More and more manufacturers have reduced particle sizes in their sunscreens to the nanoscale – causing the particles to be more transparent – and avoiding the white, pasty appearance of sunscreens that use large particles. Francis Quinn, director for sustainable development at personal care firm L’Oréal, said, “Consumers are willing to apply a product that you can barely see” and “are more likely to apply it several times per day” to reduce the risk of skin cancer. Yet nanotechnology brings questions about whether the tiny particles interact with living cells and the environment in a harmful way. There is concern about the potential for nanoscale zinc oxide and titanium dioxide to pass through the outer layer of skin, and eventually to the bloodstream. There is also concern that titanium dioxide nanoparticles act as photocatalysts and generate skin-damaging free radicals. While researchers are intensely studying the issue, and most companies have put safeguards in place for their products, there has been an uproar over nanoparticles in sunscreens. Studies have found differing levels of penetration of nanoparticles into the skin or blood, but most concur that the amounts are tiny. Other studies have found that nanoparticles tend to aggregate when applied. Andrew Maynard, director of the Risk Science Center at the University of Michigan, United States, said that in sunscreens, even though “the individual particles are very small, the clumps of stuff on your skin are not going to be that, so there’s even less chance of them getting where you don’t want them to be.” Other studies, however, found that titanium dioxide nanoparticles in sunscreen killed two to three times as many viable skin cells as UV light alone. Despite concerns about the safety of nanoparticle-based sunscreens, researchers usually caution that sunscreen should be worn to protect against skin cancer – the leading type of cancer in the U.S. As Maynard says, extended exposure to sunlight without sunscreen will always be a risk, and the best balance between risk and benefit is the continued use of sunscreen for protection.