NEW YORK -- It's supposed to make computers small enough to implant into a wrist and supply materials that strengthen and lighten bridges and airplanes. It might even cure cancer.

But some environmentalists fear that nanotechnology, the fast-advancing science of manipulating materials at the molecular scale, may create contaminants whose tiny size makes them ultra-hazardous.

"If they get in the bloodstream or into groundwater, even if the nanoparticles themselves aren't dangerous, they could react with other things that are harmful," said Kathy Jo Wetter, a researcher with the ETC Group, an environmental organization that also opposes genetically modified crops.

Scientists say such fears consist mainly of speculation.

Nanotechnology, they say, involves well-known materials such as carbon, zinc and gold -- both toxic and benign. New tools simply let researchers alter those materials at the atomic level, where the particles are measured in nanometers, or billionths of a meter.

"It may have some unexpected consequences. Some could be toxic," said Mihail Roco, the National Science Foundation's senior adviser on nanotechnology. "But this happens with larger particles and in other industries. The risks are very small in comparison with the benefits."

Nanotechnology research is one of the U.S. government's top science initiatives, fed by $604 million in federal funds this year. ETC estimates worldwide research funding at $4 billion, including government initiatives in Japan, Taiwan, South Korea and Australia.

Wetter believes the coming industrial production of nanoparticles has not been properly scrutinized for environmental or health risks.

What if the tiny, man-made particles accumulate in the liver or lungs? she asks.

Carbon nanotube molecules currently touted as a substitute for silicon in ever tinier transistors closely resemble spiky asbestos fibers, she said.

Although a pair of studies on mice and guinea pigs indicated that the carbon fibers probably posed little risk to humans, Wetter and others speculate they could damage humans' lungs.

Wants development halted

In a move that researchers believe is too dramatic, ETC is asking governments to halt development of nanotechnology until environmental and health concerns are researched and assuaged.

"Commercial applications are getting closer," Wetter said. "This is a new material, and it needs to be looked at."

In the United States, the federal Environmental Protection Agency hopes to hatch a pair of such studies this year.

"Are there going to be classes of nanomaterials that are going to pose health problems? Sure," said Kevin Ausman, director of the Rice University Center for Biological and Environmental Nanotechnology. "But those are things we'll know beforehand. We can plan around them."