Introduction

Occupational health is an interdisciplinary field dedicated to the prediction, identification (measurement), evaluation, and control of workplace hazards. These hazards encompass a broad spectrum of physical, chemical, biological, ergonomic, psychological, and safety factors that pose risks to the physical and mental well-being of employees [1]. Chemical agents hold significant importance for a variety of reasons: In nearly all occupations, employees come into contact with substances or chemicals. Annually, millions of tons of chemicals are produced and utilized globally [2]. The global chemical market held a significant value of 1.4 trillion dollars in 2021 [3]. It is estimated that 32 million people worldwide are exposed to at least one chemical in 3.5 million occupational settings. Notably, China leads the way with over 6 million workers in this sector [4]. Within these work environments, more than 650,000 hazardous chemicals are produced or utilized, with thousands of new dangerous substances being added to these figures each year [5]. This extensive level of production and utilization of chemicals within these industries presents numerous risks to the employees involved. In addition to the health hazards, these materials also introduce various safety risks that directly impact the industries themselves. According to the World Health Organization, in 2012, 1.3 million diseases, injuries, and disabilities attributed to chemical exposure were reported and 43 million years of life were affected. These numbers escalated to 1.6 million deaths and 45 million years of life impacted in 2016. In 2019, however, this figure experienced a further increase, reaching 2 million reported deaths and 52 million years of life affected by various diseases, injuries, and disabilities resulting from exposure to chemical substances [6]. In occupational settings where chemical substances are present, occupational health experts utilize risk prediction, identification, and evaluation to proceed with the hierarchy of controls in order to determine and recommend the most suitable control measures based on the nature of the work, associated hazards, and risk assessment results. Subsequently, the implemented control measures undergo periodic evaluation to ensure their effectiveness and suitability, allowing for necessary modifications or replacements if required [7].

The hierarchy of controls, as proposed by NIOSH and arranged in descending order of effectiveness and priority, consists of elimination, substitution, engineering controls, administrative controls, and personal protective equipment (PPE) (Fig. 1).

Fig. 1
figure 1

The hierarchy of controls in occupational health (proposed by NIOSH)

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Elimination is the highest priority control measure as it involves completely removing the hazardous agent from the work environment. If feasible, elimination should be pursued as the primary control measure. Substitution involves replacing the existing agent with another substance, material, or equipment that offers equal effectiveness but poses lower risks to employees [8]. Engineering controls encompass the use of specialized equipment and technical engineering principles to reduce or eliminate hazardous factors in the work environment, either temporarily or permanently. The main objective of engineering controls is to enhance the overall work environment and mitigate potential risks [9]. Administrative controls do not aim to reduce or eliminate the risk factor itself. Instead, they focus on minimizing employee exposure to risk through training programs, procedures, policies, and shift management. Unlike other control measures that rely on modifying the work environment, administrative controls primarily rely on the actions and behaviors of employees [10]. Personal protective equipment (PPE) is given the lowest priority in the hierarchy of controls. This type of equipment serves as the last line of defense between employees and the hazardous agent. Various forms of PPE, such as respiratory protection (masks and respirators), skin protection (gloves and coverings), and eye protection (glasses, goggles, and shields), are crucial in different occupational settings [11].

Nanotechnology is an emerging field experiencing rapid growth and finding diverse applications in various industries. Nanoscale materials are increasingly prevalent in work and laboratory settings. While the existing hierarchy of controls for occupational hazards appears suitable from a theoretical, scientific, and practical standpoint, a pertinent question arises: Can this hierarchy of controls, which has demonstrated acceptable efficacy thus far, be effectively applied to novel technologies like nanotechnology, which may introduce new risks?

