Ultraviolet radiation (UVR) from the sun is divided unto UVC, UVB and UVA. UVC is filtered by the ozone in the stratosphere, therefore it does not reach the earth’s surface. The amount of UVB and UVA reaching the surface is affected by altitude, latitude, season, time of day, cloudiness and ozone layer. On the earths surface, the ratio of UVA to UVB is 20:1. UVA is less effected by altitude or atmospheric conditions. UVA can reach deeper through the skin and is not blocked by window glass. It takes more UVA to induce redness than UVB. UVA redness is often immediate and peaks at 24 hours. UVA is more efficient in inducing tanning whereas UVB is more efficient in inducing redness. UVB irradiation causes redness approximately 4 hours post exposure and peaks 8-24 hours later, fading over a day or so. Therefore, the first signs of sun exposure are from UVA and then the delayed response is related to UVB damage.