It is a controlled environment.
Responsible usage in a quality assured environment allows users to gain UV exposure safely.
More than 90 percent of professional indoor tanning units emit about 95 percent UVA and 5 percent UVB in regulated dosages similar to summer sun. Recommended exposure schedules developed by the U.S. FDA in cooperation with the tanning industry allow trained indoor tanning operators to set incremental exposure times, based on the "skin type" of a patron, that deliver consistent non-burning dosages of UV light to allow a tanner to gradually build a tan.
While tanning units may be 2-3 times as intense as summer sun, the duration of exposure is controlled, and thus the total UV output is controlled, to minimize the risk of sunburn.