As swimming pools close their gates and the weather becomes too cool for the beach, some of the most fervent sun lovers are flocking to tanning beds to preserve their summertime color.

Many are aware of the health risks. Though any form of ultraviolet light increases skin cancer risk, people younger than 30 who use tanning machines increase their risk of skin cancer by 75 percent, according to the International Agency for Research on Cancer, which is affiliated with the World Health Organization. The melanoma rate among young women nearly tripled from 1973 to 2004, a period in which use of tanning beds increased significantly, a National Cancer Institute study showed.

Studies show that tanning salon customers know as much or more about skin cancer than non-tanners. Yet emerging evidence shows that frequent tanners seek out ultraviolet radiation, or UV, not just because they want to look good but because they want to feel good.

For some, tanning resembles not just a bad habit but an addiction, says dermatologist David Fisher, author of an editorial earlier this month in The New England Journal of Medicine. In an April study of 229 tanners published in the Archives of Dermatology, up to 39 percent met commonly used criteria for addiction. People who met the addiction criteria were far more likely than others to be anxious or depressed, and to use other addictive substances, such as drugs and alcohol, the study says.

The Indoor Tanning Association's John Overstreet says there's no proof that tanning causes addiction. Instead, he says, "some people who show addictive tendencies may also tan more than they should."

But Fisher says the evidence for a real addiction is growing.

The cause of that addiction could be endorphins, the body's "feel good" molecules, which are released in response to UV light, says dermatologist Steven Feldman of the Wake Forest University School of Medicine.

Endorphins help relieve pain and cause the natural "runner's high" that people feel after rigorous exercise.

Thanks to these endorphins, an experiment showed, tanners can actually tell the difference between "real" and "fake" tanning beds, even though they look identical, Feldman says. When he gave 12 regular tanners a choice between two machines in an experiment, people chose the UV bed 95 percent of the time.

Participants said they chose their beds because they made them feel more relaxed, he says.

In another small experiment with frequent tanners, Feldman found that blocking endorphins can produce symptoms similar to narcotic withdrawal, such as nausea, sweating and nervousness, Feldman says.

Yet that doesn't make people addicted, says Stuart Gitlow of the Mount Sinai School of Medicine, an addiction specialist.

Gitlow says there are serious differences between real addiction and bad habits, such as overindulging in video games or chocolate.

People who tan in spite of a potential cancer risk aren't addicted, he says. "On the other hand, if I were to do so despite having skin cancer and a narrow brush with death, now we're talking about addictive disease."

In the same way, all drinkers aren't alcoholics, he says. A person is clearly an addict, however, if he or she continues to drink even after alcohol causes serious harm, such as losing a job or spouse.

"The difference between addicts and non-addicts is that non-addicts learn that the lit match will burn them," Gitlow says. "Addicts, on the other hand, think, 'This time, the match won't burn me.' "

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SIGNS YOU MAY BE ADDICTED

Up to 39 percent of tanners are addicted to tanning, according to a study in April. Criteria in the study included:

  • Do you feel guilty about how much you tan?

  • Have you tried to cut back but failed?

  • Have you missed important activities because you chose to tan instead?