The same qualities that make nanomaterials so promising also create the potential to do harm in unexpected ways. The smaller the particle, for example, the more atoms there are on its surface rather than hidden inside. The more surface atoms, the more likely the particle is to react chemically with other compounds. As a result, elements that are harmful in conventional form can become more dangerous as nanoparticles, and even normally benign substances might become toxic when nanosized.

The limited nanotechnology information and research so far indicates that some nanoparticles can bypass the body’s defenses. For example, they can reach the brain by entering the nasal passages and traveling along the ­odor-detecting nerve. They can move from the lungs into the blood and then to other organs. Ingested nanoparticles seem to reach the organs more readily than larger particles. And at least some of them can pass through the skin.

“Nanoparticles aren’t necessarily hazardous, and most may turn out to be harmless,” but we need to identify “the bad actors,” says Gunter Oberdorster, a nanotech researcher at the University of Rochester. Here’s what’s known about the risks of the most commonly used nanomaterials in consumer products:

Fullerenes. Recent studies have shown that fullerenes, composed of spherically arranged carbon atoms, might damage cells in fish, kill other aquatic micro­organisms, and harm human liver cells and DNA. Moreover, they can penetrate the skin, at least when it’s repeatedly stretched.

But manufacturers are already using fullerenes in cosmetics, fuel cells, and many other products. Since fullerenes can act as ­antioxidants, companies using nanotechnology such as Sircuit Cosmeceuticals in Los Angeles and Zelens in London, have developed “anti-aging” facial creams containing them. Zelens boasted on its Web site in late April that the C-60 fullerenes (composed of 60 carbon atoms) in its $300-per-ounce night cream are “Nobel Prize-winning ingredients."

We asked Harold Kroto and Robert Curl, who shared with the late Richard Smalley the 1996 Nobel prize in chemistry for constructing C-60 fullerenes, whether their discovery should be used in cosmetics. “I wouldn’t put C-60 on my skin,” Kroto said. Curl agreed, saying he would not expose himself “to a new substance for essentially trivial reasons."

Asked about those comments, Zelens’ commercial director, Elliott Goldstein, said that the company has done “a wide range of tests” on the C-60 creams and “has no concerns” about their safety. Nevertheless, he said the company is replacing them with versions that don’t contain fullerenes. Sircuit’s president, Michael Wolfgeher, said research has shown that the fullerenes in his company’s cosmetics, which cost $110 to $150 per ounce, are “safe and extremely beneficial."

The Food and Drug Administration generally does not review the safety of cosmetic ingredients before they’re sold. And the industry’s Cosmetic Ingredient Review board has not evaluated whether nanoingredients are safe. But industry spokesman John Bailey says fullerenes have been added to the list of ingredients that manufacturers are supposed to identify in cosmetic labeling.

Carbon nanotubes. These tube-shaped structures are typically sealed in composites for auto-body parts, electronic equipment, and sports gear. So they’re unlikely to become airborne during normal use, though that might happen when nanotechnology products are thrown away or incinerated. Currently, lab and factory workers probably face the most exposure.

Nanotubes and asbestos have similar fibrous shapes. Preliminary findings from the National Institute of Occupational Safety and Health (NIOSH) have suggested that one type of nanotube, called multiwalled, did not act like asbestos in the body.

But several animal studies of single-walled nanotubes, the other common type, indicate that they can inflame the lungs. One study found rapid lung damage in mice exposed to the equivalent of the workplace limit for another form of carbon. An accompanying commentary on that study warned that nanotube workers might therefore be at risk for pulmonary fibrosis, or potentially fatal scarring of the lungs.

Silver. Nanoparticles of silver generate silver ions, which inhibit bacterial growth. Nanosilver products promoted for their antibacterial power include refrigerators with silver-coated interiors, bandages, and toothbrushes. Whether silver nanoparticles or ions pose health risks is not known. But the National Association of Clean Water Agencies is concerned about their environmental impact. Noting that “silver is highly toxic to aquatic life,” the group asked the Environmental Protection Agency in early 2006 to regulate as pesticides all products that release silver particles.

That action was prompted by the sale of products produced using nanotechnology such as Samsung’s SilverCare washing machine, which deposits silver ions into clothing during the wash cycle. When we tested that product last year, it inhibited the subsequent development of odors in washed clothes more effectively than other machines but was no better at removing dirt and stains.

Late last year the EPA informed Samsung that its washer would indeed be regulated as a pesticide. Samsung spokeswoman Deborah Szajngarten says the company will continue to sell the appliance and will make sure it meets government regulations, though she adds that the company’s tests indicate it is safe.

When the EPA announced the Samsung decision in November 2006, agency spokeswoman Enesta Jones said that silver-containing products would not be regulated if their maker didn’t make a pesticide claim. Around that time, certain antibacterial claims and references to silver nanoparticles disappeared from online descriptions of products such as Sharper Image’s FresherLonger Miracle Food Storage containers. The company did not answer our e-mailed request for comments.

The EPA said in December that it would publish a formal policy on silver-releasing products. That policy had not appeared at the time of this report.