 Ultraviolet, ultraviolet UV is electromagnetic radiation with a wavelength from 10 Nm to 400 Nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight constituting about 10% of the total light output of the sun. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although long-wave-length ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemically and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derived from its interactions with organic molecules. Sun tan and sunburn are familiar effects of overexposure of the skin to UV, 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-wave-length 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 specifically, UVB. The UV spectrum thus has effects both beneficial and harmful to human health. Ultraviolet rays are invisible to all humans, although insects, birds, and some mammals can see near UV.