https://orcid.org/0000-0002-1189-9643
Abstract
Research has shown that controlling worker exposure to engineered nanomaterials (ENMs) helps to reduce the exposure risk to employees in workplaces. This study aimed to identify the available evidence on the effectiveness of various control methods used in the workplace to reduce worker exposure to ENMs. The search was conducted in databases—Medline, OVID, Scopus, Science Direct, Web of Science, and Cochrane and the gray literature published from January 2010 to December 2022. The search keywords included ENM controls and their efficiency in workplace environments. Of the 152 studies retrieved, 22 were included in the review. The control measures in the review included (1) substitution controls; (2) engineering measures (i.e., isolation, direct source extraction, and wetting technologies); (3) personal protective equipment; and (4) administrative and work practices. The study results indicate that the above-mentioned control measures were effective in reducing ENM exposures. This information can be used to help employers choose the most effective controls for their workplaces.
Introduction
The prefix “nano” is derived from the Greek word “nanos,” meaning “dwarf.” Nano-scale materials are defined as having at least one dimension in the range of 1 to 100 nanometers (nm) (Oberdörster et al. Citation2005). According to the Australian National Nanotechnology Initiative, engineered nanomaterials (ENMs) are a class of deliberately designed and manufactured materials with nano-scale dimensions. Due to their nano-scale particle size and high surface area to mass ratio, ENMs have a rapidly expanding range of applications in our everyday life, and their smaller size, lower weight, and increased potential to become airborne necessitate careful considerations in applying existing control measures. ENMs are used in many different products, from construction materials to medical devices, and are often manufactured in large quantities (Pietroiusti and Magrini Citation2014). More complex nanomaterials are used in a variety of fields, including drug delivery and medicine, environmental management, electronics, information and communication technology, and defense.
Multiple approaches can be used to produce a particular ENM (Virji and Stefaniak Citation2014). ENMs can be categorized by production method, e.g., top-down (breaking down) or bottom-up (building up) (Virji and Stefaniak Citation2014). During their life cycle from production to disposal, ENMs can be released into the workplace, which can lead to worker exposure. Exposure can occur during large-scale production, synthesis, recovery, packaging, transport, and storage (Semenzin et al. Citation2019). ENMs may also be re-engineered or reprocessed during cutting, spraying, sawing, or finishing. The degree of exposure to ENMs is affected by a variety of factors, including the way they are produced, processed, removed, handled, contained, and packaged (Hristozov and Malsch Citation2009; Pietroiusti and Magrini Citation2014). Exposure to ENMs can have adverse health effects, including irritation, inflammation, fibrosis, cell death, and changes in biochemical parameters, Hence, developing and implementing effective control measures is a key aspect to protecting employee health (Virji and Stefaniak Citation2014).
To control exposure, it is important to understand how they are used and manufactured (Virji and Stefaniak Citation2014), the type of task conducted, work practices, controls, and PPE used. For example, the control measures used in small-scale production (e.g., fume hoods and PPEs in laboratories) and large-scale production differ (e.g., isolation of the process) (Brouwer Citation2010).
Designing for Safety (DfS) is a framework that helps to identify and implement control measures at every stage of the ENM process (NIOSH Citation2016). DfS can be used to tailor control measures to the specific hazards during various phases of the ENM. The DfS specifies three key stages at which control measures can be implemented are development, production, and end-of-life. By implementing appropriate controls at each stage, manufacturers can reduce the risk of exposure. The most effective control measures will vary depending on the specific situation, and the hierarchy of control principle provides a systematic approach to identifying and implementing the most effective controls.
To be effective, controls must be able to reduce risk, which can be achieved by implementing a single control or a combination of multiple controls. Elimination, substitution, and engineering controls that limit ENM exposure take precedence over administrative controls or PPE, as they ensure better risk control. However, calculating the effectiveness of control strategies is difficult, as some ENMs require high factors of risk reduction. To date, there is limited research on the effectiveness of control measures in reducing ENM exposure (Methner Citation2008; Ntlailane and Wichmann Citation2019). Thus, a better understanding of ENM control and its effectiveness is warranted. This scoping review sought to identify different types of control measures for ENMs in workplaces and evaluate their effectiveness.
