Are ultraviolet therapy instrument potentially harmful to skin?

The ultraviolet therapy instrument can emits ultraviolet ray as 311nm(because uv led strip inside). the ultraviolet ray ability to be absorbed by epidermal tissue. so the 311nm of ultraviolet has obvious biological effect for skin and other tissues. so far, a number of investigations have been launched in over the years,for example: include that detecting the light-absorbing effect of skin pigmented diseases. all result proved ultraviolet therapy instrument are relatively safe. the skin cancer have not been caused directly by ultraviolet therapy instrument.