7. How can exposure to nanoparticles be measured?
- 7.1 How can nanoparticles be detected and their specific properties measured?
- 7.2 How can exposure to airborne nanoparticles be assessed?
- 7.3 In what ways are humans exposed to nanoparticles?
7.1 How can nanoparticles be detected and their specific properties measured?
The SCENIHR opinion states:
The detection of nanoparticles and the measurement of specific properties associated with them are necessary for two distinctly different reasons. First, methods are required that reliably detect nanoparticles and measure their physico-chemical properties in the media in which humans and ecosystems are exposed to them, such as air, water, soil, consumer products and nanocomposites. Methods must also be available that support the studies to assess the risk of nanoparticles, such as with toxicological and ecotoxicological studies. Additional tools are required in this case, which are able to detect nanoparticles in the relevant medium, including cells, fluids and plant tissue.
The second issue relates to the physical or chemical properties associated with nanoparticles that are the basis of the detection of nanoparticles in these media. The range of properties of nanoparticles of potential relevance to risk assessment highlights the principal needs for extremely sensitive methods. The typical dimensions of nanoparticles are below the diffraction limit of visible light, so that they are out of range for optical microscopy. In low concentration liquids and gases however, single chromophore detection is possible and in Scanning Near Field Optical Microscopy (SNOM) sub-wavelength feature can be analysed. While the chemical composition of nanoparticles might be accessible by classic analytic methods for macroscopic amounts of nanoparticulate material, chemical analysis of individual nanoparticles in a dilute environment was for a long time impossible due to their low mass, and only recently have methods become available for this purpose, so that even surface coatings may be detected.
3.6.1 In situ and on-line detection principles for nanoparticles in gas suspension
Driven by the recent developments in atmospheric chemistry and physics, which have highlighted the role of nanoparticles in areas such as climate and health research, the measurement technologies for atmospheric aerosols offers a suite of tools specialized to the nanometer size range. An HSE Report (2004) and Luther (2004) have summarized the various types of device which might be or have been used to provide measurement information on nanometre size aerosols. Due to the lack of significant scattering or absorption by particles in the nanometre size range, particles are counted in commercially available, condensation nucleus counters (CPC or CNC), in which the particles are activated to droplets in a supersaturated atmosphere of alcohol, which can then be detected optically. Currently available instruments can detect particles as small as 3 nm, while new developments may reach the 1 nm limit (Kim et al 2003). CPCs cover a large dynamic range from a few up to 106 particles.cm-3.
A relatively simple technique involves charging particles by ion attachment and subsequent trapping of particles in a filter within a Faraday cup, which is connected to a sensitive electrometer. Although this method provides a signal which needs to be calibrated, it does give a sensitive proxy of aerosol surface area, under conditions of substantial aerosol load. This is similar to the epiphaniometer which relies on attachment and detection of a radioactive lead isotope rather than an ion, and is thus sensitive to rather low concentrations of particles.
While these instruments are not themselves size selective, they can be coupled to size selecting instruments, such as the commercially available differential mobility analyzer (DMA), which specifically covers the low nanometre size range. It discriminates charged particles with respect to their drift velocity under the action of an electric field. The combination of a DMA and CPC is often referred to as a scanning mobility particle sizer (SMPS). Low pressure impactors, where particles are separated by inertial impaction, can easily separate and count nanoparticles from larger particles, but commercially available systems do not provide a size resolution down to the nanometre range. An important advantage of the impactor systems is that aerosols with nanometre sizes can be collected for further analysis.
The recent developments of aerosol mass spectrometry, in which particles are vaporized and the resulting ions analyzed in a mass spectrometer have provided new alternative procedures. Depending on sampling inlet configuration, size separation method, vaporization method and type of mass spectrometer coupled to it, very specific, size resolved chemical composition of nanoparticles in gas suspension can be obtained.
3.6.2 In situ and on-line detection of particles in a liquid medium
Direct detection of nanoparticles in liquid media faces similar physical obstacles as in the gas phase. The most successful approach into the nanometre range involves measuring the size dependent Brownian motion of an ensemble of particles through the change of interference patterns with time. Commercially available instruments reach a lower detection limit of 3 nm. Other important techniques include optical chromophore counting, resonant light scattering and Raman scattering techniques, as well as the microscopic analysis of precipitates and cross section cuts. Highly sensitive techniques within electrochemistry and mass spectrometry and Rutherford backscattering have been used to identify compounds which have been brought into tissue in the form of particles (Penn et al 2003).