Methods
The scoping review adopted the framework of Arksey and O’Malley (Citation2005), which comprises five stages: defining the research question, identifying the relevant studies, selecting the studies, charting and collating the data, and summarizing and reporting the results.
The study aimed to understand the different types of control methods applicable to ENMs and their effectiveness in industrial settings. A search was conducted of the following databases: (1) Medline—OVID; (2) Scopus; (3) Science Direct; (4) Web of Science; and (5) Cochrane. The following keywords were used: “nanoparticle” or “ultrafine” or “engineered nanoparticle,” together with “control” or “PPE” or “enclosure” or “intervention” for exposures at “workplaces” or “factory” or “occupational” (Truncation and proximity searching were applied to some terms). The search was limited to articles written in English and published between January 2010 and December 2022. ‘The effectiveness’ of the control measures was measured during the full-text review of the articles and tabulated in and as Key findings and outcomes. Additionally, the identified keywords were used in searches of gray-literature databases (i.e., Google Scholar, Open DOAR, Open Grey, Science.gov, and WorldWideScience.org).
Table 1. Experimental studies and types of controls used.
Table 2. Descriptive studies and type of controls.
The inclusion criteria were (1) the study aimed to examine the release of airborne ENMs and provided a qualitative or quantitative description of the control methods; (2) the study was conducted in laboratory or industrial settings; and (3) the study compared the efficiency of the control methods used to reduce ENM exposure. Articles were excluded from the review if the studies examined microparticles and bulk particles, modeling exposure scenarios, or included participants aged under 16 years.
For the title and abstract screening, 152 articles were retrieved, cataloged, and screened by two reviewers. A third reviewer was consulted to resolve conflicts that arose between the two reviewers. Five studies were excluded due to duplication and 120 were excluded during abstract screening based on the inclusion and exclusion criteria using the referencing software Zotero and the web-based software platform Covidence to review the inclusion and exclusion criteria. Again, a third reviewer was consulted if any conflicts arose. The remaining 27 articles were then subject to a full-text review. Five of the full-text articles were excluded as three of them did not meet the inclusion criteria. Studies by Moeta et al. (Citation2019) and Lehutso et al. (Citation2021) focused on environmental exposures and did not directly address workplace exposures. One other study from Yokel and MacPhail (Citation2011) focused on general toxicological aspects of ENMs. Two of the studies had incorrect outcomes focusing on Material Safety Data Sheets (MSDS) (Eastlake et al. Citation2012) and agricultural environmental implications (Iavicoli et al. Citation2017), respectively. Twenty-two studies were included in the final screening process for the scoping review. presents the PRISMA flowchart for the screening process.
Figure 1. Prisma flowchart for screening process conducted for the scoping review.
Results
To address the research objective, the results of this scoping review have been divided into two sections: (1) the types of controls; and (2) the effectiveness of the control methods.
Exposures to ENMs were identified in personal and area sampling and were found to originate from a variety of sources, including combustion, automobile emissions, and outside air intrusion. These exposures primarily occurred when specific activities with ENMs were being conducted. Conversely, non-controlled exposures tended to occur when ENMs remained in the background for extended periods and were not appropriately controlled. In total, nine experimental studies were included in the review (see ). Six of these studies found that workers were exposed to ENMs, including nano titanium dioxide (Vaquero et al. Citation2016; West et al. Citation2016; Sivapirakasam et al. Citation2017; West et al. Citation2019), nano silicon dioxide (Wang et al. Citation2013), nano copper oxide (Semenzin et al. Citation2019), and nano alumina (Sivapirakasam et al. Citation2017). Three studies found that workers were exposed to carbon nanotubes (CNTs), nanowires, and graphene. Five studies found that workers were exposed to ENMs used in the manufacture of automobile parts, spray painting and sanding, construction, and weld coat products. These studies focused on the different types of control measures applicable to the manufacturing of ENMs (Ling et al. Citation2012; Boccuni et al. Citation2020) in industry settings, including at automotive and construction facilities (Vaquero et al. Citation2016; West et al. Citation2016; Semenzin et al. Citation2019; West et al. Citation2019), a pilot-scale facility (Wang et al. Citation2013), a laboratory (Sivapirakasam et al. Citation2017), and an exposure chamber (Vo et al. Citation2015).
