Navigating the minefield of airborne nanoparticle exposure

Nanotechnology—like other emerging technologies—presents a dilemma:  If you’re making new substances with uncertain health risks, how low is low enough when it comes to managing exposure?

The issue is raised in the current edition of Nature Nanotechnology by Vladimir Murashov of the National Institute for Occupational Safety and Health (NIOSH), and former NIOSH-director John Howard.  But the question has been bubbling along for some time.

And it’s an important one.  Uncertainty over safe workplace practices is bad news for nanotech businesses trying to do the right thing—especially small start-ups that don’t have the resources to work out their own bespoke solutions.  It’s not much better for regulators—as the gap between emerging technologies and solid information on their safe use widens, how do you craft new approaches to protecting people’s health and the environment?

Back in 2007, the Environmental Defence Fund and DuPont released their Nano Risk Framework… The Framework places a heavy emphasis on pragmatic exposure-based decision-making.  In a nutshell, the message was: Use the best information available. And when that runs out, use every trick in the book to come up with the best possible benchmarks for qualitatively managing risk—until better information is available.  And do all this under “reasonable worst-case” assumptions.

But the Nano Risk Framework stops short of providing practical guidelines on developing benchmarks for exposure assessment.

This gap was neatly filled by a guidance document from BSI Inc—the British Standards Organization—in January 2008.  The “Guide to safe handling and disposal of manufactured nanomaterials” (BSI PD 6699-2:2007) takes the bold step of recommending starting exposure values for four different classes of nanomaterials—benchmarks for establishing exposure decision-points in the absence of anything else.  PD 6699-2 refers to them as Benchmark Exposure Levels, and couches them in enough caveats to make the most hardened lawyer proud.  A better moniker might have been Lifeline Exposure Levels—because they quite literally throw a lifeline to anyone completely at sea when it comes to making practical decisions on making sense of airborne nanomaterial exposure measurements.

But the Benchmark Exposure Levels are based on assumptions and speculation, not hard science.  And while they are firmly grounded in recommendations within the Nano Risk Framework—using available information and reasonable worst-case solutions—they are, in the long-run, no substitute for quantitative risk assessment.

This is one of the main concerns that Murashov and Howard have about the BSI guidelines in their Nature Nanotechnology commentary.  They argue that exposure limits should be based on generally accepted principles of risk assessment—and I agree with them.  But something is needed in the interim while these limits are established, otherwise the whole emerging technology enterprise is on dodgy ground!

This is exactly what the Nano Risk Framework and PD 6699-2 address, and hopefully what additional guidance from organizations like the International Standards Organization, and even government agencies, will grapple with.

But this brings us back to the original question—how low is low enough?  Because recommendations like “keep exposures as low as reasonably practicable” simply don’t cut the mustard without some sense of how to evaluate exposure, and what the numbers mean.

PD 6699-2 makes a good stab at helping industries develop internal pragmatic guidelines on how to use airborne exposure measurements when working with new nanomaterials.  Earlier this year, I took a stab at assessing the validity and utility of the Benchmark Exposure Limits for BSI—the full assessment is available here (PDF, 168 KB).  My conclusions: the benchmark levels are far from perfect, but they are a great starting point.

Assuming that most readers will have better things to do than read through the 12-page assessment, here are the conclusions:

If effective health and safety plans are to be implemented in research laboratories and workplaces generating and using nanomaterials, guideline exposure limits are essential.  In the absence of further information, the benchmark exposure levels presented in BSI PD 6699-2:2007 appear reasonable.  Furthermore, the context surrounding the levels—which is clearly stated in the document—allows people following the recommendations to adapt the levels to their specific circumstances, depending on the best available information.  In other words, they are not binding, but rather present a clear starting point for an informed process of setting relevant exposure levels.  And thus, where evidence exists to suggest that the benchmark exposure levels are overly stringent or not measurable for a given material, it is left to the discretion of the person setting the levels to adjust the accordingly.

These suggested levels are not a substitute for workplace exposure limits, and do not remove the need for targeted research leading to the development of evidence-based limits.  But until such levels are developed, they fulfil a role that is essential to underpinning the development of safe and successful nanotechnologies.  As such, BSI should be applauded for publishing them.

The bottom line here is that industry needs practical guidelines on safe workplace practices where hard information on risks is lacking, and at some point this will mean grasping the bull by the horns and providing advice on how to measure exposures, and what the numbers mean.

Giving meaning to the numbers might simply require establishing rules of thumb for developing bespoke exposure levels.  Or it might require clear benchmark exposure levels to be suggested for different classes of materials (with suitable caveats of course).  Either way, there will be exposure data, and people will want to know what they mean, and what action to take as a result.

In the long run however, hard data are still needed to underpin quantitative and authoritative risk assessment that will supersede interim qualitative measures.  And this of course means there needs to be a research plan, plenty of funding, and a willingness to translate new information into informed oversight.

But that is a story for another day…

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Amidst the cacophony of debate swirling around the true meaning of nanotechnology, I head a voice or reason last week.  The voice was that of Dr. Bernd Sachweh of BASF, speaking at the European Aerosol Conference in Thessoloniki.

I paraphrase, but the essence of Bernd’s point was this:

‘Nano’ is not a thing or a product.  It has no intrinsic value.  Rather, ‘nano’ adds value; it changes the properties and the worth of something that already exists.

