What Is the UV Index?
The Ultraviolet (UV) Index, developed in 1994 by the National Weather Service (NWS) and the US Environmental Protection Agency (EPA), helps Americans plan outdoor activities to avoid overexposure to UV radiation and thereby lower their risk of adverse health effects. EPA and NWS report the Index as a prediction of the UV intensity at noon, though the actual UV level rises and falls as the day progresses.
Previously the UV Index was reported on a scale of 0 to 10+, with 0 representing “Minimal” and 10+ representing “Very High.” The new global scale (see below) now uses a scale of 1 (representing “Low”) to 11 and higher (representing “Extreme”), a new color scheme, revised exposure categories, and different breakpoints between exposure categories. (A UV Index of “0” is still possible, but there is no corresponding health message because there either is no UV at that level or the amount is trivially small.)
Always take precautions against overexposure, and take special care whenever the UV Index is 5 and above.