3. How well can exposure to nanomaterials be measured?
One of the major routes of exposure for humans is considered to be inhalation, for which a lot of information is available including exposure measurements. Exposure data are available for non-manufactured nanoparticles (often referred to as ultrafines) from combustion processes but these data are not specific to manufactured nanoparticles. Nevertheless, the knowledge gained from studies of combustion products may make extrapolation possible and allow tentative conclusions to be drawn for nanoparticle atmospheric transport and exposure in humans. In contrast to the situation for other exposure routes, for airborne nanomaterials, analytical instruments are generally available to determine exposure (size distribution of mass and number). This is particularly true in the context of test atmospheres. However, it remains difficult to differentiate background from incidental exposure in real life situations as these methods mainly measure the presence of (ultrafine) particles and do not discriminate between the different types of particles.
Exposure of humans and ecosystems may occur via the gas-, water-, and solid phases. The latter may include food and consumer products such as cosmetics. The uptake route, dose, and group of humans exposed must be differentiated in addition to the exposure matrix. For exposure measurements, three different groups are generally distinguished, namely workers, consumers, and the general public. In the case of workers, inhalation is generally the main route of exposure. In addition, consumers and the general public are increasingly exposed to nanomaterials in various consumer products via oral and dermal routes.
It should be noted that part of the nanomaterials taken up by inhalation will result in a gastro-intestinal uptake due to the mucociliary mechanism present in the lung for particle removal. One key point, currently very often neglected in exposure and health effects studies, is the determination of the dose which can vary significantly. Taking airborne exposure as an example, exposure to manufactured particles with a median diameter of 90 nm leads to an overall internal dose of about 30-40% of the exposure value while the same value for 20 nm particles increases to 70-80% (according to the ICRP-model for a healthy worker) (ICRP 1994).
Figure 1 summarises the different measurement techniques and approaches possible for the assessment of exposure. It also gives the outline to measurement strategies since it differentiates personal and spatial (fixed point monitors) as well as continuous and discontinuous measurements. The limitations of measurement techniques directly influence measurement strategies. Generally, quite a few measurement techniques are available to assess nanoparticles exposure including mass and number based techniques, single particle chemical analysis online/offline techniques etc. (Kuhlbusch et al. 2008a). The main handicap for making good exposure assessments is the lack of instrumentation to determine personal exposure that can continuously analyse single particles or their agglomerates/aggregates for chemical and physical properties relevant for health.
Figure 1: Exposure related measurements (adapted from Borm et al. 2006)
Only a few papers have been published on measurement strategies which are currently necessary to allow first exposure assessments towards manufactured nanomaterials (Brouwer et al. 2004, Kuhlbusch et al. 2008b).
Currently most research and measurements have been conducted to assess the exposure of workers via inhalation. Data on airborne exposure are still scarce and do not always clearly differentiate ambient from manufactured particles (Fujitani et al. 2008,; Kuhlbusch et al. 2004, Kuhlbusch and Fissan, 2006, Kuhlbusch et al. 2008a, Kuhlbusch et al. 2008b, Maynard et al. 2004,Tsai et al. 2008, Wake et al. 2002, Yeganeh et al. 2008). In most cases it was seen that agglomerates of nanoparticles with diameters >400 nm are released during handling. In one case (Yeganeh et al. 2008), significant increases of sub-100 nm particle number concentrations during the handling of carbonaceous nanomaterials were reported. The latter indicates that coordinated measurement campaigns in various work areas are still needed to derive a comprehensive overview.
No quantitative or qualitative measurements of manufactured nanomaterials in ambient air outside of workplaces are known. Investigations by Murr (Murr et al. 2004, Murr and Guerrero 2006) revealed that carbon nanotubes may originate from general combustion processes and can be found in ambient locations. This illustrates the difficulty of identifying airborne manufactured nanomaterials.
Overall, the information base for exposure assessment in workplaces is currently built on a limited database which has to be improved in volume, comparability and reproducibility. This can be achieved by working on the feasibility of routine assessments, developing reliable measurement techniques, standardising measurement techniques, developing measurement strategies and implementing the screening and monitoring of nanoscale particles in sensitive work areas. Challenges are currently seen especially in the detection and assessment of product nanoparticles in the environment.
In addition to particle size and number, other metrics can be determined to express exposure. These include particle surface area, surface charge (zeta potential), surface area reactivity (radical formation, photo-catalysis, oxidation/reduction) etc. The choice of dose metrics depends on the endpoint of interest.
Exposure estimates from food and consumer products remains difficult. Information on the presence of manufactured nanomaterials solely relies on information (claims) provided by manufacturers. In addition, exposure estimation is also hampered by lack of information on product use and use of multiple products containing manufactured nanomaterials. In a similar fashion to air measurements, determination of manufactured nanomaterials in consumer products suffers from the difficulty of discrimination between background and intentionally added manufactured nanomaterials. Coordinated efforts and research strategies for a comprehensive exposure assessment of manufactured nanomaterials still have to be defined.