Invisible light rays of very short wavelength beyond the violet end of the sun's spectrum. Ultraviolet-C (UVC) (wavelength <290 nm [nanometre – see APPENDIX 6: MEASUREMENTS IN MEDICINE]) is entirely absorbed by the earth's atmosphere and would otherwise be lethally damaging. Ultraviolet-B (UVB – 290–320 nm) intensity increases with altitude: it is greatest in midsummer and at midday and penetrates cirrhus cloud. UVB causes sunburn and also tanning. Ultraviolet-A (UVA – 320–400 nm) penetrates deeper into our skins but does not cause sunburn; it is implicated in many photochemical reactions and PHOTODERMATOSES and in CARCINOGENESIS. UVR helps the skin to synthesise vitamin D.

Ultraviolet lamps produce UVR and are used to tan skin but, because of the risk of producing skin cancer (see SKIN, DISEASES OF), the lamps must be used with great caution.