Resolving the carbon nanotube identity crisis

Twelve months ago today I held a bag of multi-walled carbon nanotubes up before a hearing of the U.S. House Science Committee.  I wanted to emphasize the discrepancy between the current state of the science on carbon nanotubes, and a tendency to classify this substance as the relatively benign material graphite from a safety perspective.  So it is perhaps fitting that on the anniversary of that congressional hearing, the US Environmental Protection Agency is making it clear that carbon nanotubes are in fact, a new substance—and should be regulated as such.

Carbon nanotubes are often described as sheets of graphite—the stuff that makes pencil lead black—wrapped into a tube; leading to nanometre-thin “fibres” that are incredibly strong for their weight, and highly conducting—thermally as well as electrically.  But perhaps because of this simple imagery, they are often handled as if they are graphite—especially when it comes to using them safely.

Given the amount of time and money researchers and industry are pouring into producing and using carbon nanotubes, you would think that they are at least marginally different from their flat-sheeted cousins.  In fact the differences are anything but marginal: Wrapping the sheets associated with graphite into tubes radically changes the physical chemical and biological properties of these carbon-based materials—just like re-arranging the carbon atoms that make up soot into diamonds leads to the formation of a fundamentally different material.

Yet many companies continue to persist in claiming “it’s just graphite” when questions arise over the possible health impacts of being exposed to carbon nanotubes.

But all that is about to change.  Hot on the heels of clarification from the European Commission that carbon nanotubes (and other novel forms of carbon) need to be registered under the new REACH chemicals regulations, the US EPA has clarified their position on the material.  According to a just-released notice in the Federal Register, the EPA

“generally considers [carbon nanotubes] to be chemical substances distinct from graphite or other allotropes of carbon listed on the TSCA Inventory.”

In effect, this means that any company wanting to manufacture or import carbon nanotubes in the United States needs to submit a Pre Manufacturing Notice (PMN) to the EPA—unless the material can be shown to be on the Toxic Substances Control Act (TSCA) Chemical Substances Inventory. And the chances of that are pretty slim—at present.

EPA actually established their position on carbon nanotubes back in 2007, in a document clarifying how the agency saw TSCA applying to engineered nanomaterials [available here].  But the agency’s stance was so unclear that the Federal Register notice clarifying the situation was felt necessary.  In the words of the notice just published:

“current pre-notice inquiries to the Agency and questions in public forums still indicate a lack of clarity on this issue.”

This is a significant step forward for the US EPA, and a very welcome one.  Research is continuing to show that some forms of carbon nanotubes are potentially dangerous if inhaled in sufficient quantities.  Earlier this year, Craig Poland and colleagues showed that long thin multiwalled carbon nanotubes are potentially able to cause the disease mesothelioma if inhaled.  And more recently Anna Shvedova and co-researchers confirmed that inhaled single walled carbon nanotubes can have a unique impact on the lungs of mice.

Neither of these studies suggests that carbon nanotubes behave anything like graphite if they get into the lungs.  Yet companies persist with treating this material like graphite.

I’ve previously noted that carbon nanotube distribution companies like CheapTubes Inc. consider all forms of the material as being like graphite for health and safety purposes.  In fact, as of October 31, the Materials Safety Data Sheet posted on the CheapTubes website noted of carbon nanotubes:

“This material is listed on the US Toxic Substances Control Act (TSCA) Inventory”

There is little doubt now that this is, in fact, not the case.

The EPA’s clarification will certainly help ensure that this innovative material is used safely, and its full potential is realized without causing undue harm.  There are though, perhaps inevitably, still some unresolved issues.   These include various material use and production quantity exemptions that could be used by some companies to justify not applying TSCA to their nanotubes (see for instance the series of articles by Richard Denison on TSCA and nanomaterials).  But smart companies are realizing that compliance is the best way to ensuring safe and sustainable products—which is why a number of PMN’s for carbon nanotubes have already been submitted to EPA (again, Richard Denison’s blog at the Environmental Defense Fund has useful comments on this point).

There are however two rather large flies in the ointment:

The EPA clarification doesn’t add anything to the question of where many other engineered nanomaterials stand on the regulations front.  Carbon nanotubes are chemically distinct from other forms of carbon, and so are easily defined under TSCA as news substances.  But if you take something like titanium dioxide or silver and form it into nanoparticles, current regulations make no distinction between the nano and non-nano forms of the material—even though research suggests the nano-form may be more harmful.

