Testing Nano: How Much Is Too Much?

Among the scores of researchers hustling to study the impact of nanotechnology on people, animals and the environment, a debate is growing: How much of the super-small stuff should they use to test for safety?

Most testing is done either on animals, both embryonic and mature, or on cells from a variety of sources. In many of these laboratory studies, the test medium is hit with large amounts of whatever nanomaterial is being tested, from carbon nanotubes to titanium dioxide. The question, though, is how much is too much—and whether such studies can be translated into the real world.

“Sometimes, you have to do that, because there’s nothing else that you can do,” said Mark Wiesner (at left in the picture), a professor at Duke and the director of the Center for Environmental Impact of NanoTechnology, in which the school is a major partner. Duke hosted a recent conference on nanotechnology and the environment.

The difficulty comes because some materials, such as super-light carbon nanotubes, are tough to measure, making testing with small amounts difficult. In other cases, bombarding a test medium with large amounts of a questionable material might help lead give a researcher an inkling about what would happen over the course of many years. 

Nanotechnology leverages super-small particles (a nanometer is a billionth of a meter) to create products with remarkable properties. These materials can make bike frames lighter and stronger and sunscreen more transparent on the skin, as well as new medical instruments and medicines that can save lives.

There is broad agreement that nanomaterials have lots of potential for a wide variety of applications. But shrinking these substances can change their properties; scientists are struggling to figure out whether, how and why that shift can make them dangerous in the process. 

Wiesner is pressing his students, and others involved with CEINT, to look at the real world whenever possible. Like others in the field, he’s increasingly convinced that the hazards of nano-based products and applications will come through repeated, small-scale exposure.

For example, Eric Money, a postdoctoral associate at Duke, has developed a mathematical model to predict how much nanosilver might turn up in wastewater treatment plants in North Carolina’s Neuse River basin. Using four scenarios to examine the 38 plants in the area, Money’s model projected that the worst-case scenario would be a silver level of 40 parts per billion. For most of the water treatment plants, according to the model, the level would be less than five parts per billion.

Those are relatively low concentrations, Money said during a presentation at the Duke conference, but still “100 to 200 times higher” than had been previously predicted.

“The answer isn’t one number that comes out, but it’s a range of numbers,” Wiesner said.

He said there obviously are instances where testing with large amounts of a nanomaterial would be useful. But “you can go to extremes, where you’re working with very high concentrations,” he said.

“I’m just not sure what we necessarily learn from that.”

Alistair Boxall, a researcher at the British University of York, does most of his work on the implications of prescription drugs in our water and soil. The hurdles to adequate risk assessment aren’t unique to nano-related substances, he said: drugs and other chemicals, such as pesticides, pose similar problems because of the difficulty of measuring chronic effects, or accumulation, he said.

“What I’d like to see the scientists do is try to put their data in some perspective,” he said.

An even thornier problem, Wiesner said, is finding the best way to measure nanoparticles: by mass, by surface area or by the number of particles.

“Each one could be relevant,” he said.

Surface area might be important, for example, because the smaller the particles, the more surface area they have. That may play a big part in how the substance behaves, as well as how it effects water, soil, air or people. A higher concentration of particles could mean more accumulation in, say, wastewater, or inside certain organs in people and animals.

Mass is already being used, for lack of a better metric, to offer guidance on occupational exposure to carbon nanotubes, for example.

Then, there is the broadest question: are those tiny silver particles in sewage sludge from your neighbor’s anti-stink socks, or some other source? If someone measures carbon nanotubes in the air, are they from gasoline exhaust or were they released by a nanotube manufacturer? And what are the implications of these options for determining what’s safe, and what’s harmful?

“Have we been—and I think the answer is yes—submerged in nanoparticles from time immemorial?” Wiesner asked. “It might be irrelevant. We don’t know. It might also be very important.”


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