Kristin Conn puts non-nano sunscreen on her son, Merrick, at Lee Street Beach in Evanston, Illinois. While nano materials offer benefits, such as clear sunscreens, some people worry about the effects they can have if absorbed into the body. (Chicago Tribune/MCT: Chris Sweda)

CHICAGO -- Zinc oxide would be the perfect sunscreen ingredient if the resulting product didn't look quite so silly. Thick, white and pasty, it was once 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, antimicrobial 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.

The particles can alter how products look or function because matter behaves differently at the nanoscale, taking on unique and mysterious chemical and physical properties. Materials made of nanoparticles may be more conductive, stronger or more chemically

"Everything old becomes new when miniaturized," said Chad Mirkin, director of the International Institute for Nanotechnology at Northwestern University. "This gives scientists a new playground, one focused on determining what those differences are and how they could be used to make things better."

But the development of applications for nanotechnology is rapidly outpacing what scientists know about safe use. The same unusual properties that make nanoscale materials attractive may also pose unexpected risks to human health and the environment, according to the scientific literature.

"We haven't characterized these materials very well yet in terms of what the potential impacts on living organisms could be," said Kathleen Eggleson, a research scientist in the Center for Nanoscience and Technology at the University of Notre Dame.

Scientists don't yet know how long nanoparticles remain in the human body or what they might do there. But research on animals has found that inhaled nanoparticles can reach all areas of the respiratory tract; because of their small size and shape, they can migrate quickly into cells and organs. The smaller particles may also pose risks to the heart and blood vessels, the central nervous system and the immune system, according to the Food and Drug Administration.

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Tiny particles: Big potential or big threat?