People in the Northern hemisphere are likely craving time in the sun these days, but visiting a tanning salon is not the answer.

Sure, it feels good and leaves a seemingly healthy glow, but research has shown that people who use tanning beds are at greater risk for melanoma, the deadliest form of skin cancer, than those who don't, often developing melanoma at an earlier age.

Researchers are still not sure, however, exactly how exposure to ultraviolet (UV) light from tanning beds causes melanoma to develop.

The findings contradict the claims of the indoor tanning industry that tanning bed use is no more harmful than sunlight.

A team led by researchers at Northwestern University found not only that tanning bed use significantly increases the risk of melanoma in those already at high risk for the condition, it also caused mutations in the DNA of melanocytes, the melanin-producing cells where melanoma develops.

The mutations were found in skin samples from areas of the body that are not usually exposed to the sun, like the lower back and buttocks. These findings contradict the claims of the indoor tanning industry that tanning bed use is no more harmful than sunlight.

The study had two parts. The first part, lead author Pedram Gerami explained, involved identifying almost 3,000 high-risk patients from the melanoma clinic at Northwestern who had used a tanning bed at least 10 times during their lifetime. In addition, almost 3,000 age-matched patients who had no history of tanning bed use were recruited from the clinic and served as a control group.

Though they had no tanning bed exposure, the control group still had other risk factors for skin cancer — such as a family history of melanoma or having had serious sunburns. As a result, Gerami, director of the melanoma program at Northwestern, told TheDoctor, their baseline risk was already greater than that of the general population.

Even after these other risk factors were taken into account, however, the tanning bed group had almost three times the risk of melanoma compared to the controls.

The likelihood of developing melanoma was also dose dependent — more time spent in a tanning bed led to a higher likelihood of developing melanoma. Patients with 10 to 15 lifetime tanning bed uses were two times more likely to develop melanoma than the control group. People who had used a tanning bed more than 100 times were six times more likely to develop melanoma, and those with more than 200 lifetime uses were eight times more likely to develop it.

In the second part of the study, the researchers looked at the damage tanning beds caused at the cellular level. “We wanted to see if we could figure out what made these patients have a high likelihood of melanoma,” said Gerami. They took a sample of normal skin, without moles or melanoma, from a subset of 11 patients from the tanning bed group and nine patients from the control group.

The researchers were able to isolate melanocytes using a technique called single cell sequencing. They found that DNA from the melanocytes of tanning bed users had double the number of mutations compared to DNA from the melanocytes of the control group. “These mutations occurred in genes we know play a role in the development of melanoma,” Gerami said.

Most melanomas are the result of three or four critical mutations in certain genes, he explained. Many areas of the skin in the tanning bed group already had one or two of these mutations, so they were that much more likely to develop melanoma.

DNA from the melanocytes of people who used tanning beds had twice the number of mutations.

The researchers also took skin samples from cadavers. Because the control group already had risk factors for melanoma, they wanted to compare skin samples from the tanning bed group to samples from those who were more representative of the general population. The researchers found that tanning bed users had significantly more DNA mutations in their melanocytes, even compared to cadaver skin samples from patients who were on average twice their age .

The researchers are currently doing studies to assess more closely the link between different wavelengths of UV light and DNA mutations. “We are trying to get a better idea of how much UV exposure it takes to develop these DNA mutations,” said Gerami, a professor of skin cancer research at the Northwestern University Feinberg School of Medicine, “and what combination of UVA and UVB are most likely to cause mutations.”

The study is published in Science Advances.