Almost all the cells in your body replace themselves as they wear out, your skin cells more frequently than any other tissue. Exposed to a lot of wear and tear, the cells of your skin divide about every 27 days to replace dead or damaged cells. In each instance, the skin sloughs off dead cells from the surface and replaces these with new cells from beneath. The average person will lose about 105 pounds of skin by the time he or she turns 70.

Where's the sunscreen?

Tans are fashionable, but they are not safe

 While your skin can be damaged in many ways, the damage that seems to have the most long-term effect is caused by the sun. The skin contains cells called melanocytes that produce a pigment called melanin when exposed to UV light. Melanin produces a yellow to brown color in the skin. The type of melanin and the amount produced is genetically determined.

Fair-skinned people have fewer melanocytes and produce melanin that is more yellow in color. People with fair skin don't produce very much melanin and so have little natural protection from UV radiation; these people sunburn easily and rarely tan. Cells on the body's surface that are badly damaged by the sun -- what we call a sunburn -- slough off.

Recall the peeling you experience after a bad sunburn? That was your body tossing off skin cells killed by the sun.

People with darker skin types have more melanocytes and produce a melanin that is dark brown in color. Protected by UV-absorbing melanin, they almost never burn. This is perhaps why so many people think tanning is healthy.

A matter of style 

People didn't always think so. Up until the early 20th century, a tan was a condition that people went to great lengths to avoid. Before the industrial revolution, a tanned body was a sign of the working class, people who had to work in the sun. The wealthy elite avoided the sun, with pale skin being in fashion. Greeks and Roman would use chalks and lead paints to whiten their skin. The women in Elizabethan England would even paint blue lines on their skin to make their skin appear translucent.

All of this changed in the 1920s, when tans became a status symbol, with the wealthy able to travel to warm, sunny destinations, even in the middle of winter. That tan, bronzed glow that people would sit in the sun for hours to achieve was thought to be both healthy and attractive.

During the 1970s, doctors started to see an uptick in the number of cases of melanoma, a deadly form of skin cancer. New cases were increasing about 6 percent each year. Researchers proposed that UV rays from the sun were the underlying cause of this epidemic of skin cancer and warned people to avoid the sun when possible and protect themselves with sunscreen.

Malignant melanoma is the most deadly of skin cancers, although treatable if caught early. Melanoma is cancer of malanocyte cells. Melanoma lesions usually appear as shades of tan, brown, and black and often begin in or near a mole, and so a color change in a mole is a warning symptom of melanoma. Melanoma is most prevalent in fair-skinned people, but unlike the other forms of skin cancer, it can also affect people with darker complexions.

But the public has been slow to respond to this warning from scientists, perhaps because the cosmetic benefits of tanning are immediate while the health hazards are much delayed. The desire to achieve that stylish tanned, bronzed body is as strong as ever.

Booths aren't better 

A good tan requires regular exposure to the sun to maintain it, so indoor tanning salons have become popular. Tanning booths emit concentrated UV rays from two sides, allowing a person to tan in less time and in all weather conditions (sun, rain, snow). The indoor tanning business has grown in the U.S. to a $2 billion-a-year industry with an estimated 28 million Americans tanning annually.

People thought that building up a tan through the use of tanning booths would protect a person's skin from burning and would reduce the time exposed to the UV radiation, both leading to a reduced risk of skin cancer. However, recent research does not support these assumptions.

A 2003 study of 106,000 Scandinavian women showed that exposure to UV rays in a tanning booth as little as once a month can increase your risk of melanoma by 55 percent, especially when the exposure is during early adulthood. Those women who were in their 20s and used sun lamps to tan were at the highest risk, about 150 percent higher than those who didn't use a tanning bed. As with other studies, fair-skinned women were at the greatest risk. In fact, tanning booths, even for those people who tan more easily, heighten the risk for skin cancer because people use the tanning booths year-round, increasing their cumulative exposure.

But, how does sunlight, whether natural or artificial through tanning booths, cause cancer? The effects have to do with how your body regulates cell division. Because skin cells are on the surface of the body, they are unavoidably exposed to sunlight. However, the desire for a tan can lead a person to deliberately seek a great deal of added exposure. The cancer problem arises because UV light -- whether from the sun or from tanning booths -- damages the DNA of skin cells. It doesn't matter if the exposure results in a healthy-looking tan rather than sunburn, your skin cells are still damaged by the UV rays.

Like a blind man shooting a shotgun at your genes, the UV rays hit and knock out individual genes at random. The unrestrained cell division that is cancer results when certain "tumor suppressing" genes -- genes that restrain cell division -- are disabled.

Because there aren't many laboratory animals that are susceptible to sunburns, most skin cancer research has had to rely on epidemiological data, studying skin cancer in the human population. In recently reported research, a Scandinavian study assessed a large population over a long period of time, and found a clear relationship between UV radiation and melanoma. Stated simply, UV radiation causes skin cancer.

How? In three ways:

1. UV radiation directly damages DNA in the nucleus, causing changes that interfere with normal cell division;
2. UV radiation produces activated oxygen molecules that damage DNA;
3. UV radiation leads to immunosuppression, inhibiting the body's ability to kill cancerous cells.

How does UV damage to tumor-suppressing genes lead to skin cancer? When tumor-suppressing genes are disabled, a cell with this damage to its DNA will continue on past the normal "stop" point of the cell cycle, producing two daughter cells with damaged DNA. These damaged cells will also divide, and their daughters after them, in unrestrained cell division--cancer.

Sunbathing in your teens and early 20s doesn't result in cancer immediately, because it takes repeated exposure to knock out all of the tumor-suppressor genes. But later in life, melanoma is the frequent result of our continued seeking of "healthy" tans.

If you love someone, don't encourage her -- or him -- to tan.

To read previous On Science columns, click on the subject: type II diabetes, heart attack, home-run physics , mad cow disease , global warming I , global warming II , platypus , IQ ,  E85 , dinosaurs at Zoo , losing weight , testing evolution , cigarette addiction