Under the DfS framework, the control methods identified in the studies fell into four broad categories: (1) substitution; (2) engineering control methods, which comprised (a) methods used to control the emission source (e.g., full process containment and isolation) and (b) methods used to control the path of transmission (e.g., local exhaust ventilation (LEV), fume hoods, tool capture controls, and wet capture controls; (3) PPE; and (4) administrative and work procedure control methods. The following five characteristics of ENMs can be modified during the process of substitution: particle size, physical properties, agglomeration, chemical properties, and conductive properties (Safe Work Australia Citation2012). In one experimental study, Sivapirakasam et al. (Citation2017) demonstrated that the use of nano-aluminum and titanium-coated welding rods significantly lowered the exposure concentration compared to traditional welding rods. One of the most common isolation systems used was glove box containment (via isolators), which was shown to be an effective method to control exposure and was used as an enclosure in small-scale powder processes, such as mixing and drying in laboratories (Boccuni et al. Citation2020). For larger-scale processes, full process containment whereby people were isolated from the hazard (rather than trying to capture the ENMs) was more effective in reducing the exposure concentration (Oksel et al. Citation2016).
Five of the experimental studies demonstrated that LEV systems were effective at capturing most of the airborne particles when handling large quantities of ENMs (Ling et al. Citation2012; Vaquero et al. Citation2015; Oksel et al. Citation2016; West et al. Citation2019; Boccuni et al. Citation2020). However, when the particle size increased or when high energy activities (e.g., drilling and shearing) were performed, larger-sized particles along with ENMs were released that could not be fully captured by the on-tool extraction systems in occupational (Semenzin et al. Citation2019) and open-work areas (West et al. Citation2016). Three studies used engineering control systems and high-efficiency particulate air (HEPA) filters in the LEV systems (Ling et al. Citation2012; Vaquero et al. Citation2016; West et al. Citation2019). The HEPA filters were effective in controlling high particle concentration levels.
Two experimental studies (Vo et al. Citation2015; West et al. Citation2016) evaluated the filtration effectiveness of exposure chambers. These studies also assessed the types of control methods used in these settings, which included a combination of control methods, such as substitution, engineering controls, administrative controls, and PPE. In two of the studies (Vaquero et al. Citation2016; Boccuni et al. Citation2020), laboratory fume hoods were used to handle small quantities of ENMs for operations, such as compaction, weighing, mixing, synthesis, transferring, and solution preparation. These studies demonstrated that the use of laboratory hoods was effective in removing the emitted ENMs.
Two studies examined the implementation of administrative controls in the workplace (Ling et al. Citation2012; West et al. Citation2016). These controls included implementing changes to working methods (Ling et al. Citation2012) and scheduling practices (e.g., work rotation practices) to limit the period of worker exposure to ENMs. West et al. (Citation2016) identified several risk communication practices that were adopted to educate the workers on ENM exposure risks. However, no information was available as to the efficiency of these administrative controls.
