The Index is conservatively calculated based on effects on skin types that burn easily . A conservative measure is appropriate because although not everyone sunburns easily, people of all skin types can experience other effects of UV exposure which include cataracts, immune system suppression and the potential for skin cancers, aging and wrinkling of the skin. The UV Index predicts the next day's ultraviolet intensity at the Earth's surface over the one-hour period around solar noon. It should be noted that the Index values are meant to be reflective of the overall problem, not solely one of exposure for one particular skin type. However, as it is necessary to give a specific number, the National Weather Service has chosen UV effects on skin Type II , that which usually burns easily and tans minimally.
Most people can stay in the noon sun for up to one hour without burning.
Unprotected exposure can result in a burn in only 20-30 minutes. UV is the shortest wavelength component of the solar spectrum. That Determine an Individual's Exposure Exposure to UV radiation varies with the time of day, season of year, latitude and altitude. Weather conditions also affect the amount of UV that reaches the Earth. Much of the UV radiation reaches the Earth's surface even on slightly overcast or cloudy days.