concluded that people who started indoor tanning before age 35 had a 75 percent greater risk of developing melanoma. Since 2003, UV radiation from any source has been listed by the U.S. National Toxicology Program as a known carcinogen (cancer-causing substance). Currently, many government agencies caution against tanning.

Laboratory research has helped us understand how tanning affects skin cells. Both UVB and UVA rays damage the cells' DNA, potentially causing mutations that may lead to cancer. This same DNA damage is the cause of tanning. In other words, tanning itself is a sign of DNA damage in the skin.

Despite the clear evidence that it's unsafe, the use of tanning beds is on the rise. Nearly 30 million people in the United States tan in salons every year, most of them women between the ages of 16 and 49. Surveys show that many people understand the risks but continue to tan because they think it makes them look healthy.

Meanwhile, the tanning industry makes misleading claims for the healthfulness of indoor tanning.

One claim is that it helps build a base that protects against sunburn. It does, but only slightly -- the equivalent of a sunscreen rated SPF 4 or less. Another claim is that tanning is a good way to stimulate the skin's production of vitamin D, a hormone that's essential to bone health and has been linked to a reduced risk for several cancers. But you can get all the vitamin D you need in a daily vitamin D supplement, which offers all the benefits without any of the risks to your skin.

-- Celeste Robb-Nicholson, M.D., Editor in Chief, Harvard Women's Health Watch

(c) 2009 Harvard Women's Health Watch