SPF (sun protection factor) indicates protection against the ultraviolet B (UVB) rays that cause sunburn. In theory, higher-number SPFs block slightly more of the sun's UVB rays. However, many studies have found that people are misled by the claim of the sun's UVB rays on high-SPF sunscreen bottles and more likely to use high-SPF products improperly. They tend to stay out in the sun much longer or may skip reapplying. As a result, they may expose themselves to more harmful ultraviolet radiation than do people who rely on products with lower SPF values.