sunscreen protective factor index (SPF), a system of evaluating the effectiveness of various formulations for protecting the skin from actinic rays of the sun. A sun protective factor of 15 means that the sunscreen provides 15 times the protection of unprotected skin. There are a number of formulations of sunscreen. Some, such as titanium dioxide or zinc oxide, reflect the sun’s UVA and UVB rays. Others, such as oxybenzone and octinoxate, are synthetic chemicals absorbed into the skin to filter and absorb UV rays.
