I know I may be a bit biased, but it is a known fact that tanning booths are bad for you. Sure the sun feels great, but those high powered bulbs cause some serious damage your skin.
Overexposure to any UV rays can cause premature aging and loss of skin elasticity. But premature aging and cancer are harmful effects of tanning booths that do not show up for many years, after damage to the skin is already irreversible.
There is no completely safe UV ray exposure. It is true that exposure to some sunlight is healthy, because it prompts the body to produce vitamin D, but the amount of sunlight needed for this health benefit is MINIMAL. UV rays are radiation, and over-exposure always causes damage of some kind. Both types of UV rays have been linked to immune system damage.