Tanning bed use is tied to almost a threefold increase in melanoma risk, and for the first time, scientists have shown how these devices cause melanoma-linked DNA damage across nearly the entire skin surface, reports a new study led by Northwestern Medicine and University of California, San Francisco.

Melanoma, the deadliest skin cancer, kills about 11,000 in the U.S. each year. Despite decades of warnings, the precise biological mechanism behind tanning beds’ cancer risk remained unclear. The indoor tanning industry, which is making a comeback, has used that uncertainty to argue that tanning beds are no more harmful than sunlight.

This new study “irrefutably” challenges those claims by showing how tanning beds, at a molecular level, mutate skin cells far beyond the reach of ordinary sunlight, according to the authors.