There's no such thing as a healthy tan. Most cultures throughout the ages tried their best to protect themselves from the sun. All that changed, however, in the enlightened era of the 20th century.
By the early 1900s, scientists discovered that ultraviolet light or sunlight, which contains ultraviolet light, could cure rickets, a disease characterized by weak and deformed bones as a result of a vitamin D deficiency. Also, the Icelandic-Danish scientist Niels Ryberg Finsen won the 1903 Nobel Prize in Medicine for his work on phototherapy, in which light of various wavelengths is used to cure disease. This set the stage for sunbathing and, in particular, deliberate suntanning with suntan lotions.
Today it is clear that ultraviolet radiation causes skin cancer. In fact, as of this year, the sun has joined the ranks of tobacco and asbestos in the highest-risk category of agents that cause cancer. No longer is ultraviolet radiation a probable carcinogen. It is a carcinogen, period. This applies to tanning booths, long thought to be reasonably safe because the radiation used is lower-energy ultraviolet A, or UVA.
A little sun is needed for vitamin D generation. How little is too much is hotly debated.