Most of these techniques are off-line and involve complicated sampling / sample preparation techniques. Using fluorescent molecules, quantum dots or magnetic nanoparticles as tracers, it is possible to count low concentrations of particles online. However, the choice of particles and chromophores puts further restrictions on the system studied.
Scanning Electron Microscopy (SEM) is the method of choice to investigate particle size shape and structure. When equipped with an Electron Dispersive Spectrometer (EDS) chemical composition can be determined, at least for larger particles and refractory components. The X-ray microanalysis system is not always suitable for chemical analysis because identification of substances can only be performed on the elemental level and cannot be quantified. Only solid, very high vapour pressure particles can be analysed due to the high vacuum of the system required for X-ray microanalysis.
The resolution of SEM and the related techniques has progressed below 10nm due to the implementation of cold electron sources in recent instruments. SEM resolution has been improved in Scanning Transmission Electron Microscopy (STEM) or High Resolution Transmission Electron Microscopy (HRTEM) techniques, which can again be combined to very powerful analytical techniques using electron probes and x-ray analysis.
Source & ©: SCENIHR
3.6 The Detection and Measurement of Nanoparticles, p. 19
7.2 How can exposure to airborne nanoparticles be assessed?
The SCENIHR opinion states:
3.9 Exposure Scenarios
Most human individuals are routinely exposed to particles in the ambient atmosphere, primarily from diesel fumes. Any combustion process produces nanoparticles in vast numbers from condensation of gases. Initially only about 10 nm in diameter, these rapidly coalesce to produce somewhat larger aggregates of up to about 100 nm, which may remain in the air for days or weeks. The air in a normal room can contain 10,000 to 20,000 nanoparticles.cm-3, whilst these figures can reach 50,000 nanoparticles.cm-3 in a wood and 100,000 nanoparticles.cm-3 in urban streets. Although the mass concentration of nanoparticles is low, it still amounts to substantial numbers. These concentrations imply that every hour, individuals breath millions of nanoparticles, and it is estimated that at least half of these reach the alveoli. At present it is not known to what extent the engineered nanoparticles contribute to these numbers.
The current best practice for measuring the exposure of an individual to a material present as an aerosol is to use a personal sampling device. Samples collected are subsequently assessed either gravimetrically or via chemical analysis to determine the mass and provide an estimate of time weighted mass concentration. Currently pollution standards are mass based; the dose by number or area will increase as size decreases. (Kittelson 1998). However for nanometre size aerosols, measurement of mass is not sufficient. Oberdörster et al (2005) showed the tremendous differences in number concentrations and surface areas for particles. (Table 1). The extraordinarily high number concentrations of nanoparticles per given mass may be of toxicological significance when these particles interact with cells and subcellular components. Likewise, their increased surface area per unit mass can be toxicologically important if other characteristics, such as surface chemistry and bulk chemistry, are the same.
In general, the use of mass concentration data alone is insufficient for the expression of dose, and the number concentration and / or surface area need to be included. Serita et al (1999) exposed rats to metallic nickel nanoparticles at around the Japanese occupational exposure level (OEL). This OEL was based on particles larger than the nanoscale, but exposure to concentrations around the OEL (1.4 mg.m-3) in the form of nanoparticles caused severe lung injury after a single exposure. This finding supports the concept that surface area is the dose measure that predicts pulmonary response, rather than mass, and this has far reaching potential consequences for occupational standards that are based on mass. Therefore the ideal sampler to measure biologically relevant exposure to nanoparticle aerosols would be a personal sampling device which collects the relevant size fraction and provides either an instantaneous measure of a sample surface area or which facilitates the off-line analysis of the sample to provide a measure of surface area.