In relation to the ENMs that were handled in laboratory or pilot-scale settings, no information was provided on PPE use (Sivapirakasam et al. Citation2017; Boccuni et al. Citation2020). Two studies reported that workers donned full-face powered air-purifying respirators and Tyvek suits with hoods (West et al. Citation2016, Citation2019); however, other studies reported that only gloves were worn when conducting the experiments (Ling et al. Citation2012; Semenzin et al. Citation2019). Vo et al. (Citation2015) compared the filtration efficiency of different types of filters used in respirators. The authors found that P100 filters were more effective at filtering particles than N95 filters. The results also showed that the performance levels of all types of respirators with filters were similar for nanoparticles and larger particles (ranging from 100 nm to 400 nm in size), and the efficiency of the filters increased as the particle diameter decreased.
Of the 13 qualitative/descriptive studies (see ), eight were based on surveys conducted in the workplace. One study extracted information from a Safety Data Sheet (SDS) to evaluate the ENM risks (Lee et al. Citation2012). Four of the studies adopted a control-banding (CB) approach to evaluate the risks at workplaces handling ENMs (Liguori et al. Citation2016; Iavicoli et al. Citation2019; Buitrago et al. Citation2021; Mohammadi et al. Citation2021). The descriptive study results were based on survey data, and the reported controls included the installation of various engineering controls, PPE, administrative measures, and substitution. The controls that were adopted in workplaces were based on bulk material properties rather than nanomaterial properties (Oksel et al. Citation2016). The controls most commonly used by the surveyed companies were LEV and fume hoods. The results of a similar survey by Conti et al. (Citation2008) indicated that fume hoods were more commonly used than other engineering controls. Further, three studies (Schubauer-Berigan et al. Citation2015; Iavicoli et al. Citation2019; Semenzin et al. Citation2019) reported formal guidance on the safe handling of ENMs was offered to workers. PPE was also found to be the main method used to control worker exposure, and PPE was more commonly used than engineering and administrative controls (Dolez et al. Citation2010).
Gray literature and worldwide limits for occupational exposure levels for handling ENMs in the workplace were included in our review and are summarized in . The gray literature provided information on exposure and guidance for controlling exposures to ENMs by various regulatory bodies. The NIOSH has published recommended exposure limits for nano-scale titanium dioxide (Dankovic et al. Citation2011), nano-silver, CNTs, and nanofibers (Hodson et al. Citation2009). Other international agencies have also adopted some of the exposure limits.
Table 3. Limits of exposure and guidance for controlling exposures to ENMs by regulatory entities.
In examining the gray literature, guidelines on the safe handling of ENMs by the World Health Organization (WHO), International Labor Organization, Organization for Economic Co-Operation, and Development (OECD), and 12 different countries were reviewed. The key concept in the gray literature was DfS (NIOSH Citation2013). DfS was reported to be the most effective method for controlling exposure throughout the life cycle of a product. The gray literature also discussed various other controls, including isolation, substitution, engineering controls, PPE, and administrative work controls. The identified gray literature included governmental guidelines from seven countries (i.e., Canada, Japan, Korea, Australia, Netherlands, Switzerland, and Singapore) that have established occupational exposure limits (OELs) for specific nanomaterials. There are not many OELs for ENMs applicable to workplaces; however, the WHO (Citation2017) recommends a stepwise approach to conduct an exposure assessment according to the guidelines proposed by the OECD (Citation2022). As part of an overall plan to reduce worker exposure to nanoparticles, the NIOSH (Citation2006, Citation2016) recommends that workplaces employ several administrative and work procedure controls. PPE is the most commonly employed control method in the workplace; however, it should not be employed as the primary exposure control method. According to the WHO guidelines (2017), PPE is less effective without administrative and engineering controls.
Discussion
Exposure patterns and controls identified
The studies included in this review establish a clear pattern of exposure to ENMs based on personal and area sampling. ENM exposure primarily occurs when activities with ENMs are being conducted. Consistent with the findings of Kumar et al. (Citation2010), non-controlled ENM exposure tends to occur when ENMs remain in the background for extended periods; thus, background concentrations of ENMs need to be controlled. The review of the experimental and descriptive studies and gray literature revealed that the methods used to control exposure to ENMs fall into four broad categories: (1) substitution; (2) engineering controls, including LEV, tool capture controls, wet capture controls, elimination at the emission source (e.g., full process containment and isolation); (3) PPE; and (4) administrative and work procedure controls.
