The emergence of nanotechnology opened new challenges in occupational health and safety. Over the last 15 years scientists have paid attention to the development of new instruments capable of measuring personal exposure to nanosize particles. New frameworks, methodologies and standards to assess and control exposure to engineered nanomaterials (ENMs) in the workplace have been published. A large effort has also gone to establish grouping approaches according to how they exert their toxicity and towards the development of occupational exposure limits (OELs). This task has been difficult due to the high diversity of ENMs, the uncertainties on their mode of action and on selecting the metric that best represents the end point.
Since 2007 some institutions, like the UK British Standard Institute (BSI), US National Institute of Health and Safety (NIOSH), the German Institute of Occupational Safety and Health (IFA) and the Dutch government started to recommend pragmatic "benchmark exposure levels” to control workplace exposures to nanomaterials. Undoubtedly, this was a necessary measure to minimize exposure. However, these benchmark values were developed following different approaches (Mihalache et al. 20161) and with the exception of those proposed by NIOSH do not represent health-based occupational limit values. Health-based inhalation OEL are time-weighted average (typically 8-h) air concentrations believed to represent a safe level of exposure for most workers over their working lifetime. Benchmark values are cautious levels to prevent exposure but do not represent safe levels to which workers can be exposed without developing adverse health effects.
It has taken time, but some advances on this front have been published recently. Visser et al.2 published this year (2022) recommendations from an expert panel to establish health-based nano reference values differentiating six groups of ENMs.
Those that have attempted to characterize NMs at the workplace know well that most of the exposures to NMs are not due to ENMs.
Incidental nanomaterials can make up the largest proportion in most workplaces. Incidental NMs include: natural particles and traffic related air pollutants that have reached the work environment; materials handled at the workplace, since many have a size distribution that covers the micro and nanorange (titanium dioxide, carbon black, etc); as well as those that are generated during industrial processes, known as process generated NMs (PGNMs) like for example from combustion (e.g. diesel exhaust), heating (e.g. metal fumes generated during welding, polymer fumes from rubber manufacturing and 3-D printing), or particles released from mechanical processes (e.g. grinding, sawing, drilling).
Some examples of exposures measured in occupational settings are peak concentrations in the breathing zone of 2.5 × 105 particles cm-3 during friction stir welding (FSW) (Pfeffrkorn et al. 2010)3; 8-TWA ranging between 1.4 - 5.3 × 105; particles cm-3 during laser sintering of ceramic tiles (Fonseca et al. 2016)4. The concentration of these incidental nanomaterials in most cases exceeds that of the ENMs. And although most of them have a mass based Occupational Exposure Limit (OEL) there is not a legally binding OEL for the number of particles.
The direct reading instruments used to measure the number of particles only discriminate by size and not by chemical identity. The recommendations to differentiate between both include assessment of the background particle concentration. For example, by measuring before and after the ENMs are handled or measuring in a place not influenced by the handling of the ENMs. However, in industrial sites background concentrations are not stable and can vary for multiple reasons introducing a large uncertainty on the estimation of this background concentration and on any calculation where this background concentration is used.
In addition, a fraction of the nanomaterials is the result of heterogeneous agglomeration between background particles and ENMs, since the latter tend to agglomerate upon release. Therefore, background particles act as scavengers of ENMs. This can cause a drop in the number of particles measured, accompanied by an increase in their mean size.
Another example of a hybrid particle is during the manipulation of nanocomposites in mechanical processes where polymer particles that have embedded or protruding ENMs are released.
Thus, quantitatively discriminate ENMs, from the non-ENMs and to assess their particle size distribution to account for the agglomerates is very challenging and costly for routine occupational hygiene monitoring. Sometimes requires equipment that is not available at most of the analytical laboratories like electron microscopy techniques (e.g. SEM, TEM).
Considering the scientific evidence on the size related effects of nano particles, or ultrafine particles (as incidental particles in the nano range are known in public health studies) on causing adverse human health effects, in addition to the specific limits for each type of ENM, setting up a legally binding OEL for the total number of particles regardless of their origin seems a logical step to protect workers’ health.
Regarding the upper size limit, considering that nanoparticles tend to agglomerate, and the health effects of the respirable fraction (< 4.25 µm) are well documented, it seems reasonable to establish the cut off size here.
Adopting legally binding OELs has been proof to be a useful regulatory tool to protect workers health from exposure to chemicals. These limits are useful for employers to set up exposure controls. Certainly, within an effective risk management strategy where the first measure is the development of safer processes and the use of safer materials.
Dr Araceli Sánchez works at the Spanish Institute of Health and Safety (INSST). She received a PhD on Air pollution and Health from Strathclyde University and a MSc on Occupational Hygiene by Manchester University. Her expertise is on the assessment of exposure and risk of chemicals, especially on occupational settings. Over the last 12 years she has been actively involved on the development of methodologies and models for the risk assessment of engineered nanomaterials.
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