Sampling of nanoparticles is a challenging task for several reasons. First, the sampling strategy should ensure that the particle collection methods, including location, represent as accurately as possible the real exposure at the site in question and methods should be developed and chosen according to the size and nature of the particles under investigation. Secondly, because of their small mass, separation of nanoparticles from larger particles by inertial impaction can only be achieved at a relatively high pressure drop. Thirdly, considering that typical ambient atmosphere nanoparticle concentrations are less than 1 µg.m-3, collection of filter samples for gravimetric analysis and chemical characterization is only feasible with certain high volume sampling techniques (Sarnat et al 2003). In addition, the discrimination between existing ambient particles and engineered nanoparticles is an important factor in the sampling strategy.
Analysis of relatively large particles in a scanning electron microscope requires relatively little preparation of the samples. At the opposite end of the spectrum, nanometre-diameter particles to be analysed in the transmission electron microscope or scanning transmission electron microscope must be presented without contaminants on a suitably thin electron-transparent support. Inertial collection methods such as gravitational settling and centrifugal collection, are suitable only for particles greater than 1-10 µm in diameter, and are impractical to implement for nanoparticles.
Inertial deposition in impactors is achieved by increasing particle momentum in a high velocity air flow, enabling deposition onto a substrate by rapidly changing the flow direction. Use of low pressure stages in cascade impactors allows the collection of particles as small as 50 nm in devices such as the electrical low pressure impactor. Recent developments in nozzle design have led to hypersonic impactors capable of collecting particles down to 50 nm, and focusing impactors capable in principle of operating below 10 nm. However, deposition forces are necessarily high, leading to the possibility of particle damage. Aerosol samples collected by impaction are generally restricted to a small region of the substrate, thus increasing the probability of particle coincidence, and may be non-uniform with respect to particle size.
Electrostatic deposition allows relatively high deposition velocities, especially at high particle charge-to-mass ratios. Where particles are unlikely to be damaged by the charging mechanism used or the electric fields encountered, relatively gentle and uniform deposition is possible. If particles are charged to their theoretical limit, electrostatic deposition velocities are relatively independent of particle size. However, this limit is difficult to achieve under practical sampling conditions. Under conditions where positive and negative ions may freely attach to aerosol particles, a charge equilibrium is reached that is highly size dependent. The fraction of nanoparticles having a minimum of one charge drops off rapidly with decreasing size, leading to a dramatic fall in deposition velocity. Diffusional or photoelectric charging can be used to increase the average particle charge at small diameters, and electrostatic precipitation can be used effectively for particles larger than 20 nm in diameter.
Below 10-20 nm, diffusion begins to dominate other deposition mechanisms. For particles smaller than 10 nm diffusion is ideally suited to obtaining uniform particle deposits on a range of sampler substrates, although samples may be highly biased towards smaller particles, and are unlikely to contain a significant fraction of particles larger than 20-30 nm. Thermophoresis, the movement of aerosol particles in the presence of a temperature gradient, has the advantage that for a given particle composition, deposition velocity is constant below a size of around 100 nm. Achievable deposition velocities are relatively low, but deposition is gentle and unlikely to influence the physical nature of the particles. Implementation of thermophoresis in a uniform temperature gradient between two horizontal surfaces has enabled uniform deposits of discrete particles from below 5 nm to nearly 1 µm directly on to transmission electron microscope support grids (Maynard 2000).
Source & ©: SCENIHR
3.9 Exposure Scenarios, p. 34
There is no clear opinion on which parameter(s) should be measured as a most appropriate measure of assessing exposure (mass/number/surface area). There is inadequate portable instrumentation for nanoparticles exposure available. New sampling techniques and strategies for exposure assessment at workplace and environment should be elaborated. The possibility of establishing of Occupational Exposure Limits for chemicals in the form of nanoparticles should be considered.
Source & ©: SCENIHR
3.9.3 Conclusions, p.41
7.3 In what ways are humans exposed to nanoparticles?
The SCENIHR opinion states:
3.9.2 Exposure Assessment Approaches
The following section is based on human health experiments and observations although some observations may also be relevant for other species.
The respiratory tract acts as a serial filter system and in each of its compartments (nose, larynx, airways, and alveoli) the predominance of characteristic physical mechanisms of particle deposition may change. In addition, these mechanisms significantly change with particle size. Nanoparticles are primarily displaced by Brownian motion and therefore underlie diffusive transport and deposition mechanisms. In practice it means that the smaller the particle the higher is the probability of a particle to reach the epithelium of the lung.
