The UV index is a scale that measures the intensity of ultraviolet radiation emitted by the Sun and the risk it poses to health (sunburns, cataracts, and skin cancer). The higher the index, the greater the risk. The Zip, just like your skin, receives ultraviolet rays, but it reacts immediately by changing color. Therefore, the more purple the zipper becomes, the stronger the ultraviolet rays are, indicating the need for protection.