Scientists studying titanium dioxide and zinc oxide, however, have found that the compounds are most effective at blocking UV rays when nanosize. And they are among the only chemicals capable of shielding both UVA (skin-wrinkling) and UVB (cancer-causing) rays and, therefore, of fending off potentially deadly skin cancer. This disease, which affects some one million Americans each year, a number that continues to grow, according to the Centers for Disease Control and Prevention, is primarily caused by UV exposure.

Therefore, the Environmental Working Group (EWG) recommends using sunscreens that contain zinc oxide or titanium dioxide, nanosize or not. EWG also prefers these formulations because studies show that sunscreens containing alternative organic chemicals such as oxybenzone and octinoxate seep through the skin into the body where they can sometimes behave like the hormone estrogen, prompting abnormal activity in the human reproductive system, among other things. In addition, the group says, the zinc oxide and titanium dioxide formulations utilize fewer other chemicals, some of which are known or suspected to cause cancer and damage the brain or immune system.

After testing some 800 different sunscreens, the EWG found that those without zinc and titanium "could accelerate by an average of 20 percent the skin damage, premature aging wrinkling and UV-induced immune system damage linked to UVA exposure."

The fundamental problem is a lack of information: No one other than the manufacturers know the exact ingredients in their products and little safety testing has been done or documented on the vast majority of chemicals commonly used in sunscreens. Because they are classified as over the counter drugs, the Food and Drug Administration (FDA) has the authority to regulate the active ingredients (like titanium dioxide or zinc oxide that protect the skin) in a sunscreen but it does does not regulate inactive ingredients, such as preservatives or emollients. Rather, manufacturers must "substantiate" safety with their own clinical trials, or avoid the whole process by simply placing a warning on product labels noting safety has not been determined.

The Royal Society (the U.K.'s national science academy) three years ago advocated peer-reviewed, publicly accessible safety studies for any cosmetics containing nanoparticles, and some U.S. lawmakers have also urged stricter safety rules—a call that has yet to be heeded.

The FDA last month nixed new regulations governing the use of nanoparticles in personal care products noting that previous studies showed larger versions of the same compounds were safe. "Current regulations dealing with cosmetics are pretty weak, because they don't require any pretesting" on humans, says physicist Andrew Maynard, chief science advisor to a project on emerging nanotechnologies at the Woodrow Wilson International Center for Scholars (a Washington, D.C., think tank created by Congress in 1968). "If you bring a new technology along that has even more uncertainty than common chemicals, it really focuses a spotlight on that issue."

The EPA reviews new chemicals for toxic implications, but lacks comprehensive data to assess nanosize ones in the 90 days it has to do so. "We have developed a growing need for data on the environmental and health implications of nanomaterials to help us review them. As we looked at the data available [from companies and elsewhere], it has become clear that it is widely variable," says Jim Willis, director of the EPA's chemical control division. "The number one issue is data availability."

The EPA and several programs of the National Institutes of Health are working to fill that gap by studying the health and environmental impacts of various nanoparticles. But funding is scarce. "I would like to look at it in animals," EPA's Veronesi says. "For that we really need technical support and resources so we're shelving a lot of ideas right now."

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