Motor vehicle emissions usually constitute the most significant source of nanoparticles in an urban environment. The relationship to traffic volumes indicates that the accumulation mode particles are associated with emissions from heavy-duty traffic (mainly diesel vehicles) whilst particles in the range 30–60nm show a stronger association with light duty traffic. Both of these size fractions show the anticipated dilution effect with increasing wind speed (Charron and Harrison 2003).
Combustion of fossil fuels, especially in diesel engines, produces waste by-products, including nanoparticles. Today, these combustion waste nanosized particles constitute the most important source of anthropogenic nanoparticles.
Exposure, uptake, distribution and degradation of nanoparticles from the environment have been recently discussed by Oberdörster G et al. (2005). They believe that nanomaterials are likely to enter the environment for several reasons. With nanomaterials now being manufactured in large quantities, it is argued that manufacturing effluent and spillage, use and disposal through landfill, will inevitably result in environmental exposure. Moreover these materials are being used in personal- care products such as cosmetics and sunscreens, which can enter the environment on a continual basis from washing off of consumer products. However, it should be said that currently very little is known about the behaviour of nanoparticles in the environment. One study has shown that iron nanoparticles can travel within ground water over a distance of 20 m and remain reactive for 4-8 weeks (Zhang 2003).
Based on the systematic study by the Institute of Occupational Medicine for the UK Health and Safety Executive it may be assumed that there are a few main industrial activities in which exposure to nanoparticles may occur (HSE 2004):
- Nanotechnology sector, primary research development (universities and other research groups and spin-offs);
- Existing chemical and pharmaceutical companies;
- Powder handling processes including paints, pigments and cement manufacture;
- Other processes where the nanoparticles are by-products.
The potential risks following occupational exposure were also discussed in the same report, summarised in Table 2 (HSE 2004)
Also in the same report (HSE, 2004) it was estimated that the number of workers in the UK who may be exposed to manufactures nanoparticles in the work environment in the university sector and in emerging nanoparticle companies may be as high as 2,000. Around 100,000 individuals may potentially be exposed to fine powders through various powder handling processes. It is not possible to say what proportion of these may be exposed to nanoparticles. More than 1,000,000 workers in the UK may be exposed to nanoparticles via incidental production in processes such as welding and refining (Aitken et al 2004). In the U.S., an estimated 2 million people work with nanometre-diameter particles on a regular basis in the development, production, and use of nanomaterials or products, this being based on national industry-specific occupational employment estimates by the U.S. Department of Labor’s Bureau of Labor Statistics for the year 2000. If growth in nanotechnology-related industries meets expectations, a similar number of additional workers will be required globally (NIOSH 2005). It should be emphasized that although some industrial processes have involved nanoscale compounds (e.g. carbon black, welding, etc) for decades, occupational exposure data, including size and mass of the particles, is very scarce.
The aim of one major workplace study (BIA 2003) was to gather and catalogue technical measurement information on nanoparticles occurring at different work processes, where those nanoparticles had been released occasionally as by-products of technical processes. Typical examples include welding fumes, metal fumes, soldering fumes, plasma cutting fumes, plasma spraying emissions, polymer fumes, vulcanisation fumes, amorphous silicic acids, powder coating emissions, oil mists, aircraft engine emissions, bakery oven emissions, meat smokery fumes, and particulate diesel motor emissions. The particles were for the most part the products of condensation in thermal and chemical reactions, the primary particles created having a size of only a few nanometres. The most frequently-occurring particle size was between 160 and 300 nm. The total concentration of all particles in the measurement range 14 to 673 nm was between 500,000 and 2,500,000 particles per cm³. A comparison of the occurrence of nanoparticles in different workplace atmospheres is given Table 3 after Möhlmann (2004).
With respect to carbon nanotubes, Maynard et al (2004) carried out a laboratory based study, then complemented by a field study, in which airborne and dermal exposure to single-walled carbon nanotube material (SWCNT) was measured in 4 sampling sites where workers handled unrefined material. Estimates of nanotube concentrations ranged from 0.7 to 53 µg.m-3. Filter samples indicated that many of the particles may have been compact, rather than having an open, low density structure more generally associated with unprocessed SWCNT. Glove deposits were estimated at between 0.2 and 6 mg per hand.
