Current guidance for the exposure assessment of nanomaterials
Assessment of exposure is a key element in relation to understanding the potential risks of nanomaterials. There are a number of reasons why it may be useful to assess exposure:
- Identification of emission sources;
- Evaluation of the performance of a control approach;
- Compliance with an exposure limit;
- Development of an exposure scenario as a requirement of the REACH Regulations;
- Quantification of exposure as part of the calculation of risk;
- Quantification of personal exposures in a working population for an epidemiology study.
Each of these needs may require a different study design (e.g. choice of measurand, instrument selection and location, number and duration of samples, analytical sensitivity and method). Provided below is an overview of the key issues and a strategy for assessing exposure to nanomaterials, primarily occupational exposure by inhalation, followed by links to some key guidance documents.
For nanomaterials, assessment of the exposure of workers presents a number of significant challenges. These have been widely discussed in the literature, and are outlined in brief below.
The first issue concerns selection of the most appropriate metric to assess exposure. For other substances, it has been the case that assessment of exposure has been in terms of the mass concentration in the air, usually expressed as an 8-hour time weighted average concentration (TWA) in units of mg/m3 or similar. Methods for assessment typically involve the assessment of the aerosol in terms of the “inhalable” or “respirable” fractions. For fibrous aerosols, such as asbestos, the general approach is to assess in terms of respirable fibre number concentrations in air. In relation to nanomaterials there has been significant discussion regarding the advisability of using surface area concentration as a measurement metric. At the current time there is no clear single advice that can be given as to the appropriate choice of metric. It is very likely that in terms of compliance with limit values, the requirement for mass concentration measurements will continue for the foreseeable future. However, additional relevant information about exposure to nanomaterials can also be gained from number and surface area concentration measurements. A range of instruments are available which can be used to measure these different metrics is available. Details are provided within the guidance documents outlined below.
Discrimination from background
One of the critical issues in relation to measurement of airborne nanomaterials is the presence of background aerosols. Normal ambient air, whether in the environment or indoors already contains a large number of particles in the nanometre size range.
These are primarily from industrial emissions or traffic generated or arising in the air from condensation processes. Typically in an urban setting, number concentrations in air of the order of 20,000 particles per cc can be found. In order to assess exposure to nanomaterials it is necessary to try to take account of the background level, which may or may not be stable in any given situation. Various approaches have been suggested to overcome this. One approach involve measurement of the background concentration either before and after an event in which it is considered that nanomaterials may be released into the air (e.g. before the start of a synthesis process and after it has been completed). An alternative approach is to try to make a parallel measurement of the concentration due to the process (sometimes called the near field) and of the background concentration where it is expected that there will be no contribution from the process (sometimes called the far field). The assumption in both cases is that that the background concentration will remain stable. In practice however this is often not the case and neither of these approaches is entirely satisfactory in all cases. However careful study design and interpretation can enable discrimination from the background in some circumstances.
A third issue relates to the assessment of personal exposure. In general, assessment of exposure in occupational settings requires the use of personal exposure measurements that are derived from instruments, which are worn by or carried by the subject for whom it is intended to assess exposure. However, most of the devices which have been used for measurement of nanomaterials are large scale, relatively static, not personal devices. This has made assessment of personal exposure problematic. In the literature, most of the published figures relate to measurement of workplace concentrations (from static instruments), rather than personal exposure. New devices are being developed which may in the relatively near future enable assessment of personal exposure for some types of nanomaterials for some metrics. Until then, the approach needs to be based on comparisons, where possible, between workplace concentrations and personal samples in order to try to establish some relationship between these in the given scenario.
