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Zinc oxide would be the perfect sunscreen ingredient if the resulting product didn’t look quite so silly. Thick, white and pasty, it once was seen mostly on lifeguards, surfers and others who needed serious sun protection.

But when the sunscreens are made with nanoparticles, the tiniest substances that humans can engineer, they turn clear – which makes them more user-friendly.

Improved sunscreens are just one of the many innovative uses of nanotechnology, which involves drastically shrinking and fundamentally changing the structure of chemical compounds. But products made with nanomaterials also raise largely unanswered safety questions – such as whether the particles that make them effective can be absorbed into the bloodstream and are toxic to living cells.

Less than two decades old, the nanotech industry is booming. Nanoparticles – measured in billionths of a meter – are already found in thousands of consumer products, including cosmetics, pharmaceuticals, anti-microbial infant toys, sports equipment, food packaging and electronics. In addition to producing transparent sunscreens, nanomaterials help make light and sturdy tennis rackets, clothes that don’t stain and stink-free socks.