Exposure to carbon black has been a major concern for decades. Furnace black account for 98% of the worldwide production and has an average aggregate diameter of 80-500 nm and an average primary particle diameter of 17-70 nm. (IARC 1996). The exposure to carbon black dust has been measured in two phases of a large multi-national study (Gardiner et al 1996). The highest mean exposure was experienced by the warehouse packers and they are also most likely to exceed the OES 3.5 mg.m-3 The range of means for 14 job titles varied from 0.3 to 10.4 mg.m-3. In another study, exposure to inhalable dust in carbon black manufacturing industry was measured during three sampling periods (from 1987 to 1995) in several European countries. Prior to the exposure measurements, all workers were categorized into 14 job titles, which were amalgamated into eight job categories. Average inhalable dust exposure (directly calculated using the exposure data, dropped from 1.3 mg.m-3 in Phase I (1987-89) to 0.8 mg.m-3 in Phase II (1991-1992) and 0.7 mg.m-3 in Phase III, 1994-95, (van Tongeren et al 2000).
There is very little data in the literature on potential exposure through the skin, even though nanomaterials have been used in cosmetics and pharmaceuticals for many years. Currently, most of the dermal exposure concerns skin preparations that use nanoparticles. A recent review of dermal exposure issues concluded that there was no evidence to indicate specific health problems are currently arising form dermal penetration of nanoparticles (HSE 2004).
In theory, harmful effects arising from skin exposure may either occur locally within the skin or alternatively the substance may be absorbed through the skin and disseminate via the bloodstream, possibly causing systemic effects, although there is no evidence of this as yet. Most studies concerning penetration of nanoparticles into the skin have focused on whether or not drugs penetrate through the skin using different formulations containing chemicals and/or particulate material as a vehicle. The main types of particulate materials commonly used are liposomes, poorly soluble solid materials such as TiO2, ZnO2 and polymer particulates and submicron emulsion particles such as solid lipid nanoparticles.
There is only limited data on the fate of nanoparticles of titanium dioxide when used in sunscreens and other products on the skin and it appears unlikely that this does not penetrate beyond the dermis. The investigations of Schulz et al. using optical and electron microscopy proved that neither surfacecharacteristics, particle size nor shape of the micronised pigments result in any dermal absorption of this substance. Micronised titanium dioxide is solely deposited on the outermost surface of the stratum corneum and has not been detected by light and electron microscopy in deeper stratum corneum layers, the human epidermis and dermis (Schulz et al 2002).
It was already recognized in 1926 (by Kumagai cited by Salata 2004) that particles could translocate from the lumen of the intestinal tract via aggregations of intestinal lymphatic tissue (Peyer’s patches), containing M cells. It is now known that uptake of inert particles can occur not only through immune cells present in Peyers’ patches but also through enterocytes, and to lesser extent across para-cellular pathways (Aprahamian et al 1987). However, once again data in the literature on potential exposure through the GI tract are very scarce.
Szenkuti (1997) observed that cationic nanometre-sized latex particles became entrapped in the negatively charged mucus, whereas repulsive carboxylated fluorescent latex nanoparticles were able to diffuse across this layer. The smaller the particle diameter the faster they could permeate the mucus to reach the colonic enterocytes; 14 nm diameter permeated within 2 min, 415 nm particles took 30 min, while 1000 nm particles were unable to cross this barrier.
After oral gavage for several days, a sparse accumulation of charged latex particulates in the lamina propria was found compared to uncharged latex nanoparticles in the same size range (Jani et al 1989). The same authors (Jani at el 1990) investigated the body distribution after translocation of polystyrene particles ranging from 50 nm to 3000 nm. Rats were fed by gavage daily for 10 days at a dose of 1,25 mg.kg-1. It was found that as much as 34% and 26% of the 50 nm and 100 nm particles were absorbed, respectively. Those larger than 300 nm were absent from the blood. No particles were detected in heart or lung tissue.
Source & ©: SCENIHR
3.9.2 Exposure Assessment Approaches, p. 37