he great Gray Goo debate is beginning to matter.

The controversy involves the potential perils of making molecular-size objects and devices, a field known as nanotechnology.

From its earliest days, nanotechnology has had its fear-mongers, warning of novel and terrifying risks.

Who could be sure how products so small that they would be invisible to the human eye would behave, particularly when the nanoworld's basic design elements — atoms and small molecules — are governed by the surreal laws of quantum mechanics rather than the more familiar Newtonian physics of large objects?

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The ultimate nightmare was the so-called Gray Goo catastrophe, in which self-replicating microscopic robots the size of bacteria fill the world and wipe out humanity.

Until recently, though, the debate was restricted to the relatively small community of nanotechnology researchers and experts. The risks they discussed often seemed cartoony or vague compared with the dazzling breakthroughs they projected in fields like medicine, supercomputing, energy and environmental cleanup.

Now, though, nanotechnology is toddling into commercialization, with nanoscale particles being embedded in consumer products like sunscreens, stain-resistant khakis and wound dressings.

A number of companies are racing to scale up production of carbon nanotubes — molecule-size cylinders of carbon with unusual electrical, thermal and structural properties.

For the first time, nanotechnology is encountering the kind of real-world headwinds that have impeded biotechnology.

Nanotechnology is becoming a new organizing focus for groups like the Science and Environmental Health Network, a World Wide Web-centered research center for environmental and public health groups. The network is a leading proponent of the go-slower approach to new technology, sometimes known as the precautionary principle.

The principle has become a potent force in European regulation in recent years and is frequently discussed, if less adhered to, among regulators in the United States.

The most conservative backers of the principle tend to look for proof beyond a reasonable doubt that potential risks have been examined, as well as evidence that less risky ways of reaching the same or similar goals have been weighed. And such critics do not necessarily accept industry's definitions of accepted science. For instance, the Science and Environmental Health Network gives much more weight than do most industry scientists and government regulators to theories that chemicals in the environment are disrupting the human endocrine system and contributing to a wide range of ailments.

One notable addition to the go-slow bandwagon is the ETC Group (so named for Eco-Equity Erosion, Technology Transformation and Corporate Control), based in Winnipeg, Manitoba. In the 1990's, under its previous name — the Rural Advancement Foundation International — the group became a well-known opponent of agricultural biotechnology, pasting the science-fiction label "Terminator Technology" on research supported by Monsanto, the Agriculture Department and several other companies interested in genetically engineering plants that would be incapable of producing fertile seeds.

The inventors of the technology saw numerous potential benefits from such sterility, including a reduced risk that other genetically engineered characteristics in plants — like resistance to herbicides — could escape into weeds. But the Rural Advancement Foundation and others saw the effort as an attempt by big business to make it impossible for small farmers to plant crops from seed they saved the previous year.

Whatever the merits on either side, Monsanto and others interested in the technology were pummeled by the foundation's advocacy skills and forced to retreat from the seed research. Now, reborn as the ETC Group, the organization has broadened its concerns to include nanotechnology.

"We've had a file on it since 1988," said Pat Mooney, executive director of ETC, "but it was on the back burner until we did a patent search two years ago. We were shocked at the number of patents, how fast it was accelerating and the range of big companies involved."

Nanotechnology's skeptics, like Mr. Mooney, and the field's advocates can sound indistinguishable on some points. Both want a sharp increase in financing for research to determine the relevant questions and begin rigorous assessments of nanotechnology's risks. And both say there is enough time to create a dialogue and consensus that could prevent the kind of confrontations that have plagued biotechnology. "We recognize that nanotechnology's potential for being useful is phenomenal," Mr. Mooney said.

But the addition of groups like ETC to the nanotechnology debate brings in experienced skeptics of industry's motives and strong backers of increased government regulation. The questions about risk management are becoming much more specific and, as the science-fiction aspect of the debate recedes, pressure is mounting for regulators to step in.

Mr. Mooney's group recently began homing in on the dearth of published research about how nanoparticles interact with living cells. That issue is already on the radar of scientists at the Environmental Protection Agency, which in March invited experts at Rice University's Center for Biological and Environmental Technology to brief them on such issues.

Seizing on a report about the meeting in Small Times, a nanotechnology trade publication, ETC began telling its network of advocacy groups that reputable researchers had deep concerns about the environmental risks.

ETC asserts that research suggests that the characteristics that make carbon nanotubes and similar nanoscale particles attractive candidates for carrying drugs into the brain could also allow such particles to transport toxins. It quotes Dr. Mark Weisner, a Rice professor, as warning that nanotubes, because of their needle-like shape, could become "the next asbestos." ETC also worries about a lack of research into how nanoparticles absorbed by bacteria might enter the food chain.

ETC's critics say the group has taken the concerns of Dr. Weisner and others out of context.

Undaunted, Mr. Mooney is urging allies to join ETC's campaign to persuade heads of state who attend the World Summit on Sustainable Development in Johannesburg next week to declare a moratorium on commercial production of nanomaterials. He wants them to set up "a transparent global process" for evaluating the social, health and environmental risks of nanotechnology.

But some nanotechnology experts consider that plea naοve and self-defeating. Because nanotechnology is gaining momentum from the convergence of advances in genetics, robotics and artificial intelligence, it has become an unstoppable force, said Douglas Mulhall, author of the recently published "Our Molecular Future" (Prometheus Books). "A moratorium isn't going to happen," Mr. Mulhall said, "so it's the wrong thing for environmentalists to focus on."

It would be better, he said, for them to join in the push for far more research on potential benefits and risks.

Whatever happens, it is clear that the crosscurrents of opinion that will shape nanotechnology's commercialization are bound to become stronger — and more unpredictable — as a broader range of participants join the Gray Goo debate.