Columbia Sportswear's Omni-Shade clothing protects you from damaging UV radiation by blocking the majority of the sun's harmful rays, so you can stay out longer. Omni-Shade blocks Ultraviolet A and Ultraviolet B rays (UVA and UVB), helping to prevent sun burn and long-term skin damage through tight weave construction, UV reflectors, and UV absorbing technology. Unlike SPF (Sun Protection Factor), which is a measure of sunburn reduction from sunblock, and does not indicate any protection from UVA rays. UVA rays are much more harmful than UVB, and are present even on cloudy days. Omni-Shade provides protection from both UVB and UVA rays. Like sunblock, Omni-Shade is given a numeric value to indicate its level of sun protection. Unlike sun block, Omni-Shade does not wear off.