Substitution
Substitution with technical alternatives was one of the most effective controls used to reduce the release of ENMs in workplaces. Semenzin et al. (Citation2019) found that nano-scale copper oxide was used as an alternative antimicrobial coating in the automotive industry to reduce the release of more harmful ENMs. However, substitution primarily occurs at research facilities and occurs less commonly in commercial and industrial environments (Jackson et al. Citation2010; Charitidis et al. Citation2014). Once the experimental phase for an ENM has been completed, the ENM can be used in larger quantities in production in industrial facilities (Safe Work Australia Citation2012). Changing products in industrial facilities may require additional quality controls and process changes, which can affect substitution as a primary control measure (Jackson et al. Citation2010).
Engineering controls
In the management of worker exposure, engineering controls are preferred to administrative controls and PPE (NIOSH Citation2015). Engineering controls remove ENM exposure at the source or along the pathway before worker contact. Engineering controls have been shown to manage worker safety more effectively than administrative controls or PPE in a variety of workplaces (Methner Citation2008; Oksel et al. Citation2016). LEV is an engineering control that is effective in decreasing ENM concentrations (Methner Citation2008). LEV is often employed in industrial environments to control particle concentrations at the source and along the pathway (Conti et al. Citation2008; Liu et al. Citation2014). LEV was shown to reduce particle mass concentration and number concentration to 75–96% during a cleaning activity (Methner Citation2008). However, the effectiveness of LEV is not only dependent on its ability to capture the particles but also on how the workers use the existing workplace systems. Schmid et al. (Citation2010), acknowledged that in addition to engineering controls, worker handling factors (e.g., personal behaviors, experience, hood/ventilation maintenance, and housekeeping procedures) also require consideration when evaluating control measure effectiveness.
Administrative controls
The administrative controls identified included standard operating procedures (SOPs), general or specialized housekeeping procedures, training in handling ENMs, spill prevention and control, and appropriate labeling and storage. Employee training on the appropriate use and handling of PPE was identified as an important administrative function (Dolez et al. Citation2010). Administrative controls are a qualitative metric and little research on their effectiveness in controlling exposure to ENMs has been conducted. However, SOPs (e.g., labeling) are effective in controlling risk by increasing worker awareness while handling ENMs (Amoabediny et al. Citation2009). However, these should be used as part of a multifactorial approach to managing risks and in combination with other controls, rather than in isolation. For example, high-risk industries, such as construction (Borys Citation2012; O’Neill et al. Citation2022), automotive (Battini and Boysen Citation2013), and process industries, adopt multifactorial approaches that include engineering controls and administrative controls (Yuan et al. Citation2022). Experienced industrial hygienists and toxicologists often use a checklist as a measure to determine the presence of administrative controls; however, the effectiveness of the checklist is not easily measurable (Ramachandran et al. Citation2011).
PPE
Concerning PPE, the commercially available filters used in PPE have been shown to be effective against ENMs (Dolez et al. Citation2010). Vo et al. (Citation2015) evaluated the effectiveness of different filter types and found that filters provided adequate protection against ENMs if the PPE was worn properly (Trehan et al. Citation2021). However, if the mask is not properly fitted, the protection for an individual may be compromised. A compromised respirator seal is a more likely source of ENM inhalation risk than penetration through respirator filter media, and thus the appropriate fit and seal of the mask are critical factors affecting efficiency (AIHA Citation2018). Fit-test procedures for PPE masks are mandated by local and federal government requirements (OSHA Citation2004) and specific industry requirements.
