The sun emits a broad spectrum of radiation that includes harmless visible light as well as ultraviolet radiation. The bands of wavelengths that damage human skin are categorized as UVA and UVB. UVB rays are the main cause of the skin turning red, or exhibiting a sunburn. They can fracture the DNA in skin cells, resulting in mutations. UVA rays penetrate deeper into the skin and are the main cause of wrinkling, and discoloration. They cause the formation of damaging atoms called free radicals, and can inhibit the immune system from doing its job to prevent cancer. It is important to protect the skin from both UVA and UVB.