Ultraviolet radiation (with a wavelength in the range of 290 to 400 nanometers), visible light (400 to 760 nm), and infrared radiation (760 to 3,000 nm) are the three forms of energy that the earth receives from the sun. Within the ultraviolet spectrum, the three sub-categories are UVC (200-290 nm), UVB (290-320 nm), and UVA (320-400 nm). Most UVC rays do not reach the earth because ozone and other gases in the upper atmosphere absorb them, but exposure to this type of radiation from germicidal lamps and mercury lamps may still be hazardous. Excessive exposure to visible light is also thought to be harmful, but it is not of extreme importance with respect to prevention of skin damage. Most skin damage is caused by UVA and UVB radiation. The amount of UVA that reaches the earth is ten times greater than UVB, but the amount of UVA need to produce sunburn in human skin is 800 to 1,000 times higher than UVB. Nevertheless, UVA intensifies the sunburn effects of UVB through a delayed erythema and aids in cancer formation. Thus, effective sunscreens and sun blocks must protect throughout both the UVB and UVA ranges.