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Sometimes environmentalists get accused of being against everything, so it’s important to speak out when the science suggests that a new technology is not obviously harmful to the environment. Right now, I think that is the case with most forms of manufactured nanoparticles. I expected the worst, but the available science is more reassuring.
Just what are nanoparticles? Like the name suggests, these are very small particles—less than about 300 nanometers in diameter. At the moment, the EPA regulates air pollution by particles that are less than 2.5 micrometers in diameter, known as PM2.5, which includes nanoparticles, but also particles that are as much as 8X larger. Particles less than 2.5 um in diameter can be inhaled deep into our lungs, where they can lead to undesirable effects.
Very small particles are ubiquitous in nature. Many of the clay minerals in soils are found in particles that fall into the size range of nanoparticles. Woodsmoke also contains particles of that size and larger.
Manufactured nanoparticles are produced with specific purposes in mind. For example, nanoparticles of titanium oxide form the basis of many sunscreens, because they increase the reflection of sunlight away from a treated surface. Nanoparticles produced with silver are useful as antimicrobial agents that have revolutionized the treatment of burns. Silver nanoparticles are also added to athletic-wear, such as socks to kill the microbes associated with “foot odor.” (My mother would have certainly appreciated that when I was growing up.)
Their small size is one reason nanoparticles are so effective. Most of the atoms in a nanoparticle are found at or near the surface, so they deliver an effective dose of reactive material to the target. Already, huge quantities of nanoparticles are in industrial production. A 2011 assessment by Dr. Christine Hendren and her collaborators at Duke’s Center for the Environmental Impacts of Nanotechnology (CEINT) reported production of titanium nanoparticles alone is likely to exceed 38000 tons/year. Of course, most of the nanoparticles in sunscreens and antibacterial clothing are likely to be washed off in water and enter the sewage-treatment stream of modern society.
One might expect a host of environmental impacts of antimicrobial particles in nature, particularly when they enter wetlands. Silver and copper have biocidal properties that have been recognized for centuries. At high concentrations, titanium nanoparticles can alter bacterial composition in freshwater environments. Nano-particles are taken up by phytoplankton and passed to higher levels of the food chain. Nanoparticles can also be taken up by larger wetland plants and accumulate in their tissues. Silver nanoparticles are known to kill bacteria in wetland sediments, even at the low dosage that is carried in sewage waters that have been diluted by other effluent. The purposeful and inadvertent impacts of nanoparticles are often determined by their surface affinity to other components of the environment. Ben Colman, also at Duke, has shown that nanoparticles are altered and degraded in nature, making them less effective in their anticipated role.
In an ideal world, the best available science would inform policy. Certainly, policy should be responsive to change as new scientific results are delivered. But, my read at the moment suggests that nano-particles have a benefit/cost ratio significantly greater than 1.0. Let’s not stop studying the potential impacts of nanoparticles; something bad may turn up. But let’s not sound premature alarm either.
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