According to the definition, working and laboratory environments involving nanomaterials encompass settings where nanomaterials are produced, manipulated, or utilized. Nanomaterials refer to materials possessing at least one external dimension, internal structure, or surface structure within the nanoscale range (1 to 100 nm) [12]. Despite the widespread notion that nanomaterials are produced in limited quantities for specific purposes, the production and consumption of various nanomaterials globally have reached significant volumes, amounting to thousands of tons annually. Examples of such nanomaterials include carbon nanotubes, titanium oxide (TiO2), and zinc oxide (ZnO) [13]. With the expanding utilization of nanomaterials across diverse scientific and industrial domains, the prevalence of working and laboratory environments involving these materials is steadily on the rise [14]. As the scope and volume of nanomaterial usage continue to increase, so does the number of individuals exposed to these materials within work settings. Projections show that by 2029, more than 8 million people will be occupationally exposed to nanomaterials [15]. Furthermore, numerous studies indicate the significant nature of exposure to nanomaterials within these environments [16]. The release and subsequent exposure to nanomaterials can occur in various working environments, including those involved in the production, consumption, or recycling of these materials, as well as research laboratories [17]. The specific methods employed for the production and consumption of nanomaterials, along with their physical state, significantly influence the likelihood of release and subsequent exposure. Among different physical states, nanomaterials in a “dry powder” state have the highest probability of release and exposure, while those “bonded with a solid” exhibit the lowest likelihood [18]. Nanomaterials in states such as “suspension/emulsion,” “paste,” and “gel” fall within a moderate range of probability for release and exposure. The amount of energy applied to nanomaterials during industrial and laboratory processes is another crucial factor in determining the potential for their release and subsequent exposure. Processes involving significant energy input, such as “synthesis in a reactor,” “spraying,” and “machining,” are more likely to cause the release and exposure of nanomaterials. Conversely, processes like “packaging” and “cleaning” (when performed using wet wipes instead of air pressure) introduce less energy to the nanomaterials, resulting in a lower likelihood of release and exposure [19]. The energy levels employed during various processes play a significant role in determining the risk of nanomaterial release and subsequent exposure. Furthermore, several studies have indicated that the release rate of nanomaterials can be influenced by their physicochemical characteristics [20]. It has been observed that nanomaterials can exhibit distinct physicochemical properties compared to their larger counterparts, even when they share the same chemical composition [21]. The unique properties of nanomaterials stem from two main factors: their reduced size and their increased surface area. When materials reach the nanoscale, approaching the size of electrons or photons, the principles of quantum mechanics become applicable, leading to energy-level dissociation and the manifestation of particle-like behavior. Additionally, at the nanoscale, the surface-to-volume ratio becomes significantly higher, meaning that the properties of these materials are predominantly governed by their surface characteristics rather than their volume properties. Apart from these, in the nanoscale, the ratio of surface to volume is very high; Therefore, the properties of these materials are more controlled by their surface properties than their volume properties. Moreover, at this scale, gravitational and inertial forces can be disregarded in comparison to van der Waals and electromagnetic forces. The influence of nanomaterials’ size and surface area is evident in various properties, including optical, electrical, mechanical, chemical, physicochemical, thermal, and magnetic properties [22]. The substantial increase in biological activity of nanomaterials, relative to larger-sized materials, can be attributed to their high surface-to-volume ratio resulting from their reduced size. For instance, in nanoparticles with a diameter of 300 nm, only 5% of the atoms reside on the surface, while the remaining atoms are inside the particles. Conversely, in nanoparticles with a diameter of 30 nm, approximately 50% of the atoms are situated on the surface. It is important to note that surface atoms exhibit significantly higher biological activity than that of their internal counterparts [23]. In addition to the heightened biological activity, nanomaterials exhibit distinctive interactions with biological systems compared to their larger counterparts. Research indicates that nanoparticles possess unique biological properties, such as translocation to secondary target organs, limited clearance by macrophages, and the ability to traverse sensory neuron axons and reach intracellular structures like mitochondria and nuclei [24]. Furthermore, investigations have revealed that at least 11 out of the 28 characteristics mentioned by Stefaniak et al. [21] are essential for assessing the toxicological risks associated with nanomaterials. Therefore, comprehensive information on these specific characteristics is crucial in determining the potential hazards posed by nanomaterials. These 11 properties include specific surface area, chemical/elemental composition, surface chemistry, particle size, particle size distribution, shape/form, surface charge, stability, agglomeration, crystal structure, and solubility in water. However, a big challenge associated with nanomaterials is the limitation of standardized and universally accepted methods for characterizing these materials. Consequently, in numerous cases and for various nanomaterials, accessing information on all these 11 characteristics is quite difficult, leading to a lack of information during the risk assessment process [25]. Numerous studies have provided evidence of the potential dangers associated with occupational exposure to nanomaterials, affecting various parts of the body [26, 27]. It is widely acknowledged that the primary route of exposure in working and laboratory environments involving nanomaterials is through inhalation. However, skin contact and ingestion (via contaminated hands during eating and drinking) are also possible routes of exposure in these environments. Due to the small size of nanomaterials, their initial deposition site is typically the pulmonary alveolus [28]. Subsequently, they can disseminate throughout the body via the lymphatic and circulatory systems [29] or induce toxicity at the initial deposition site. Regarding lung toxicity, inflammation and the generation of reactive oxygen species are recognized as the primary mechanisms of nanomaterial-induced damage [30]. Consequently, nanomaterial exposure control remains a pressing concern for occupational health professionals.

Hierarchy of controls in the working and laboratory environments of nanomaterials

Elimination

This level of the hierarchy of controls is not applicable in the working and laboratory environments involving nanomaterials [31]. As mentioned earlier, elimination involves completely removing the hazardous agent, in this case, nanomaterials. However, in the context of working and laboratory environments for nanomaterials, the nanomaterials themselves are the main material being used, and removing them would render the environment no longer specific to nanomaterial-related work. Essentially, eliminating the hazardous agent would entail eliminating the core material, resulting in the complete loss of the work process. But measures such as changing the work process, containment and prevention through design (PtD), have been proposed by some guidelines as an alternative to elimination [32, 33].

