There is an unrecognized epidemic of skin cancer underway in the United States, the American Academy of Dermatology warns.

One in five Americans will develop skin cancer, and a person’s risk of the disease doubles if he or she has had five or more sunburns, according to a report in the April issue of the Mayo Clinic Health Letter.

Basal and squamous cell carcinomas, the most common and treatable types of skin cancers, had long been considered a problem only for people over 50, according to the report. But Mayo Clinic researchers found that the percentage of women under 40 with the more common type, basal cell, tripled between 1976 and 2003, while the rate of squamous cell cancers increased four-fold.

In the same study, the researchers found that just 60% of the cancers they identified occurred on skin frequently exposed to the sun, such as the head and neck, rather than the normal 90%. Most of the remaining cancers were seen on the torso. The researchers suspect this may be due to more widespread use of tanning beds.

Two types of ultraviolet (UV) light are implicated in skin cancers, the article explains. UVA, which penetrates deeper into the skin and impairs its immune defenses, is more responsible for melanoma, the most deadly type of skin cancer. UVB exposure causes sunburn, as well as squamous and basal cell skin cancers.

Tanning beds chiefly release UVA, although some also use UVB. According to the Mayo Clinic report, “occasional yet intense UVA exposure poses a greater risk of melanoma skin cancer than does spending long hours in the sun.”

The report offers the following tips for effective sun protection:

-Apply sunscreen 30 minutes before you are going to be outside, even if the weather is cloudy or hazy.

-Spend as little time as possible in the direct sun between 10 a.m. and 4 p.m.

-Use about an ounce of sunscreen - roughly a shot glass full - and reapply it every two hours.

-Always wear sunglasses and a broad-brimmed hat, and wear clothing made from tightly woven fabric to protect your skin.

SOURCE: Mayo Clinic Health Letter, April 2006.