Tanning is the skin's reaction to ultraviolet (UV) radiation. When skin is exposed to UV rays, cells called melanocytes produce the brown pigment melanin, which darkens the cells of the epidermis. This darkening of the skin cells is the skin's natural – if imperfect – defense against further damage from UV radiation.

The sun's UV rays damage the DNA of the skin's epidermal cells, triggering enzymes that race to repair the damage. However, these enzymes do not always repair the DNA successfully, and all this unrepaired damage can lead to mutations that increase the risk of skin cancer. Also, repeated unprotected sun exposure can cause photoaging – wrinkles, sagging skin, and spots associated with sun damage.

Scientists divide the solar UV spectrum into three wavelengths - UVA, UVB and UVC. Once, UVA and UVC were thought harmless, and only UVB was believed dangerous. UVC is still deemed no threat, since it is absorbed by the ozone layer. But UVA accounts for up to 95 percent of solar UVR reaching Earth. Though far less capable of causing sunburn than UVB, UVA is present during all daylight hours year round, while the amount of UVB in sunlight varies by season, location and time of day.

By the 1990's, scientists knew that UVA exacerbates the cancer-causing effects of UVB, and is the main wavelength behind photoaging. Recently, an Australian-U.S. study found that UVA may be more carcinogenic than UVB. It penetrates more deeply and causes more genetic damage in the skin cells (keratinocytes) where most skin cancers arise. The National Institutes of Health and the World Health Organization now designate both UVA and UVB as causes of cancer.

A year-round tan is a staple of appearance-conscious celebrities and the ordinary men and women who want to emulate that bronzed look.

Darker skin does offer greater protection than light skin against sunburn and skin cancer. However, that applies only to people with naturally darker skin. Tanning, like sunburns, attacks the skin's DNA, producing genetic defects that may cause skin cancer.

But a new study suggests the pursuit of a perma-tan through sunbeds may be as addictive as drugs or alcohol.

The study, carried out by two New York-based researchers – Dr Sharon Danoff-Burg, of the University of Albany, and Dr Catherine Mosher, of the Memorial Sloan-Kettering Cancer Centre – found that people who have used indoor tanning facilities may meet criteria for addictive behaviour. They can also be more prone to anxiety symptoms, excessive alcohol consumption and drug abuse, said the study.

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