Exposure assessment of nanomaterials
Developing and commercialising products and technologies utilising nanomaterials needs risk management, with an understanding and evaluation of the potential exposure to novel materials throughout their lifecycle. Exposure assessment of nanomaterials in the occupational, consumer and environmental settings are active and challenging areas of scientific research. Routes of exposure are diverse, with exposure to nanomaterials via the inhalation route being the most likely route and subsequently the most studied, particularly in occupational settings. Although not discussed in this in the know article, other routes of exposure, such as ingestion, are also actively being studied and use similar approaches to assess nanomaterials in media such as solids and liquids.
The challenges related to nanomaterial exposure assessment can be classified under the following themes, as reviewed by Brouwer et al. (2012):
- Instrument limitations
- Background particles
- Spatial and temporal profile of the aerosol
Exposure assessment is important for risk assessment as it is used as a proxy for the potential dose an organism could receive. However, there are many uncertainties concerning the best parameter to describe the dose of nanomaterials. Particle mass was the original metric used for the exposure assessment of coarse materials. However, this is not necessarily applicable or sensitive enough for nanomaterials due to their very small size i.e. for an equivalent mass there would be considerably more nano-sized particles present compared to the larger coarse-sized particles. It is increasingly evident that physico-chemical characteristics (e.g. shape, surface reactivity, solubility) can play a major role in the toxicity of nanomaterials and the debate continues about the most relevant metrics to describe the “biological active dose” of nanomaterials. Toxicity is an important driver of the need for exposure assessment and will therefore have a continuing influence with regards to the most relevant exposure metrics to measure (Brouwer et al. 2012).
Accepting that information on the shape, size distribution, particles number and surface area is valuable to exposure assessment, the current instruments available present limitations. Firstly, not all information can be collected at once and by a single instrument, with instruments often utilising different physical principles to give either absolute, relative or computed measurements. Multiple instruments may need to be used in combination in order to gain comprehensive insight of an aerosol’s characteristics. Sampling onto suitable filter media for off-line chemical, morphological and/or gravimetric analysis in the laboratory complements direct reading instrumental data, but can have some limitations in assessing the dynamic or transient nature of emissions of nanomaterials. Whilst uncertainty and unresolved issues regarding the measurement of the most relevant exposure metric remain (e.g. sensitivity, method of detection etc), it is recognised that an exposure assessment strategy based on multiple detection systems and complementary methods can build a good profile of a given aerosol to assess the potential for exposure and inform the selection and evaluation of control measures (Kuhlbusch et al. 2011 and BSI 2010).
When studying the potential release of nanomaterials during their handling and use, it is important to understand and characterise the contribution from the ‘background’. Background airborne particles can arise from several sources including the natural environment, combustion sources, and incidental release from, for example, electrical motors and hot metallic surfaces. It is crucial to characterise this background and its influence on the measurements being gathered to assess the exposure(s) of interest. Strategies developed to address the background have been described by Kuhlbusch et al. (2011) and include time series, spatial, and comparative approaches based on studies with and without nanomaterials, and chemical and/or morphological analysis. Simulated approaches can also be undertaken using a “clean room” in order to study a specific process under controlled conditions. This simulation, although artificial, can give valuable information which can help the interpretation of complex data gathered from workplace measurements.
Spatial and temporal profile of the aerosol
The concentration and nature of aerosols can vary over time and also location. In close proximity to a source of emissions, the high concentration of particles emitted can result in the formation of agglomerates and/or aggregates, the stability of which can be complex and may be influenced over time by a variety of environmental factors and the physico-chemical properties of the particles. This behaviour is important to consider for exposure assessment where the spatial location of instruments and personal monitors may influence/bias the nature of the aerosol being measured or sampled (Kuhlbusch et al. 2011). Therefore, multiple measurement campaigns are recommended as good practice, as well as recording the ventilator set up of the room, temperature and humidity which might influence the characteristics of exposure (Kuhlbusch et al., 2011).
Strategies to overcome these challenges...
At the moment, although there is not yet consensus in terms of nanomaterial release assessment, there is a general recommended scheme as reported in Kuhlbusch et al. (2011):
- Assessing the background particle concentration and reducing it to allow sensitive subsequent measurements and analysis;
- Process in real-workplace or process simulation;
- Nanomaterial release and aeroslisation;
- Aerosol studies using multiple instruments and sampling (multi-metric study);
- Aerosol/particle characterization: on line and off-line analysis.
A practical strategy for exposure assessment of nanomaterial has been proposed as a decision tree, in a tiered approach, by the German industry (IUTA et al. 2011) and NIOSH (Methner et al. 2010). A "Guide to assessing airborne exposure in occupational settings relevant to nanomaterials" was also published by the British Standards Institution (BSI, 2010). Moreover, further need for harmonization in terms of protocols, reference materials as well as data reporting and interpretations and the development of nanomaterial databases have been highlighted by Brouwer et al. (2012).
Exposure assessment of nanomaterials is an invaluable part of the risk assessment process and despite the technical limitations and uncertainties, a multi-faceted approach involving instrumental measurements, sampling and contextual observations can provide a robust exposure analysis to inform risk management and support the safe use of nanomaterials.
Brouwer D, Berges M, Virji MA, Fransman W, Bello D, Hodson L, Gabriel S, Tielemans E. 2012, “Harmonization of measurement strategies for exposure to manufactured nano-objects; report of a workshop,” Ann Occup Hyg., vol. 56, no. 1, pp. 1-9.