Recent studies have shown that women under the age of 40, who will be using tanning beds regularly, are at increased risk for skin cancer, specifically melanoma.
The exact cause of this is not well understood. We do know that the U. S. government does not regulate the types of bulbs, and the types of energies used in these tanning booths. This means the type of bulbs they use may have a variety of different UV lights, more UVB, or more UVA, and the dosage that they emanate are not regulated, so you don't know what type or how much UV light you're getting.
That's why it's important to avoid getting a tan at a tanning salon. Going outside for 20 minutes, either before 10:00 a.m. or after 4:00 p.m. when the UV Index is lower, is a much better way to make sure that you get a safe tan in stages.