PPE also includes protective clothing, such as suits, aprons, and gloves; however, there was little data on the protection provided by such PPE for ENM (Park et al. Citation2011). The use of chemical protective clothing is effective for firefighters (Rabajczyk et al. Citation2021) and researchers handling ENM suspensions (Sundarrajan and Ramakrishna Citation2007) where dermal exposure is a high-risk parameter. However, further studies need to be conducted to examine the specific protection provided by different types of PPE.
Risk assessment tools
Several risk assessment tools were used to assess workplace risks, including CB IVAM Technical Guidance, ANSES CB Tool, NanoSafer, and MARINA Stoffenmanager Nano tools. These tools are used to rank processes as low to high risk (Riediker Citation2015; Liguori et al. Citation2016). A limitation of many of these tools is that they do not consider particle behavior when assessing the risk of ENMs. The Swiss government (FOPH Citation2008) recommends the use of a precautionary matrix that allows for an initial evaluation of the hazards associated with ENMs to be made without having detailed knowledge of the toxicity. Such contextual data are used in CB approaches where hazards are grouped into broad categories with control measures based on the bands. Similarly, Oksel et al. (Citation2016) finding that a whole new risk management paradigm is not required to control ENM hazards supports the gray literature (Hodson et al. Citation2009; Lee et al. 2010). However, the various studies advise that existing techniques must be expanded to better address ENM-related challenges and ensure the safe manufacture, processing, and use of ENMs.
Semenzin et al. (Citation2019) proposed the use of Technological Alternatives and Risk Management Measures, which is similar to the implementation of DfS in workplaces. The study also recommended that specific hazards be appraised to enable appropriate controls to be implemented. To evaluate the effectiveness of control measures in workplaces, data on the quantity of material used, ventilation, PPE, worker activities, and sampling locations must be collected.
Future direction of exposure evaluation and control efficiency
A comprehensive exposure assessment should include the observance of appropriate contextual information, such as airflow visualization and measurement, and quantitative containment test procedures. In another study, Tsai et al. (Citation2012) found that engineering controls, such as improved ventilation and the enclosure of releasing sources, can significantly reduce the concentration of particles in the air. This, in turn, makes the controls more effective. Newer technologies (e.g., video exposure monitoring) could also provide data on essential task-based exposures, which could be used to detect high-exposure activities. Appropriate evaluations of controls need to be completed to determine the most effective way to reduce worker exposure to ENMs. According to Schulte et al. (Citation2010) performance-based engineering controls, control bands, and interim OELs are effective steps in minimizing exposure and can be used as tools to determine the efficacy of exposure control methods and other risk management choices. The management and documentation of these exposure-related variables are crucial in exposure assessments, as, in the absence of such measures, the designs might lead to biased exposure control efficiency findings.
Limitations
This scoping review identified several experimental/descriptive studies and gray literature that discussed different types of workplace controls for ENMs and their effectiveness. However, the efficiency of the controls described in the studies could not be evaluated; thus, this area requires further research. The reviewed literature on ENM life cycles has focused on the manufacturing and usage stages. However, there is very little data available on end-of-life treatment and the effectiveness of the implementation of controls. The information needed to evaluate the effectiveness of ENMs throughout their life cycles can be obtained by identifying the efficiency or effectiveness of controls during disposal or recycling.
Conclusion
There is limited evidence on the effectiveness of ENM controls in workplaces. Substitution and engineering methods were the most effective interventions for ENM exposure. It is important to develop and evaluate the effectiveness of these two controls throughout the ENM life cycle. Future studies should identify the adequacy of the hierarchy of workplace controls and assess their implementation and effectiveness. Such research would help workplaces reduce the risks associated with ENM exposure by selecting and managing appropriate controls. Regulators should also consider these findings when developing guidelines for the use of ENMs in various industries.
Research involving human participants and/or animals
This was a review article, and the authors did not conduct any studies with human participants or animals.
Disclosure statement
The authors have no conflicts of interest to declare.
Additional information
Funding
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