Measurement of HARN
Measurement of High Aspect Ratio Nanomaterials (HARN) presents a particular problem at the current time. Examples of HARN include carbon nanotubes (CNT), graphene, nanoclays and other novel 2D platelets. In occupational assessment for other types of High Aspect Ratio Particles, such as asbestos, measurement is made according to a counting of numbers of fibres collected onto a sample filter so as to derive a fibre concentration in air. This approach is based on optical microscopy according to the published WHO method, "Determination of airborne fibre number concentrations. A recommended method, by phase-contrast optical microscopy (membrane filter method)
While this approach may be applicable to other fibrous aerosols, it has so far not been used to any extent to test or evaluate exposure to HARN. One of the main challenges is the consideration that aerosols of CNT (if such aerosols are produced) are likely to form highly aggregated clumped entities, which would not lend themselves to counting according to traditional WHO counting methods. While it is important that attempts should be made when using these materials to determine the nature (in terms of size and shape) of the particles which are released in the atmosphere, at this point it is not possible to provide clear guidance for the quantification of these concentrations according to fibre counting methods.
For CNT and measurement, a NIOSH report “Occupational exposure to carbon nanotubes and nanofibres
”, NIOSH 161-A (2010), suggests that until better methods are developed, the best approach is to attempt to measure the mass of CNT. The NIOSH Current Intelligent Bulletin 65 (2013) advocates the use of elemental carbon determination as a practical method for the measurement of carbon nanotubes (CNT) and carbon nanofibres (CNF) (and as a proxy for the detection of graphene and graphene-family nanomaterials) in the workplace with a recommend an exposure limit (REL) of 1 µg/m3 8-hour TWA when the predominant workplace exposure to elemental carbon (EC) material is CNT or CNF. Due to the many possible sources of organic carbon in workplace atmospheres, and where bulk CNTs and graphene family nanomaterials contain little organic carbon, elemental carbon is a better indication of the presence of graphene in the atmosphere than total carbon assessment.
In the published literature, there is now an emerging strategy in which a tiered approach is taken in relation to assessment of potential exposure to nanomaterials. Broadly this adopts the following steps.
1. Initial inspection of the workplace to determine potential sources of nanomaterials and any confounding sources (such as heaters) which may also generate atmospheric release of nanoparticles;
2. Assessment of the release of nanomaterials in to the air from these sources using relatively simple handheld particle counters such as CPCs which enable the assessment of particle number concentration with a very narrow time interval.
These two first steps may in themselves be sufficient to demonstrate the presence or otherwise of nanomaterials in the air (as a form of release), and may provide adequate information to adjust or adapt the particular control approach being used in order to try to minimise this exposure.
3. If release has been observed, and cannot be easily reduced by application of a control approach, then it may be necessary to further characterise the nature of this release. In this case a range of possibilities are available. One would include collection of the emissions on to a filter for subsequent analysis using electron microscopy techniques. The second would be assessment of the particle size distribution using instruments such as the Fast Mobility Particle Sizer (FMPS) and Aerodynamic Particle Sizer (APS). A further approach would be to collect samples for chemical analysis. This does not indicate a comprehensive listing of the possibilities at this point. It is intended to illustrate that by application of various techniques, a more complete picture of the nature of the emissions can be built up.
4. If such emissions have been characterised and cannot be controlled, then it may be necessary to carry out assessment of personal exposure. In practice this is likely to involve the collection and comparison of paired samples, one of which being a measurement of personal exposure (i.e. from a personal sampler worn by the worker in the workplace), with simultaneous measurements of the detailed nature of the emissions using the range of instruments (described in Step 3). Based on this tiered approach, a more comprehensive picture of the likely exposure can be developed. This kind of approach has been identified in a range of published studies, some of which are referred to in the guidance documents described below.
Although this section focuses on exposure by inhalation, other exposure routes such as dermal may also be important and should be minimised where possible. Methods for assessing dermal exposure such as wipe sampling or assessment of glove contamination have been used for assessing exposure to nanomaterials.
A range of guidance documents are available which provide information about the various measurement possibilities which are available, including instrument selection and the kind of strategies which could be adopted in order to quantify exposure.
Key guidance resources for the exposure assessment of nanomaterials are summarised below.
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