Unless sun lovers continually reapply sunscreen, agents in the sunscreen itself can attack the skin and leave the body susceptible to ultraviolet radiation, researchers say.

Scientists at the University of California at Riverside have found that molecules that block UV radiation in sunscreen can, over time, penetrate into the skin, leaving the outer body vulnerable.

"Sunscreens do an excellent job protecting against sunburn when used correctly," Kerry Hanson, a research scientist in the Department of Chemistry at UCR, said in a news release.

"This means using a sunscreen with a high sun protection factor and applying it uniformly on the skin. Our data show, however, that if coverage at the skin surface is low, the UV filters in sunscreens that have penetrated into the epidermis can potentially do more harm than good."

Harmful compounds created  

Unprotected skin exposed to UV radiation generates harmful compounds called reactive oxygen species or ROS. These molecules can react with cells, membranes and DNA, causing skin damage and signs of aging.

Hanson's research team found that three UV filters (octylmethoxycinnamate, benzophenone-3 and octocrylene) widely used in sunscreens generate ROS when skin is exposed to UV radiation. Only a few UV-filters are capable of blocking UV-A, the wavelengths that penetrate deeply into the skin.

"For now, the best advice is to use sunscreens and reapply them often — the Skin Cancer Foundation recommends every two hours, and especially after sweating or swimming, which can wash away sunscreen — to reduce the amount of UV radiation from getting through to filters that have penetrated the skin," said Christopher Bardeen, an assistant professor of chemistry at UCR. "This, in turn, would reduce ROS generation."

Hansen's study will be published in an upcoming issue of Free Radical Biology & Medicine.