A study conducted in part by Chicago's Northwestern Medicine found that tanning beds not only triple the risk of melanoma, but can also damage DNA across nearly the whole skin surface.

Northwestern Medicine and the University of California, San Francisco launched the study after researchers noticed an unusually high number of women under 50 who had skin cancer more than one time.

Dr. Pedram Gerami, professor of skin cancer research at Northwestern University Feinberg School of Medicine, and his research team compared roughly 3,000 tanning bed users with a control group of 3,000 age-matched people who did not have a history of indoor tanning.

The researchers' methods involved single-cell DNA sequencing on melanocytes — the pigment-producing skin cells where melanoma begins.

The researchers sequenced 182 melanocytes altogether, and found skin cells from tanning bed users had nearly twice as many mutations as those from the control group, and were more likely to carry mutations linked to melanoma.

The team found that melanoma was diagnosed in 5.1% of tanning bed users compared with 2.1% of non-users. After adjusting for age, sex, sunburn history, and family history, tanning bed use was still associated with a 2.85-fold increase in the risk for melanoma, Northwestern said.

The researchers also found that tanning bed users were more likely to develop melanoma on parts of the body that are covered in the sun, such as the lower back and buttocks. This supports the hypothesis that tanning beds may cause broader DNA damage than sun exposure, researchers found.

Gerami said only about 20% of a person's skin gets the most damage as a result of outdoor sun exposure, while tanning beds resulted in dangerous mutations across the entire skin surface.

Gerami called for policy changes, including making indoor tanning illegal for minors "at the very least."

"What I'm hearing from most of my patients is that they this started, this exposure started at a time when they were young, vulnerable, didn't have the same level of knowledge and education that they did as adults, obviously," said Gerami, "and they feel wronged by the industry, and they regret the mistakes of their youth."

Gerami said tanning beds should also carry warnings like cigarettes.

He added that anyone who tanned frequently earlier in life should have a total body skin exam from a dermatologist, and find out if they need routine skin checks.

The study said 11,000 people die from melanoma in the U.S. each year.