9. Conclusion – Are existing methodologies to assess the potential human health and environmental risks associated with products of nanotechnology appropriate?
The SCENIHR opinion states:
4. COMMITTEE OPINION
The following conclusions may be drawn from this analysis of the appropriateness of existing methodologies to assess the potential human health and environmental risks associated with engineered and adventitious products of nanotechnology.
It should be recognised that there is insufficient data available at the present time to allow the identification of any systematic rules that govern the toxicological characteristics of all products of nanotechnology. It follows therefore that risk assessment will need to be made on a case by case basis. In order to perform a risk assessment for the application of nanotechnologies, identification of the methodological issues requires consideration of both the exposure and the hazard.
In considering the potential of adverse health risks associated with nanotechnology products, two separate types of nanostructure may be identified, those where the structure itself is a free particle, and those where the nanostructure is an integral feature of a larger object. Although all nanostructures may interact with living systems in ways that may be influenced by the nanoscale characteristics, it is not considered that these nanoscale features of larger objects (for example nanotopographical features on medical devices) pose any additional human health and environmental risks. The situation with free nanoparticles, including agglomerates, is quite different. It is the generation, application, distribution, persistence and toxicological characteristics of free nanoparticles that give rise to concerns over possible human health and environmental risks. These concerns include the physical, chemical or biological degradation of nanocomposites, which potentially releases nanoparticles. For environmental risk analysis, these concerns imply the necessity for life cycle evaluation of these products.
Free nanoparticles may occur naturally, or be the unintentional products of an industrial or domestic process, or they may be specifically engineered for applications which depend on their unique properties. These properties will primarily be influenced by the high surface to volume ratio associated with nanoparticles and the quantum effects that occur in the nanometre range. Careful characterisation of the physico-chemical properties is essential, for which appropriate methodologies must become available for routine use.
Several different exposure scenarios can be identified with respect to nanoparticles. Nanoparticles of natural origin and those generated unintentionally by human activity ensure that all individuals are routinely exposed to nanoparticles throughout life. The principal route of human exposure is by inhalation in view of the presence of nanoparticles in air. The rapidly increasing use of manufactured nanoparticles in consumer products such as cosmetics, and pharmaceutical preparations and food technology implies that dermal, gastrointestinal, and parenteral routes of exposure are becoming more significant. For the environment, release and distribution of nanoparticles may occur through air, water and soil. As such, species living in all environmental compartments may be exposed to these particles. There is an urgent need for exposure data on humans (consumers and workers) and environmental species including micro-organisms.
The evaluation of exposure of individuals and the environment in general to nanoparticles, and therefore of the associated health risks, has been impeded by the difficulty of routine sampling, and of counting and measuring particles that are below the limit of detection by visible light. The use of mass concentration data alone for the expression of dose is insufficient, and number concentrations and surface area metrics are generally more relevant in exposure and risk assessment. This is not incorporated in current regulations. The development of methodologies and equipment that enables routine measurements in various media for representative exposure to free nanoparticles is an important priority.
In considering the hazards associated with nanoparticles, the size, shape and composition, including surface charge and adsorbed species, of the nanoparticles are important. The phenomena of surface modification, aggregation and dissolution or degradation are also significant. Since nanoparticles that are readily soluble in the physiological environment lose their particle specific effects, they only remain of concern if they dissolve into harmful molecules . For particles that are essentially insoluble, there is the possibility of biopersistence, resulting in long term exposure and associated nanoparticle-specific effects. So the characterisation of nanoparticles used in biological evaluations is essential.
There is little published data on the biological behaviour of nanoparticles, including the distribution, accumulation, metabolism and organ specific toxicity. Much of the data that is available concerns the respiratory system where there are experimental data to show that nanoparticles often exert greater toxic effects than larger particles of the same substance at the same mass concentration. Interactions of nanoparticles with biomolecules such as DNA, RNA or proteins are also more likely with decreasing particle size. Although no mechanisms unique to nanoparticles have yet been identified, a mechanism of toxicity for some nanoparticles is the induction of reactive oxygen species and the consequential oxidative stress experienced by cells.
Nanoparticle translocation can occur to a greater extent and to different sites than occurs with larger particles. There can therefore be a systemic distribution and accumulation of such particles. There is evidence that nanoparticles can translocate from their portal of entry and can reach other parts of the body, including the blood and the brain, although again very few studies have been performed and the extent and significance of this translocation is unclear. It is uncertain whether nanoparticles can reach the foetus. Obviously, in medical applications involving parenteral administration of nanoparticles, systemic distribution is probable. At this stage, the evidence of toxicity in man arising from such systemic exposure to manufactured nanoparticles is sparse. Current testing guidelines for the hazard identification and characterisation of chemicals and products do not yet require the identification of the systemic distribution of nanoparticles, although some potentially suitable methods do exist.
