[link|http://www.sciencedaily.com/releases/2006/08/060828211528.htm|Sunscreens Can Damage Skin, Researchers Find]:

"Sunscreens do an excellent job protecting against sunburn when used correctly," said Hanson, who works in the laboratory of Christopher Bardeen, an assistant professor of chemistry at UCR. "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. More advanced sunscreens that ensure that the UV-filters stay on the skin surface are needed; such filters would reduce the level of UV-induced ROS. Another solution may be to mix the UV-filters with antioxidants since antioxidants have been shown to reduce UV-induced ROS levels in the skin."

[...]

"For now, the best advice is to use sunscreens and re-apply 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," Bardeen said. "This, in turn, would reduce ROS generation."


ROS is "reactive oxygen species".

This study was funded by the Worldwide Association to Promote Maximum Sunscreen Consumption. (Not really.)

FWIW.

Cheers,
Scott.
(Who hates sunscreen, but who rarely goes to the beach.)