It seems that significant nanoparticle risk studies are just like London buses, you wait ages for one to arrive and then two come along together.
Last month we had the Poland et al. study concerning the hazardous nature of MWCNT. This highly reported study was the first to explicitly test the hypothesis that long carbon nanotubes behave like long asbestos fibres in the body. As described in Andrew Maynards blog, they uses an established method to test whether a fibrous material has the potential to lead to the disease mesothelioma—a cancer of the outer lining of the lungs that can take decades to develop following exposure.
The results showed that fibres longer than 15 micrometers to 20 micrometers (whether asbestos or carbon nanotubes) led to a positive response; short/compact particles did not. This is the first study to demonstrate that carbon nanotubes that physically resemble harmful asbestos fibres, can also behave like harmful asbestos fibres.
However, as Andrew stated “What the study does not address is whether exposure to long straight carbon nanotubes will occur or, if it does, whether these fine fibres will reach the mesothelium surrounding the lungs, and go on to cause mesothelioma.”
Now we have a study from the other side of the risk equation, exposure
A new study published in inhalation toxicology is the first published attempt to measure exposure to CNT using methods similar to those used to measure asbestos. The study
“Monitoring Multiwalled Carbon Nanotube Exposure in a Carbon Nanotube Research Facility “ from a team led by Prof Il Je Yu measured exposures in the post production recovery of MWCNT and in a blending activity, part of a composite formulation process.
The authors use real-time systems (SMPS, APS) to measure airborne concentrations (number and mass) and size distributions, but measured the concentration of fibre-like structures by collecting samples onto cellulose acetate filters and analysing using a transmission electron microscope. All objects, identified as MWCNT with an aspect ratio greater than 3 were counted and measured (length and diameter).
High airborne concentrations of fibres were found in the blending activity (maximum 194 fibres/cc) well over the current fibre TLVs (asbestos 0.1/cc). However, the authors also report that the MWCNT lengths that were shorter than 5 μm, (1760.2 ± 1198.3 nm) and so if conventional fibre counting protocols were followed, all would have been excluded from the count. Clearly then all of the fibres measured in this case would fall into the short category as described by Poland and therefore would not be expected to exhibit the same pathogenicity as long CNT or long asbestos.
However, the study does demonstrate that in a relatively simple industrial process, mixing and blending, it is possible to generate high levels of airborne MWCNT. This is extremely significant given the widely held view that generation of an aerosol of this material is almost impossible. Just because in this case, all of the fibres generated were short, there is no guarantee that a different batch of material or a different process would not produce longer fibres
The study provides a very clear warning about the need for effective exposure control in facilities where MWCNT are being processed. I can only echo Andrews earlier comments,
“action is also needed now to ensure carbon nanotube exposures to workers and users are kept as low as possible. This means developing appropriate exposure measurement methods, applying effective control and containment protocols, and agreeing on benchmark exposure levels to use in the absence of more formal exposure limits.”
To help with this you may wish to refer to the recent BSI Guide to safe handling and disposal of manufactured nanomaterials (PD 6699-2:2007,) which recommends a benchmark exposure level of 0.01 fibres/ml for carbon nanotubes in the absence of any other information —this would seem to be good advice for long carbon nanotubes, until more is known about their exposure potential and hazardous nature.
So in answer to my earlier question, yes I think we are getting somewhere. I’m just not sure its where we want to be. Those working with MWCNT, please take note.