Many sunscreens contain nanoparticles of zinc oxide or titanium dioxide, even though you probably won’t see the term “nano” on the labels. Whether smearing those nanoparticles on your skin can harm your health is not clear. But you don’t generally need them to get superior protection, our tests found.

Nanoparticles of the two compounds are used in sunscreens because the normally white substances, which absorb ultraviolet radiation, become more transparent when the particles are nano-sized. We asked an outside lab to test for those nanoparticles in eight sunscreens that listed either compound on their label. All contained the particles, yet only one, Keys Solar RX, disclosed that use of nanotechnology.

Lab studies indicate that both of those nano-ingredients create free radicals that damage the DNA of cells and possibly cause other harm as well. And even low exposure to nanoparticles of titanium dioxide can damage the lungs of animals if inhaled.

But whether those particles in sunscreens pose direct health risks to humans depends mainly on whether they penetrate the protective outer layers of dead skin. Studies suggest they don’t reach live tissue under normal circumstances. But it’s not known whether skin damaged by acne, eczema, sunburn, or nicks from shaving is more vulnerable to penetration, says Kristen Kulinowski, director of the International Council on Nanotechnology, which promotes responsible development of nanotech. And studies of other nanoparticles show they can penetrate the outer skin layers through the hair follicles or when the skin is repeatedly stretched.

Bottom line. Until there’s adequate safety assessment, people who wish to avoid exposure to those nanoingredients could choose sunscreens that don’t list titanium dioxide or zinc oxide on their label. That wouldn’t require settling for less sun protection: Our tests found no correlation between effectiveness and the presence of those ingredients. Moreover, the top-scoring U.S. product, Neutrogena Ultra Sheer Dry-Touch SPF 45, did not contain either zinc oxide or titanium dioxide.