Novelties though they were, these feats proved that with new tools in hand scientists could arrange atoms as methodically as masons arrange bricks -- and in doing so build materials never made in nature.

Now the field is taking off.

Last year alone, hundreds of tons of nanomaterials were made in U.S. labs and factories. Microscopically thin sheets of tightly woven carbon atoms are being wrapped around the cores of tennis balls to keep air from escaping. New fabrics have been endowed with nanofibers that keep stains from settling in. Some sunscreens have ultraviolet-absorbing nanoparticles so small they cannot reflect light, making them invisible. Tennis rackets and airplane bodies are being made with nanomaterials whose atoms have been carefully arranged to make them especially strong.

"This technology is coming, and it won't be stopped," said Phillip J. Bond, the Department of Commerce's undersecretary for technology.

Bond may be right. But it won't be for some people's lack of trying.

Foremost among those activists is Pat Mooney of the Winnipeg-based ETC Group, which has called for a moratorium on commercial production of nanomaterials until its risks are better elucidated and regulations put in place.

It is a radical stand , but industry knows it ignores Mooney at its peril. He spearheaded much of the opposition to agricultural biotechnology -- opposition so successful that it made biotech giant Monsanto Co.'s name synonymous with "PR failure" and resulted in European restrictions on imported crops that continue to cost the United States hundreds of millions of dollars in lost trade every year.

"I do think there is a growing sense that they have to address these issues more seriously than they did in the past," Mooney said.

Scientists have known for years that tiny particles such as soot or metal powders can, when inhaled, cause lung disease, cancer and other ailments. But the laws of chemistry and physics work differently when particles get down to the nanoscale. As a result, even substances that are normally innocuous can trigger intense chemical reactions -- and biological damage -- as nanoscale specks.

Gold, for example, is a famously inert metal. But nanoparticles of gold are extremely chemically reactive, with the potential to disrupt biological pathways.

"The smaller the particles, the more toxic they become," said Vyvyan Howard, a University of Liverpool pathologist who studies the health effects of environmental aerosols.

The first two studies to look for such problems appeared in the January issue of the journal Toxicological Sciences, and the results, experts said, are less than reassuring.

In the first study -- sponsored by NASA, an agency that hopes to make great use of nanomaterials -- Chiu-Wing Lam of Wyle Laboratories in Houston and his colleagues washed three kinds of carbon nanotubes into the lungs of mice and examined them as much as three months later. Nanotubes are incredibly strong, microscopic tubules made of carbon atoms; some are already being produced in factories.

All three types caused lung granulomas -- abnormalities that interfere with oxygen absorption and can progress to fatal lung disease. And although each mouse got just one exposure, the lesions got worse over time, with some progressing to tissue death. On average the reactions were worse than those in mice given equal amounts of quartz particles, which toxicologists use as their "serious damage" standard.

Carbon nanotubules, the team concluded, "can be more toxic than quartz, which is considered a serious occupational health hazard in chronic inhalation exposures."

The other study was led by David Warheit at DuPont Co.'s Haskell Laboratory near Newark, Del., and involved similar exposures in rats. Surprising the scientists, 15 percent of the animals getting the highest dose died from lung blockages within 24 hours -- an outcome the group had never seen for any lung toxin. Warheit said in an interview he did not believe the deaths were indicative of any "inherent pulmonary toxicity" of nanotubes. But his other results were surprising, as well: All the surviving rats developed granulomas, yet without the inflammatory responses that usually accompany those lesions.