Years ago, Ultraviolet B radiation (UVB) was considered the dangerous portion of the UV spectrum, because it caused sunburns. More recently, UVA got a reputation for being the more dangerous portion of the spectrum, because it penetrates deeper than UVB, and suntan lotions have scrambled to provide adequate UVA protection. But it looks like UVB is responsible for deadly melanomas:
Dr. Edward C. De Fabo of The George Washington University Medical Center, Washington DC, and colleagues, exposed mice to light of various wavelengths. This included UVA, UVB, solar simulation, an unfiltered sunlamp, and a sunlamp filtered to remove more than 96 percent of the UVB spectrum.
Melanoma was triggered most strongly by the UVB lamp, followed by the solar simulator and the sunlamp, which both had comparable melanoma-inducing properties, De Fabo’s group reports in the September 15th issue of Cancer Research.
In contrast, mice exposed to UVA light or to the filtered sunlamp had responses no different from those of a comparison group of animals that were not exposed to any of the light sources.
When it comes to tanning, UVB triggers melanin production, and UVA oxidizes (and browns) the melanin once it is produced.