8. What research is still needed?
- 8.1 What needs investigation for characterising nanomaterials?
- 8.2 What needs investigation to improve measurement of human exposures?
- 8.3 What needs investigation to improve identification of human hazards?
- 8.4 What needs investigation to improve measurement of environmental exposures?
- 8.5 What needs investigation to improve identification of environmental hazards?
8.1 What needs investigation for characterising nanomaterials?
The research needs as indentified by SCENIHR in its Opinion on the evaluation of the Technical Guidance Documents (TGDs) are still valid (SCENIHR 2007a). They include amongst others the availability of validated in vitro assays, development of an approach for using quantitative structure activity relationship (QSAR), studies on potential cardiovascular effects, evaluation of bacterial genotoxicity assays, methodology on prediction of environmental concentrations (PEC), and environmental species used for ecotoxicity testing. Some progress has been made in the areas of properties needed to determine for nanomaterial characterisation, toxicokinetics and genotoxicity testing with mammalian cells, and environmental behaviour of nanomaterials. Recent research has also identified new concerns in the areas of protein fibrillation, potential hazards of certain nanotubes, and potential for transfer across the food chain in environmental species.
8.2 What needs investigation to improve measurement of human exposures?
There is a need for comparable, reproducible and repeatable harmonised methods for measuring and characterising nanomaterials (SCENIHR 2006), especially for measuring concentrations and characteristics of nanomaterials in biological and environmental media. Being able to address these gaps is important for providing meaningful data which can produce a system of reliable risk assessment. This requires also defining the metrics most appropriate for hazard characterisation and exposure, including the methodology to perform the measurements.
There is an urgent need for the development of reference nanomaterials for the evaluation of both the quality of measurement techniques and to compare biological responses.
8.3 What needs investigation to improve identification of human hazards?
The effect of nanoparticles on protein behaviour as demonstrated in vitro needs further investigation. It is necessary to elucidate whether the in vitro observed effects on protein fibrillation processes (both enhancement and retardation) also occur in an in vivo situation or in more complex biological fluids where competitive binding may take place.
There are indications that after deposition at the olfactory mucosa of the nose, ambient air and nanoparticles may translocate into the brain. This may offer a potential route of entry for medicinal products into the brain. This observation may also raise some concern in view of the amyloid diseases of the brain in the context of the potential of nanoparticles to cause protein fibrillation in vitro. This is certainly an area for which additional research is urgently needed.
Additional studies on the potential hazards of nanofibers/nanotubes need to be performed.
8.4 What needs investigation to improve measurement of environmental exposures?
The estimation of relevant environmental exposure concentrations is seriously hampered by lack of the two essential pieces of information/knowledge. Firstly, there is no quantitative knowledge on the rates of release of nanomaterials to the environment. Secondly, there is neither knowledge nor theory that can be used to predict concentrations of nanomaterials in the ambient environment from release rates. Well- established knowledge of distribution and fate of chemical substances, as it is applied in the current EU guidelines for environmental risk assessment of conventional chemicals, cannot be used for nanomaterials without modification. It is recommended that research be initiated to develop quantitative theory and models that predict residual concentrations of free nanoparticles from release rates, and implement such models in the current EU guidelines for environmental exposure assessment of nanomaterials.
One of the unknown processess relevant to the environmental exposure assessment of nanomaterials is the extent/rate of dissolution of nanomaterials in water. It is unlikely that the standard OECD methods for measuring solubility of nanomaterials in water can provide the required information and it is recommended to revise these methods to accommodate the measurement of the rate of dissolution of nanomaterials in the natural environment.
Most urgently needed are analytical methods to detect and measure ambient concentrations of free nanomaterials. Currently, no standard methods exist for this purpose, although efforts are being developed in this area and environmental exposure levels are still unknown.
8.5 What needs investigation to improve identification of environmental hazards?
Studies on soil systems and terrestrial species in general, including primary producers are still lacking. There is also a general paucity of studies on marine species.
Further work on the establishment of standard protocols is required. The use of mechanical or chemical means to suspend nanomaterials may lead to changes in the physical-chemical properties of the test material. It is unclear what the extent of these may be and how they may impact any effects observed.
Arguably, dispersants/surfactants/solvents may need to be used in certain situations; however, it is important that they must not add to the toxicity of studied materials. It is suggested that results of studies where THF was used should be treated with caution. The same caution may apply to other dispersants for which there is lack of knowledge regarding their interaction with the test material (e.g. SDS). Further work with humic and fulvic acids, as well as widely used detergents (which are likely to be encountered in the environment) should be undertaken.
Related to this topic is the use in hazard assessment of ready-made (off-the-shelf) suspensions of nanomaterials. It is not clear how the dispersants used in the preparation of the nanomaterials might interact with the test material, and what effects they may have on the properties (and thus behaviour) of the test material (as described above). Thus any reported effects might not be comparable with effects observed on exposures of the same species to the same component material but which is in a different form (i.e. solid and suspended nanomaterials in the laboratory vs nanomaterials obtained as a suspension).
Regarding experimental design and approach, characterisation of exposures, via appropriate method(s) should be carried out and chemical analyses undertaken, as possible. The assessment of the solubility of the nanomaterials being studied is very important in this context so that any observed effects can be attributed to the different fractions. This is particularly important in the case of certain metal nanomaterials, as well as in the case of CNTs and quantum dots.
The importance of assessing contamination of the nanomaterials has been highlighted. The comparison of effects between nano and equivalent, larger, material needs to be undertaken. This has not been consistently incorporated in the published studies and would allow the correct attribution of effects. There is lack of information regarding the fate and form of the test nanomaterials within biological systems following in vivo exposures. It is unclear what particular form (e.g. soluble or particulate) is preferentially taken up into tissues and cells. It is likely that this would depend on the material composition; nevertheless these studies are not routinely carried out.
Studies should be conducted on a range of guilds and endpoints, with fate within the body and tissues assessed and depuration quantified, as possible. Micro/mesocosms studies should be undertaken. Furthermore, dietary studies, the role of nanomaterials’ coatings in uptake and translocation within the body, should be conducted, as well as the assessment of the role, if any, of their interaction with other environmental contaminants.
In this context it is crucial to ascertain the fate of nanomaterials in the environment so that their availability for environmental exposure can be assessed. Environmental fate and load assessment of nanomaterials must, therefore, be undertaken. The use of the current approach to the derivation of Kow in the assessment of environmental fate is unlikely to be beneficial to risk assessment. Nevertheless, the derivation of alternately approaches may be useful and may allow the development of appropriate predictive modelling. Finally, further information on the degradability (bio and abiotic) of nanomaterials should be derived.