Sunblock shields your skin from the harmful ultraviolet rays. The Sun Protection Factor (SPF) rating you find on a sunblock bottle is based on how it blocks UVB rays, which cause sunburn. Dr. Oz says you should wear sunblock with an SPF of at least 15. Many sunblocks also block harmful UVA rays—which penetrate deep into your skin and cause aging and cancer. Dr. Oz says that the SPF refers to the protection from UVB rays—not UVA—and that the SPF number doesn't tell you if a sunblock protects you from both.
Dr. Oz says people with darker skin have already evolved a sort of natural SPF. Too much sun can break down important nutrients and other elements your body needs, and the melanin in darker skin protects from this loss. "Black-skinned individuals naturally adapted to protect themselves because they had too much sun," Dr. Oz says.
According to Dr. Oz, most sun damage occurs before you are 20 years old, so it is very important to protect children from too much sun. His rule of thumb is to make sure your shadow is bigger than you when you go outside because that means the sun is rising or setting, and to wear sunblock most of the time.