Indoor tanning is associated with a 74 percent higher risk of melanoma, researchers said.
And, in a large case-control study, some forms of indoor tanning machines were associated with a four-fold increase in the risk of skin cancer, according to DeAnn Lazovich, of the University of Minnesota, and colleagues.
The risk rose depending on years of use, hours of use, or number of sessions, Lazovich and colleagues said online in Cancer Epidemiology, Biomarkers & Prevention.
The finding "provides strong support" for a recent declaration by the World Health Organization's cancer research agency that the devices are carcinogenic, the researchers concluded.
"The take-home message is that indoor tanning is not safe," Lazovich said in a video prepared by the University of Minnesota.
Tanning salons are increasingly popular in the U.S. and their rise has been paralleled by increasing incidence of melanoma, the researchers noted.
But evidence that might show a link has been inconclusive, they said, largely because studies of the issue have not accounted for confounders such as sun exposure or measured dose response.
To help fill the gaps, Lazovich and colleagues undertook a case-control study in Minnesota, where there is a high prevalence of indoor tanning. Cases were 1,167 people ages 25 through 59 who were diagnosed with invasive cutaneous melanoma between July, 2004, and December, 2007.
Participants took a self-administered questionnaire, followed by a detailed one-hour telephone interview. The data collected included such things as the age when indoor tanning started, its intensity in terms of duration and time of use, and the types of machines used, as well as sun exposure and a range of risk factors for melanoma.
Four types of machines were used -- conventional, high-pressure, high-speed and intensity, and sun lamps. The researchers found that cases of skin cancer were significantly more likely than controls to have used any type of machine.
The risk of melanoma also increased with use, regardless of how it was measured. Specifically, compared with those who had ever used the devices.
People who had used an indoor tanning machine for between one and nine hours in total were more likely to get melanoma, but less likely than those with more than 50 hours of use.
Over time, different types of machines, with differing frequency characteristics, have been used. However, the risk of disease was significantly elevated for use before 1990 and after, as well as for both periods together, the researchers found.
They cautioned that the study is unable to determine which form of ultraviolet light -- A or B -- is responsible for the results.