Under intense summer rays sunscreen can help protect against a wicked burn, but some of the common active ingredients in these sprays and lotions can also seep through the skin and enter the bloodstream. Although it is unclear whether this poses any risks, Yale University dermatologist Michael Girardi thinks it is worthwhile to develop alternatives. In collaboration with the university's bioengineering department, he has developed a sunscreen formulation designed to keep chemicals on the skin's surface.

The sunscreen chemicals that absorb the sun's dangerous ultraviolet radiation are typically organic molecules (as opposed to the metal oxides that block the sun's rays in sunblock). There is no evidence that these absorbing molecules can directly harm humans. A small body of research that includes animal and cell-culture studies, however, shows that some of them can bind to hormone receptors. The results may suggest a potential to disrupt the body's endocrine system, the traffic of hormones that regulates reproduction and other functions.

In an attempt to develop a sunscreen that would not soak into the skin, Girardi and his team encapsulated the molecules of the common UV absorber padimate O in nanoparticles of a biodegradable polymer that binds to proteins on skin cells. The nanoparticles stick to those cells even when wet and come off only when toweled. The new formulation protected mouse skin from UV damage just as well as a conventional padimate O sunscreen, according to Girardi's recent study in Nature Materials.

Kenneth Kraemer, a dermatologist at the National Cancer Institute who was not involved in the project, is impressed by the results. “If you can minimize the risk of sunscreen entering the bloodstream, it's probably a good thing,” he says. Still, it will be a long time before the formulation appears in beach bags. This summer Girardi will carry out a pilot study involving an estimated 25 people to establish the SPF of the nanosunscreen at various concentrations. Meanwhile any sunscreen is better than going outside unprotected when it comes to avoiding sunburns, wrinkling and UV damage that can lead to cancer.