The invisibly small particles and fibers that scientists are producing in the hot new field of nanotechnology pose health and environmental risks great enough to justify banning, for now, certain cosmetics now found on the U.S. market and also halting the deliberate release of nanomaterials into the environment, according to an independent report commissioned by the British government.

Most products of nanotechnology, including atom-scale electronic components and super-strong materials, will probably prove harmless, the report concludes, offering a modicum of reassurance for the nascent research field that has been buffeted by science-fiction scenarios of techno-doom.

But the joint report by Britain's Royal Society and Royal Academy of Engineering also strongly warns that the manufactured specks at the heart of nanotechnology -- tens of thousands of which can fit on the tip of a needle -- behave in unpredictable ways and in some cases appear surprisingly toxic.

The report recommends that increasingly popular nanoparticle-laden cosmetics be kept off the market until proven safe for use on skin -- a policy more restrictive than the one in place at the U.S. Food and Drug Administration.

The report is also critical of experiments performed in the United States in which nanoparticles have been spread on the ground -- experiments, the report says, that could pose serious risks to organisms in soil and groundwater.

Its advice to manufacturing facilities is to presume that nanoparticles are hazardous until proven otherwise. That means minimizing the release of the particles into standard waste streams and ensuring that workers inhale as few of them as possible.

And it suggests that consumers would be best served by the labeling of products made with nanomaterials -- and by being educated about the science's benefits, lest it meet the same fate of negative public perception that has hobbled nuclear power and genetically engineered foods.

"Nanoparticles can behave quite differently from larger particles of the same material and this can be exploited in a number of exciting ways," said Ann Dowling, a professor of mechanical engineering at the University of Cambridge and chairman of the Royal Society working group that produced the highly anticipated report. "But it is vital that we determine both the positive and negative effects they might have."

The societies advise the British government. Although influential, they have no enforcement powers.

The report is not downbeat on nanotechnology overall. In fact, it echoes the enthusiasm that has been expressed by manufacturing interests and U.S. Commerce Department officials, who have predicted that the technology will power the "next industrial revolution."

Hundreds of tons of nanomaterials were manufactured last year in the United States and the U.S. market is expected to top $1 trillion within a decade.

The field is an outgrowth of a newfound ability to manipulate individual atoms and create molecular devices smaller than human cells. At that scale, materials have different chemical and physical properties than those of identical materials in bulk. Carbon atoms, for example, when woven into hollow microscopic threads, can conduct electricity and are stronger than steel. Other nanomaterials look promising as drug delivery vehicles, environmental cleanup tools and in computer hard drives. Nanoparticles boost the ultraviolet-light-blocking power of sunscreens and cosmetics.

But because of their sometimes extreme chemical reactivity -- and because they are just the right size to integrate themselves into living cells -- nanoparticles make some scientists and activists nervous.

Preliminary experiments in animals have found nanoparticles capable of moving into and damaging the lungs, brain and other organs. And while some nanomaterials may be able to neutralize poisons in soil or groundwater, others appear environmentally toxic themselves.

U.S. regulations relating to worker safety, environmental protection, cosmetics sales and drug approvals have not been adapted to address the novel traits of these materials, in part because it has proven difficult to predict which nanomaterials pose risks. But several agencies have begun to conduct safety studies.

"I think we have an appropriate level of research and development underway to look into potential risks associated with nanoscale materials," said E. Clayton Teague, director of the National Nanotechnology Coordination Office, which advises the federal cabinet on nanotech.

Among the projects underway, Teague noted, is one on titanium dioxide, tiny particles of which are now found in some sunscreens. The report says that is the one nanomaterial already adequately tested in cosmetics. Cosmetics containing others, including zinc oxide and iron oxide, should not be sold until similarly tested, the report recommended.

Linda M. Katz, director of the FDA's Office of Cosmetics and Colors, said the agency is examining the issue and expects results within two years. She said the agency is not aware of any reports of illness or injury associated with nano-containing cosmetics.

Cosmetics manufacturers are responsible for ensuring the safety of their products, Katz said, and the agency does not have a list of products that contain nanomaterials.

Environmental Protection Agency spokesman Michael Brown said his agency will review the report as it devises new regulatory strategies for nanomaterials.

"EPA is aware of this emerging technology," he said, "and we agree that it is important to develop the appropriate regulatory oversight to ensure protection of public health and the environment."

Nanomolecules, tens of thousands of which can fit on the tip of a needle, can be surprisingly toxic and are just the right size to integrate themselves into living cells.