The problem isn't with the sunscreen, but how people apply it. Most people don't apply enough, or they don't apply it frequently enough to be effective. A major problem, according to the researchers, is failure to apply more sunscreen after swimming. The American Academy of Dermatology (AAD) recommends reapplying sunscreen immediately after swimming, or every two hours if you stay out of the water. In fact, the academy says that people who wait more than two and a half hours to reapply have five times the chance of getting sunburned as those who reapply every two hours.
The AAD also advises people to use about a shot glass full of sunscreen for adequate coverage of exposed areas of the body and to apply sunscreen about 15–30 minutes before going outdoors.