Risk Assessment of Manufactured Nanomaterials - Environmental and Human Health Impacts of Nanotechnology - page 398

raises the concern that the use of concentration as the dose metric may be inappro-
priate (Crane et al. , 2008 ; Wardak et al. , 2007). This might be resolved by reporting
hazard as a function of particle number or surface area. Nonetheless, the uncertain-
ties are cumulative in the risk analysis and uncertainty in predicted environmental
concentrations would compound uncertainty about predicted effects concentra-
tions. These issues require further research to understand, quantify and model.
The diversity of nanomaterials (discussed in Chapter 9) includes different par-
ticle sizes, morphologies, surface chemistries, surface charge heterogeneities, capping
formulations, surface ligands (functionalities) and impurities for any one given
nanomaterial (Gogotsi, 2006; Ozin and Arsenault, 2006; Roduner, 2006). There are
also nanocomposites made of layers of different chemicals (e.g. quantum dots) and
nanomaterials incorporated into a matrix with other chemicals in product manu-
facturing processes (Hansen et al. , 2008). The issue of nanomaterial diversity,
problem formulation and prioritisation in risk assessment of nanomaterials has
been considered by SCENIHR, where the description of the material (including
the size of particle, exposure, homogeneity, chemical composition) are determinants
of the extent to which risk assessment is required and the component methods that
should be used (Figure 10.4).
The diversity of functionalities (chemical ligands on the surface of the nanomate-
rial), geometries, crystal structures and impurities of individual nanomaterials, and
the diversity of nanomaterials as a ' group ' of substances, must be reconciled with
the challenges of understanding and quantifying the complexity of behaviour for
any one material and its variants. Ideally, detailed and validated models for each
type of nanomaterial are needed to minimise uncertainty. It is clear that this will
not be achievable for every nanomaterial or its variants, nor should such detailed
studies be advocated for every nanomaterial. Instead, attention should focus on
how nanomaterials are prioritised for quantitative risk assessment, and in particular
on the essential phase of problem formulation within risk assessment outlined in
the section above (Owen and Handy 2007).

10.4 Assembling Evidence for Safety and Intervention

It is clear from the above discussion that risk assessors need to assemble evidence
of varying quality for nanomaterials risk assessments. One context in which risk
assessors collate evidence is that of the ' safety case '. A case is made by an operator
to a regulator for the safety of an activity, such as the operation of a large integrated
refi nery, chlorine production facility or radioactive waste repository. Safety case
legislation is widely used across the transport, aerospace and chemical process
sectors, and the risk assessment that support safety cases must draw together evi-
dence for operational safety across a plethora of fi elds, integrating qualitative and
quantitative data with the objective of presenting an overall case for safety based
on the full set of evidence. These situations are analogous to that currently faced
by the manufacturers of nanomaterials. Partial, contradictory and equivalent evi-
dence must be collated, assessed and weighed in its entirety, so as to draw con-
clusions on the signifi cance of risks posed by the release of nanomaterials in