Just as importantly, submitting a PMN for a specific type of carbon nanotube material opens the way for that material being added to the TSCA Chemical Substances Inventory.  And once there, other companies are free to make, use and sell the material.  As Richard Denison writes,

“Once reviewed and placed on the TSCA Inventory, any company can manufacture and use the nanomaterial without even having to notify EPA it is doing so.” (unless EPA simultaneously issue a Significant New Use Rule)

Yet researchers are only just beginning to discover what might make different carbon nanotubes harmful, and how to avoid that harm.  What are the chances therefore of carbon nanotubes being added to the TSCA inventory before we have a good handle on how to use them safely?

The bottom line here is that resolving the regulatory status of carbon nanotubes is an important step forward.  But there is still some way to go before this material is regulated in a way that will encourage innovation while preventing undue harm—whether to people or the environment.

And while carbon nanotubes can perhaps leave the couch feeling a little more confident about themselves, we shouldn’t forget that there are still plenty of other materials out there that are suffering from a nano-induced identity crisis.


5 Responses to Resolving the carbon nanotube identity crisis

  1. Andrew: Thanks for a very interesting post. While I agree EPA’s more prominent clarification of its policy is welcome, I am dismayed by EPA’s lax approach to enforcement of what is a basic violation of federal law. As you note, EPA has been stating for years that carbon nanotubes (CNTs) are new chemicals under TSCA requiring notification. EPA has also been alluding to there being considerable non-compliance with such requirements.

    Yet even this latest Federal Register notice merely says that in March 2009, EPA “anticipates” it will once again look at whether there is better compliance.

    If EPA was serious about ensuring compliance, all it would need to do would be to bring and publicize a single enforcement action,
    and I’d predict this issue would be resolved in a heartbeat!

    I’ve just added a post to our own blog ( ) that highlights yet more evidence of the ability of carbon nanotubes to pose risks to human health, which makes it all the more essential that EPA know who is producing or importing such materials.

  2. Andrew Maynard says:

    Thanks Richard.

    I guess my bar for an enthusiastic response has been suppressed to a rather low level when it comes to nanomaterials and regulation. You are right though EPA could have acted long before now…


  3. Martin Griffin says:

    Excellent summary Dr. Maynard. However one more ‘fly in the ointment’ that needs elaboration is the lack of specifics on how CNT will be listed. As we all know one manufactured CNT could be completley dirrerent in purity, checmial make up etc to another maufactured CNT. And what about other nanomaterials embedded in the CNT interstitial spaces? Since CNT is such a general thing it is unclear how EPA will proceed when they get 20 PMN for 20 different variations of CNT. EPA also made no mention of SWCNT or the difference in SWCNT vs. DWCNT toxcicity in how they are synthesized. To use your analagy, CNT can be as different from one another as different cuts, qualities of diamonds, and as any jewler will tell you all diamonds are not the same. Keep up the good fight!

  4. Andrew Maynard says:

    Thanks Martin.

    This is a pretty significant issue. Vicki Colvin’s (Rice University) back-of-the-envelope estimation for the number of different “flavours” of carbon nanotubes is around 50,000 – and we don’t yet know which attributes associated with these variations are important in initiating an adverse response.

    While major differences in nanotubes are likely to trigger the “new substance” part of TSCA – say, the difference between single walled and double walled carbon nanotubes – there are likely to be subtle differences that are not caught on the basis of unique molecular identity.

    Does this mean that broad categories of carbon nanotubes will appear on the TSCA inventory that allow materials of unknown hazard to slip through the net? Quite possibly, while EPA sticks to a rather out-moded approach of determining when a substance is new, and when it is not.

  5. Martin Griffin says:

    Another thought. Even if on a perfect world all variants of CNT are considered new under TSCA and PMNs are required, it appears to me that more than likely these CNT variants would be listed under the confidental portion of TSCA and this not available to anyone unless you are a manufacturer or an importer, so just listing something in TSCA still won’t give the public or state government or anyone the crucial information that is submitted in the PMN. So even though it appears that EPA is moving forward and actually gathering necessary information to protect the environment and public healtht via the PMN requirements, who can actually use it to create policies to actually protect the environment and human health from unknowns. Any thoughts on this conundrum?

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