Indoor tanning can trigger skin cancer even when it causes no burns, research has confirmed.
Scientists stressed that tanning was a biological response indicating damage by ultraviolet (UV) radiation - with or without burns.
The study compared data on 1,167 patients with malignant melanoma, the most dangerous form of skin cancer, and 1,101 cancer-free individuals.
Melanoma patients who had never suffered from sunburn were nearly four times more likely to tan indoors than the control group participants.
Writing in the Journal of the National Cancer Institute, the authors led by Dr DeAnn Lazovich, from the University of Minnesota, US, said: “A common reason stated for tanning indoors is to prevent sunburn, with the implication that, by avoiding sunburn, skin cancer risk is reduced. However, whether this is the case has not been reported.
“Our results expand upon the current scientific evidence by demonstrating that indoor tanning, even when used in a way that does not produce burns, is a risk factor for melanoma.”
Study participants answered questions about their age, sex, income, education, eye, hair and skin colour, lifetime history of melanoma, sun exposure and sunscreen use.
Melanoma patients with no history of sunburn reported starting indoor tanning younger and continuing it for longer than those who had experienced burns.
Among the total study population, 56.8% had suffered five or more episodes of sunburn while just 5.3% claimed never to have been burned by the sun.
The researchers added: “Several possibilities exist to understand our findings. First, tanning of the skin is the biological response to indicate that DNA damage from UV radiation has occurred; burns are not required to elicit this response.
“Second, tanning indoors without burning may allow for greater cumulative exposure to the damaging effects of artificial and/or solar UV radiation.
“Third, the intensity and proportion of UV-A and UV-B (rays) emitted by tanning devices have been shown to differ from the sun in ways that could increase risks associated with indoor tanning.”