The changes in physical properties that make nanomaterials so useful can also alter their toxicity profiles. Vicki Colvin, a chemist at Rice University, says that the usual way to assess toxicity, by measuring a toxin's mass, won't work at the nano level. Nanomaterials have a much higher surface-to-mass ratio, and while this makes them good for such purposes as water filtering, it also could cause them to interact with body cells that their tiny size allows them to infiltrate.
The implications of such characteristics for health and for the environment remain unclear. "One problem with nanomaterials is that how toxic they are depends on a lot of things," says Jennifer Sass of NRDC. "It depends on surface coatings, whether the nanoparticles are functionalized, whether they agglomerate" -- in other words, whether they are modified in ways that make them more easily absorbed by the body and whether they tend to clump together into bigger-than-nanoscale clusters. These variables make it difficult to issue blanket safety standards for nanoparticles.
In late 2006, Andrew Maynard of the Project on Emerging Nanotechnologies in Washington, D.C., along with 13 colleagues, pointed out in a commentary in Nature the unique challenges of nanotoxicology. "We have known for many years that inhaled dusts cause disease, and that their harmfulness depends on both what they are made of and their physical nature," they wrote. With asbestos, for instance, the material's likelihood of causing respiratory problems is directly related to its size and shape: "Thin, long fibers of the material can lead to lung disease if inhaled, but grind the fibers down to shorter particles with the same chemical makeup and the harmfulness is significantly reduced." Sometimes it's more dangerous for particles to be small, sometimes it's less dangerous. While the toxicological profile of a nanoparticle might depend on its size, Maynard and his colleagues wrote, it might also depend on other factors, such as "surface area, surface chemistry, solubility and possibly shape."
And it's not just the characteristics of a nanomaterial that might be relevant but also its provenance. For instance, there are four different processes for manufacturing nanotubes (above), five different purification methods, and 10 different surface coatings, not to mention 20 different structural types, ranging in length from 5 to 300 nanometers. In all, calculates J. Clarence Davies in the recent report "EPA and Nanotechnology," there are 50,000 different versions of single-walled carbon nanotubes. Of these 50,000, "each version may have different chemical, physical, and biological properties."
Nanotoxicology is filled with uncertainties. Even though a growing number of toxicity studies show that nanoparticles behave differently from larger particles, different does not necessarily mean more dangerous. "Unknown risks do not translate to high risks in every case," Maynard says. "There are many nanomaterials where the materials are likely to be relatively safe."
So if the mind reels trying to conceptualize the smallness of nanoscale, it has almost as much trouble trying to comprehend the vastness of the task of getting a handle on the health and safety ramifications of nanotechnology.