Sunscreens are products applied to the skin to protect against the harmful effects of the sun's ultraviolet (UV) rays. While everyone needs a little sunshine (15 minutes a day helps the body make vitamin D), longer exposure may cause problems from wrinkles to skin cancer. Sunscreens help protect against the sun's damaging effects--but just how much protection they provide is a matter of debate.

The sun gives off two kinds of ultraviolet radiation — UV-A and UV-B. For many years, experts thought that only UV-B was harmful, but recent research suggests that UV-A may be just as dangerous, although its effects may take longer to show up. In particular, UV-A may have a role in causing a particularly deadly form of skin cancer called malignant melanoma, which has been on the rise in recent decades as tanning has become more popular. Over the same period, the thin layer of...