I’m breathing in nanoparticles, so why aren’t I dead already?

Read some accounts of nanotechnology risks, and you might be forgiven for concluding that a single engineered nanoparticle can kill you.  Of course, a little critical thinking soon dispels this notion—we are constantly bombarded with incidental nanoparticles from sources that include cars, incinerators and fires; we have been since birth.  And as critics of “risk extremists” often point out, we seem to be doing just fine in this nano-rich environment.  But does this mean that the potential risks associated with engineered nanoparticles are little more than a myth?

This was the question I faced while writing an opinions piece for the latest issue of Nano Today.  It’s a question that’s constantly popping up, either because someone has forgotten (or never realized) that nanoparticle exposure is a fact of life, or as a justification for not worrying about the engineered varieties of nanoparticles.

As you might expect, the truth is somewhat more complex than either of these extremes, and still remains unclear.  But to get back to the article; as an “ambience-hack” (the literary equivalent of a “character actor”), I felt it important to start off in a place particularly laden with nanoparticles—my local coffee shop.  Armed with a model 3007 portable condensation particle counter, kindly on loan from TSI Incorporated, I resolutely set out to sample the local nano-aerosols over a good cappuccino.

As coffee and breakfast were being prepared, the particle counter indicated I was inhaling somewhere around four billion particles per minute.  That’s not far off one nanoparticle for every man, woman and child on the planet entering my lungs every sixty seconds.  Yet I was feeling fine.  Clearly my body was doing a good job of handling them—thanks to millennia of Darwinian natural selection giving me lungs that know a thing or two about airborne nanoparticles.

But I don’t buy into the idea that my surviving the coffee shop naturally means all nanoparticles are safe. The trouble is; all nanoparticles are not created equal, and to generalize will be to make mistakes—perhaps costly ones.

And the idea that we are perfectly adapted to breathing in particles is somewhat flawed. Consider these rather sobering facts associated with inhaling particles having a range of sizes: Between 1990 and 1999, there were over 30,000 deaths in the U.S. associated with occupational exposure to airborne materials [1]. Estimates of worldwide deaths from asbestos exposure lie between 250,000 and 400,000; and in the U.K., deaths due to asbestos-related mesothelioma are not expected to peak for another ten years—despite imports and use of asbestos peaking in the 1960’s [2].  In the general environment, estimates of the number of people who died from inhaling particles in the London Smog of 1952 are as high as 12,000 [3]. At a more subtle level, exposure to fine airborne particles has been associated with an elevated likelihood of dying, and there is increasing evidence linking nanoscale particle exposure with impacts on the cardiovascular system [4].

The bottom line is that our lungs, incredible as they are at dealing with each day’s dust burden, have their limitations. Our knowledge of airborne particles in general and incidental nanoparticles in particular can illuminate our approaches to engineered nanoparticles.  But just as the health risks from asbestos, vehicle emissions and welding fume differ, we will not be able to derive everything we need to know about engineered nanoparticles just by looking at the incidental varieties.

It’s interesting to push this idea of differences between particle types further.  Clearly our lungs have evolved to handle naturally occurring nanoparticles.  But does this mean we also have the ability to deal with engineered nanoparticles never previously encountered, and as a species have not had the chance to acclimatize to?  We know that our bodies have a hard time dealing with chemicals that do not occur naturally—will the same hold true for engineered nanomaterials?

And then there is the comparison between the veritable cocktail of ambient nanoparticles we all breathe, and the precision of many engineered nanoparticles. Does exposure to a complex mixture of particles cause harm through synergistic interactions, or does the “soup” we breathe dilute the impact of the relatively few dangerous particles that might be present?  And—if a manufacturer hits on a particular combination of physical and chemical properties that is less than compatible with a long and healthy life—how much more dangerous is an aerosol of this “pure nanomaterial” than the nanoparticles you and I are breathing now?

This leads to the tricky issue of dose—how much material is needed to cause damage.  “The dose makes the poison” is the mantra of toxicologists worldwide—acknowledging that the most toxic substances can be harmless (or even beneficial) at low enough doses, while nothing is good for you in excess.  Four billion particles per minute might sound like a lot, but it is a minuscule amount of material when you consider how much mass there probably is in those particles.  Scribbling out some rather crude back-of-the-envelope calculations, I am probably inhaling no more than 50 nanograms of nanoparticles per minute in the coffee shop.  In contrast, a highly toxic dust like crystalline silica has an occupational exposure limit that equates to inhaling around 1,000 nanograms per minute over eight hours, and the equivalent limit for a material like titanium dioxide is a whopping 300,000 nanograms per minute.  Yet which is the appropriate way to measure dose—the mass of particles, their number, or something else; like surface area?

At the end of the day, I can drink my coffee and inhale the local nanoparticles with no obvious ill effects because I’m not exposed for that long and my body knows how to deal with them.  And there are probably plenty of engineered nanomaterials I could do the same with.  I know that a single nanoparticle won’t kill me—probably a few billion wouldn’t be enough to do much damage.  But I’m under no illusion that all engineered nanoparticles will be safe, just because I’m breathing in incidental nanoparticles all the time.  It all comes down to understanding what causes a new material to be harmful, and how to avoid harm—which means we need to get on and do more research if questions like the ones above are going to be answered.

Now, back to my four billion particles a minute with a cappuccino on the side…

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[1]  NIOSH, DHHS(NIOSH) Publication Number 2003-111 (2003).
[2]  European Environment Agency, ISBN 92-9167-323-4 (2001).
[3]  E. E. Dooley, Environ. Health Perspect. 110 (12), A748 (2002).
[4]  N L Mills, H Tornqvist, M C Gonzalez et al., New England J. of Med. 357 (11), 1075 (2007).

The full Nano Today article, “Living with nanoparticles”, can be accessed here

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This post first appeared on the SAFENANO blog in April 2008

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