I must confess, I rather like the idea of ‘nano’ as adding value, rather than being an entity in and of itself.  It’s hard to come up with of an example where engineering something at the nanoscale leads to behaviour or functionality that is independent of the starting material.  Rather, the great potential of nanotechnology would seem to be in taking raw materials and engineering them in ways that lead to the emergence of novel scale-related properties, which can then be used in new and innovative ways. 

But what I really like about the concept of added-value is that it provides insight into how nanotechnology might be approached from an oversight perspective.  

Just as ‘nano’ adds value to products and processes, it can also be seen as changing the potential of something to cause harm; an “added-risk” to counterbalance the “added-value.”

As soon as ‘nano’ is seen in terms of both added-value and added-risk, it becomes easier to think through some of the more knotty questions associated with using nanomaterials and nano-products safely.  

First off is the question of whether all products of nanotechnology are uniquely harmful.  

Unique nanoscale-related functionality features in many definitions of nanotechnology—this is where the added value comes from.  And it is often assumed that this unique functionality will always equate to unique risks.  Yet unlike added-value, added-risk is not intentionally built into the products of nanotechnology.  Rather, it is a by-product of the technology.  

As a result, added-risk may be significant in some cases, while in others it may be negligible.  It is even conceivable that engineering a material at the nanoscale could reduce the risk it presents to human health and the environment—leading to negative added-risk.  From an oversight perspective, functionality and potential to cause harm sometimes need to be disentangled—something that the concepts of added-value and added-risk might help to achieve.

Following this line of thought, effective nanotechnology oversight will depend on identifying whether engineering a material at the nanoscale results in added-risk.  And implementing such oversight will mean identifying, measuring and controlling those aspects of a new product or material that add to the risk—whether they are related to particle size, material surface area, surface chemistry, or other nano-relevant characteristics. 

But does nanotechnology demand a brand new set of regulations, or can the existing ones cope?  Where existing regulations work for conventional materials and products, the concept of added-risk would seem to support developing new rules on applying current regs to nanotech materials and products, rather than formulating a new set of nanotechnology regulations.  After all, if ‘nano’ has no intrinsic value or risk, what will a brand new set of regulations actually regulate?

The caveat here of course is that the existing regulations need to be sufficiently robust yet flexible to address the added-risk that some nanotechnology applications will embody.  And the evidence is that this isn’t the case for every material or product out there! (See for instance, “Managing the effects of Nanotechnology” by J. Clarence Davies)

Sticking with existing regulations, the concept of added-risk is useful when it comes to defining what is ‘nano’ and what is not from an oversight perspective.  

If the aim is for regulations (in the broadest sense) to address the added-risk rather than the added-value of nanotech materials and products, should definitions of nanotechnology be used that emphasize added-value?  Probably not.  Definitions that depend on the uniqueness and “added-value” of nanotechnology are great for guiding and inspiring research and investment that will lead to new nanotechnology-based products.  But where they do not embody the concept of “added-risk,” they are at best inadequate and at worst seriously misleading when it comes to ensuring the safety of new nanotechnologies.  For instance, gold nanoparticles can bring significant added-value to products when incorporated into heterogeneous catalysts, but if release and exposure are low, added-risk is likely to be minimal.  On the other hand, reducing the size of silver particles to 20 nanometers brings only marginal added-value from a nanotechnology perspective (the physical and chemical properties of the silver do not alter appreciably from the bulk material at this size), yet the increased possibility for release, dispersion and exposure most likely leads to significant added-risk in some cases.

For regulatory purposes, something else is needed—a point hammered home by Mike Taylor in his 2006 assessment of the US Food and Drug Administration’s ability to regulate the products of nanotechnology.  In this respect, it would be far more useful to have a definition of nanotechnology that incorporates the idea that nanoscale engineering can lead to significant changes in the potential risks associated with a material.  Something like: 

For regulatory and oversight purposes, nanotechnology is the control of matter at dimensions between approximately 1 and 100 nm, where the behaviour of the resulting material or product differs sufficiently from the component materials to lead to significant changes in potential risks to human health and the environment.

This is a definition that is based on added-risk, not added-value.  And unlike the more commonly used definitions of nanotechnology, it would encompass engineered nanomaterials where the predominant change in moving from the macroscale (or molecular scale) to the nanoscale is an increased potential for release, transport, accumulation, exposure dose, and biological impact.  

Developing an added-risk based definition along these lines (and this is just an example of what a definition might look like) would include a broad range of materials and products that have an altered risk profile because of how they have been engineered; not just those that lie within the somewhat artificial boundaries of 1 to 100 nm.  In effect, there would be no more need for lengthy arguments about whether a 99 nm particle is a nanoparticle for regulatory purposes but a 101 is not; or whether large molecules should be treated as nanomaterials.  Under such a definition, the determiner of relevance would be added-risk, NOT size.

This all sounds great.  But I do have one niggling concern about this idea of added-risk.  And that is how will it apply to the more esoteric products of nanotechnology that are coming along—the increasingly complex second, third and even fourth generation materials that have multiple components, multiple functionalities, and can respond and adapt to their environments and other stimuli.  Here we are moving from adding value to existing materials and technologies, to building brand new materials and technologies.  Will we still be able to think of oversight in terms of added-risk, or will we need to go back to the drawing board?  

That’s a tricky one and I’m not sure the answer is clear yet.  But given the current rate of progress being made in nanotechnology, we could do with some answers sooner rather than later.  In the meantime, seeing nanotechnology in terms of the added-value and added-risk it brings to materials, processes and products might just help deal with the nanotech which is out there now.

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This post first appeared on the SAFENANO blog in September 2008