Rubbing sunscreen on the skin may decrease its protective effect. Dermatologists applied ample sunscreen on skin samples and exposed them to a light source that mimicked summer sun. When they coated the skin with a thin film of sunscreen without rubbing, free radicals produced in the skin by exposure to UVA light decreased substantially. But when the cream was rubbed on the skin—possibly scattering it into sweat glands, fine lines, and pores—the defense against these free radicals was minimal. Since sunscreen's UVB protection prevents burning, people may feel falsely shielded and stay in the sun longer, exposing themselves to damage if insufficient cream is applied, says Rachel Haywood of Mount Vernon Hospital in England. "We need to test more sunscreens with different types of UVA filters," she says. "But our preliminary findings suggest spray-on sunscreens might be more effective than rub-on creams." With a cream sunscreen, she says, it's necessary to apply two layers for even coverage.