California chemists suggest that unless people out in the sun apply sunscreen often, the sunscreen itself could become harmful to the skin.

When skin is exposed to sunlight, ultraviolet radiation, or UV, is absorbed by skin molecules that then can generate harmful compounds, called reactive oxygen species, or ROS, which are highly reactive molecules that can cause “oxidative damage,” according to Kerry M. Hanson of the University of California at Riverside.

Over time, though, these filters penetrate into the skin below the surface of the epidermis, leaving the body vulnerable to UV radiation, according to the study published in the journal Free Radical Biology & Medicine.

“Sunscreens do an excellent job protecting against sunburn when used correctly,” said Hanson. “This means using a sunscreen with a high sun protection factor and applying it uniformly on the skin. 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.”

For now, the best advice is to use sunscreens and reapply them often — the Skin Cancer Foundation recommends every two hours 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, says Christopher Bardeen, an assistant professor of chemistry.