text: Ultraviolet radiation (UVR) is part of the electromagnetic spectrum, just beyond visible light. Ultraviolet wavelengths are 100 to 400 nanometres (nm, billionths of a metre) and are divided into three bands: A, B and C. UVA wavelengths are the longest, 315 to 400 nm; UVB are 280 to 315 nm, and UVC wavelengths are the shortest, 100 to 280 nm.
  About 95% of the UVR that reaches the earth from the sun is UVA and 5% UVB; no appreciable UVC reaches the earth. While tanning systems before the 1970s produced some UVC, modern tanning devices produce no UVC, a small amount of UVB and mostly UVA. Classified by the WHO as a group 1 carcinogen, UVR has "complex and mixed effects on human health". While it causes skin cancer and other damage, including wrinkles, it also triggers the synthesis of vitamin D and endorphins in the skin.