The safety evaluation of nanoparticles and nanostructures cannot rely solely on the toxicological profile of the equivalent bulk material. Nanomaterials need to be evaluated for their risk on a case by case basis for each preparartion including the intended use of the material. In carrying out the risk assessment for products of nanotechnology, new testing strategies will be required that will address the product specification, the intended use and the identification of potential exposure scenarios, both human and environmental. Conventional toxicity and ecotoxicity tests have already been shown to be useful in evaluating the hazards of nanoparticles. However, some methods may require modification and some new testing methods may also be needed. It appears that nanoparticles can exacerbate certain pre-existing medical conditions and may increase susceptibility to some diseases, which may require modification of testing strategies.
There are regulatory and risk management implications of the above analysis, for example, in toxicological testing guidelines, the setting of occupational and environmental quality standards, and in the classification and labelling of products.
International cooperation is needed to address the multiple issues in nanotechnology. Standardisation including the avalability of reference materials/particles is the key issue to come to a mutual understanding in what we are dealing with in terms of risk assessment for the use of nanotechnology.
NOTE The standardisation process for nanotechnology has already been initiated by organisations like ISO and CEN.
In relation to the specific questions asked of SCENIHR:
Are existing methodologies appropriate to assess potential and plausible risks associated with different kinds of nanotechnologies and processes associated with nanosized materials as well as the engineered and adventitious products of nanotechnologies?
Although the existing toxicological and ecotoxicological methods are appropriate to assess many of the hazards associated with the products and processes involving nanoparticles, they may not be sufficient to address all the hazards. Specifically, particular attention needs to be given to the mode of delivery of the nanoparticle to the test system to ensure that it reflects the relevant exposure scenarios. The assays may need to be supplemented by additional tests, or replaced by modified tests, as it cannot be assumed that current scientific knowledge has elucidated all the potential adverse effects of nanoparticles.
For exposure, the use of mass concentration data alone for the expression of dose is insufficient, and the number concentration and/or surface area need to be included. Equipment that enables routine measurements in various media for representative exposure to free nanoparticles is not yet available. The existing methods used for environmental exposure assessment are not necessarily appropriate for determing the distribution, partitioning and persistence of nanoparticles in the various environmental compartments.
Given the above uncertainties, the current risk assessment procedures require modification for nanoparticles.
If existing methodologies are not appropriate to assess the hypothetical and potential risks associated with certain kinds of nanotechnologies and their engineered and adventitious products, how should existing methodologies be adapted and/or completed?
Three different situations can be identified where existing methodologies are considered unsuitable:
- Routine methodologies have not yet been made available and / or have not been included in the testing guidance and/or achieved regulatory acceptance.
- Scientific research has identified a phenomenon to be evaluated and existing methodologies need to be adapted.
- Advances in nanotechnology may require additional methodological principles and developments.
Included in the areas of requirements for new or modified methodologies are:
- Appropriate methodologies must be made available for the routine and careful characterisation of the physico-chemical properties of nanoparticles.
- Methodologies and equipment need to be developed that enable routine measurements, in various media, of representative exposure to free nanoparticles.
- Although conventional toxicity and ecotoxicity tests have been shown to be useful in evaluating the hazards of nanoparticles, some methods may require modification and some new testing methods may also be needed in order to optimise this process of hazard evaluation, including the assessment of whether nanoparticles can exacerbate pre-existing medical conditions.
- In this context, although again some potentially suitable methods exist for the detection of nanoparticle translocation, these need to be developed further and incorporated into new testing strategies and guidelines for the assessment of the systemic distribution of nanoparticles.
More specifically the above mentioned methodologies need to provide information on how nanoparticles distribute in human tissues and in environmental compartments. This information can then be used in the exposure assessment algorithm provided in figure 6 in section 3.10.5 of this opinion.
In general terms, what are the major gaps in knowledge necessary to underpin risk assessment in the areas of concern?
In general, and in spite of a rapidly increasing number of scientific publications dealing with nanoscience and nanotechnology, there is insufficient knowledge and data concerning nanoparticle characterisation, their detection and measurement, the fate (and especially the persistence) of nanoparticles in humans and in the environment, and all aspects of toxicology and environmental toxicology related to nanoparticles, to allow for satisfactory risk assessments for humans and ecosystems to be performed.
The major gaps in knowledge that need to be filled in relation to improved risk assessment for the products of nanotechnology include:
- The characterisation of the mechanisms and kinetics of the release of nanoparticles from a very wide range of production processes, formulations and uses of the products of nanotechnology.
- The actual range of exposure levels to nanoparticles, both to man and to the environment.
- The extent to which it is possible to extrapolate from the toxicology of non-nano sized particles and other physical forms e.g. fibres of the same substance to the toxicology of nanosized materials, and between nanoparticles of different size ranges and shape.
- Toxicokinetic data following exposure, so that target organs can be identified and doses for hazard assessment determined. This includes dose response data for the target organs, and knowledge of the subcellular location of nanoparticles and their mechanistic effects at the cellular level.
- Information from the occupational exposure and associated health effects on workers involved in the manufacture and processing of nanoparticles.
- The fate, distribution and, persistence and bioaccumulation of nanoparticles in the environment and environmental species including micro-organisms.
- The effects of nanoparticles on various environmental species, in each of the environmental compartments and representative of different trophic levels and exposure routes.
5. MINORITY OPINION
Source & ©: SCENIHR
4. Committee Opinion, p. 58