Biomedical Engineering Reference
In-Depth Information
(with several well-conducted studies) to initial reports of tumor promotion for other
forms. Although the nature and amount of free CNTs from actual uses in relation
to these purer forms is not clear, the potential for exposure to a form of CNT that
may be toxic should be addressed cautiously (e.g., through appropriately validated
dust control and personal protection equipment in occupational settings where dust
is released) as methods and data are developed to characterize the exposure and
toxicology of the released material. In particular, in the short term, evaluation of
release likelihoods and the persistence of released material as polymer bound-CNT
fragments is needed to understand the relationship between released materials and
toxicology studies of pure CNTs.
Before such studies can be undertaken, funding must be applied to the develop-
ment of methods to routinely and reliably characterize the “added and persistent
nanoscale properties” of the released materials. Evaluations of methods presented
and discussed by experts of the NanoRelease project at workshops in 2012 and
2013 concluded that technology and standard approaches are not available to more
than qualitatively detect (e.g., electron microscopic evaluation of nonrepresentative
dust samples) the presence of CNTs in material released to exposure pathways.
Quantitative methods needed to ascertain dose in relationship with measures of tox-
icity may not even be possible with current technology, and are certainly not in
standard practice. Without such methods, the risk assessment of CNTs in polymer
composites can only be done in a binary or bounded fashion, either demonstrating
that there is no exposure and no risk or that there is some exposure bounded by
crude measures of mass of added CNTs. Differentiation within the bounding is pos-
sible, given the differences in toxicity expressed in the literature for different CNT
types, with greater attention for materials that are clearly asbestos-like in initial form
and release, and a greater expectation of safety for materials that do not have this
initial form.
Considering the likelihood of exposure to free CNT fiber, the US National
Institute of Occupational Safety and Health (2013) has issued a “Current Information
Bulletin” with a “recommended exposure limit (REL)” of 1.0 µg/m
3
for CNT fibers
based on evaluation of a number of studies on various forms of CNTs and carbon
nanofibers. Based on consideration of the material chemistry and the rare observance
of free CNTs in studies of released particles from abrasion, it would appear that this
safety level is not likely to be exceeded in direct exposure scenarios for users of con-
ductive CNT-polymer composites. However, this qualitative comparison should be
confirmed with quantitative methods sufficient to show the upper boundary level for
free fiber release. Furthermore the lack of evidence to expect that CNTs embedded in
polymer particles would be inherently more toxic than particles without embedded
CNTs indicates that, again, the use scenarios with CNTs are no more hazardous than
those without CNTs. Other routes of exposure (e.g., dermal or oral) have not been
evaluated for CNT exposure, with most of the research community focused on the
inhalation route of exposure for CNTs.
Risk evaluation for direct exposure pathways through disposal and recycling is
less well understood, although for each there would be a similar expectation of low
likelihood of pure CNT exposure. Risk evaluation for subsequent environmental
transport, degradation of polymer matrix, and exposure to free CNTs is in need of
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