<P> Suntan, freckling and sunburn are familiar effects of over-exposure, along with higher risk of skin cancer . Living things on dry land would be severely damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere . More - energetic, shorter - wavelength "extreme" UV below 121 nm ionizes air so strongly that it is absorbed before it reaches the ground . Ultraviolet is also responsible for the formation of bone - strengthening vitamin D in most land vertebrates, including humans . The UV spectrum thus has effects both beneficial and harmful to human health . </P> <P> Ultraviolet rays are invisible to most humans, although insects, birds, and some mammals can see near - UV . </P> <P> Ultraviolet rays are invisible to most humans: the lens in a human eye ordinarily filters out UVB frequencies or higher, and humans lack color receptor adaptations for ultraviolet rays . Under some conditions, children and young adults can see ultraviolet down to wavelengths of about 310 nm, and people with aphakia (missing lens) or replacement lens can also see some UV wavelengths . Near - UV radiation is visible to insects, some mammals, and birds . Small birds have a fourth color receptor for ultraviolet rays; this gives birds "true" UV vision . </P> <P> "Ultraviolet" means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the highest frequencies of visible light . Ultraviolet has a higher frequency than violet light . </P>

What are the harmful effects of the portion of uv radiation called uvb