Toxicology experiments on nanomaterials often seem to run the same way: put some nanoparticles, carbon nanotubes, quantum dots, or other kind of nanosized structures in a petri dish, water column, soil sample, or lab test tube of choice. Then expose daphnids, microbes, zebrafish, pig lung cells, human skin cells, or other model organisms to the new and exciting materials. Sit back and see what happens.

The peer-reviewed literature contains thousands of articles documenting results from these kinds of tests, all conducted in an effort to determine the health and safety of nanomaterials. Yet the scientific community has yet to determine which nanomaterials are hazardous to the environment or humans, because of a lack of methodology, metrology, and other basics, including how to actually monitor nanoparticles in air, for example. The diversity of nanomaterials, both existing ones and those to come, also presents a challenge.

Researchers say that the field of ecotoxicology and environmental risk assessment of nanomaterials is still in its infancy after less than a decade of concerted effort. And while snapshots from short-term exposure studies are yielding tantalizing glimpses now, the whole picture provided by long-term data on more subtle effects of nanomaterials is completely missing. New methods and collaborations could bring more definitive information soon. Until then, efforts to understand the hazards of nanomaterials continue in a piecemeal fashion.

Experiments in a bucket

Scientists studying nanomaterials have been “learning on the job,” says the U.K. Environment Agency’s Richard Owen, who recently published a viewpoint article in ES&T about environmental risk assessment of nanomaterials (2007, 41, 5582-5588). They mix organisms and nanoparticles, thrown into distilled water or seawater, under conditions that may or may not be similar to natural settings. Then they ask, “‘Did the daphnids die, did the fish die?’ . . . Researchers use standard methods in simple systems,” Owen says, “taking a reductionist approach using tools that are available to them.”

Such tactics may be the only option for now. “People are looking for paradigms, and they’re starting with research they can do,” says Patricia Holden, an environmental microbiologist at the University of California Santa Barbara. “Similar experiments, slightly different materials, similar results-that’s a positive thing in a way,” she continues. “We’re seeing consensus on some aspects of the research, across labs-and a little bit of consistency means the potential for paradigms.”

But nailing down behaviors and mechanisms remains tricky. For example, TiO2 can be innocuous in soils on its own but problematic in water or once a coating is added. “People are still grappling with size versus functionality, size versus surface chemistry, and the characteristics that go with each one of those subdivisions,” Holden adds. Once those characteristics can be identified, “then maybe we’re getting somewhere.”

Even if progress is made in the laboratory, extrapolating lab results to the real world will be difficult. Most lab experiments require incredibly high concentrations of nanoparticles or other nanomaterials to kill an organism. For example, a paper published January 16 in ACS Nano (2008, DOI 10.1021/nn700185t) reported that a lethal dose for rats of single-walled carbon nanohorns, or tiny tubules that agglomerate into beautiful dahlia-shaped spheres, topped 2000 milligrams (mg) per kilogram of the animals’ body weight. The rats’ lung tissues remained undamaged after a 90 day test, despite some blackening from the nanohorns.

But if that test had been conducted for longer, the researchers conjecture, they may have observed some deleterious effects. And such longer term tests are currently lacking in the literature. “It may be that chronic impacts will be more important,” Owen says, “or interactions of nanomaterials that change their properties.”

For example, bucket experiments have shown that the LC50-or lethal concentration for half a population-for daphnids in water exposed to nanosized TiO2 is 100 mg per liter, a level highly unlikely to occur in the environment. But sometimes effects occur at a smaller, subacute scale: subtle changes to fish gills exposed to copper nanoparticles (Environ. Sci. Technol. 2007, 41, 8178-8186), inflammation in human lung tissues in contact with nanoparticles (e.g., Environ. Sci. Technol. 2007, 41, 331-336), or potential changes over generations because of damage to embryos’ or microbes’ DNA when nanoparticles get into the cells (e.g., Nano Lett. 2007, 7, 3592-3597).

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