Indoor tanning units use a carefully controlled dosage of ultraviolet light in each session. Successive session times are engineered to deliver tanning results while minimizing your risk of sunburn.
Anti-tanning lobbyists miss the point when they state that tanning units are more intense than sunlight. Intensity is only part of the equation. Exposure times in tanning units are much shorter than outdoor tanners would experience outdoors.
Total UV = Intensity x Duration
Because indoor tanning times vary from 5-30 minutes, and because outdoor tanners might spend four hours or more in the sun, total UV received indoors in probably less than one would receive tanning outdoors.