Substitution

The application of this control measure is likewise impractical in nanomaterials working and laboratory environments [31]. The utilization of specific nanomaterials in these environments is dictated by their unique properties. Substitution involves replacing an agent with another agent possessing similar characteristics but presenting reduced risk. However, given the current understanding, it is not feasible to substitute a nanomaterial with another nanomaterial that exhibits identical characteristics while posing lower risks to employees. The complex nature of nanomaterials and the diverse range of properties they possess make it challenging to identify suitable alternatives that meet the same performance requirements while minimizing potential hazards. Thus, substitution as a control measure is not currently applicable in nanomaterial-related work and laboratory settings.

Engineering controls

Numerous research studies have indicated that conventional engineering controls, including general ventilation, local ventilation, and fume hoods, do not offer sufficient protection against nanomaterials in workplaces and laboratories [34]. Despite efforts by various organizations to provide guidelines for working with nanomaterials [35], a comprehensive examination of these guidelines reveals a lack of novel recommendations specifically addressing engineering controls for nanomaterials. Most guidelines recommend the use of local ventilation systems and complete containment measures for activities involving nanomaterials, similar to those applied for other chemicals, without taking into account the unique behavior of nanomaterials in the workplace air [36]. Consequently, these conventional engineering controls may not adequately address the potential risks associated with nanomaterial exposure in occupational settings. In addition to the general recommendations provided in these guidelines, one important measure is the utilization of HEPA and ULPA filters at the outlets of general and local ventilation systems in nanomaterial work environments. This ensures that the air released from these environments undergoes filtration before being discharged into the surroundings [32]. However, it is worth noting that advanced engineering controls, which have been proven to effectively reduce nanomaterial exposure (such as advanced glove boxes and specialized enclosures for handling particles), are limited in availability and highly costly. Consequently, these controls are not widely accessible in most nanomaterial working and laboratory environments, particularly in developing countries [37].

Administrative controls

Research indicates that in the working and laboratory environments of nanomaterials, there is inadequate emphasis on administrative controls, which encompass measures such as reducing exposure time, providing training, housekeeping, and implementing proper working methods. Furthermore, it has been observed that the contribution of administrative controls in reducing and effectively managing exposure to nanomaterials is minimal [37]. An analysis of the guidelines about nanomaterial work reveals that training and the adoption of appropriate working methods hold greater significance among these administrative controls [38]. It is recommended that training programs encompass several key topics, including the safe utilization of nanomaterials, standard procedures for handling nanomaterials, understanding the risks and toxicity associated with nanomaterials, familiarity with safety data sheets, appropriate selection and use of personal protective equipment, equipment maintenance protocols, nanomaterial waste management, and emergency response procedures specific to these environments [39]. However, there is a lack of information regarding the presentation and specific content of these materials. Surveys indicate that researchers and employees concerning nanomaterials receive very limited training in these areas, leading to a lack of awareness about the potential hazards associated with these materials. Consequently, many individuals perceive nanomaterials as safe or less dangerous than it actually is. This attitude contributes to a significant shortfall in adhering to safety principles when working with nanomaterials in both the working and laboratory environments, thereby failing to sufficiently reduce exposure to Nanomaterials [26]. Proper working procedures encompass a range of actions and behaviors that individuals should follow when working with nanomaterials. These procedures are outlined to varying degrees in the guidelines for working with nanomaterials, including considerations such as using nanomaterials in liquid, solid-bonded, gel, or paste forms instead of solid powders, guidelines for selecting and using personal protective equipment and safety data sheets, and proper protocols for cleaning environments contaminated with nanomaterials, among others [35]. Research has revealed a significant lack of adherence to appropriate working methods among laboratory employees and those working in nanomaterials environments, mainly attributed to their limited awareness of these methods [40]. In summary, both conducted studies and provided guidelines emphasize that training and educating personnel working in these environments are crucial control measures for effectively managing personal safety, adopting safe work practices, implementing proper cleaning procedures for environments contaminated with nanomaterials, and complying with internal regulations specific to the work environment, such as the movement control regulations of employees [35, 41].

Personal protective equipment

PPE has garnered significant attention in the guidelines provided by various safety and health organizations for ensuring safe work practices with nanomaterials. These guidelines categorize PPE into three groups: respiratory protection, skin protection (gloves and other coverings), and eye protection [33]. Most guidelines advocate for the utilization of N100, N95, FFP3, and FFP2 masks when working with nanomaterials, asserting that when used correctly, these masks provide adequate respiratory protection against nanomaterials. Furthermore, in cases where engineering controls are absent or administrative controls are not adhered to within the designated environment, the guidelines suggest the preference for respirators equipped with HEPA filters or equivalent filters over the aforementioned masks [42,43,44,45]. It is important to emphasize that the use of surgical masks is prohibited in nanomaterials working and laboratory environments [46]. Regarding hand protection, commonly used laboratory gloves such as vinyl, nitrile, latex, and neoprene have been deemed suitable for nanomaterial-related environments. However, it is recommended to wear two pairs of gloves simultaneously and replace them promptly upon detecting any signs of contamination or wear. The proper attire for other body parts is consistent with that recommended for chemical laboratories: closed-toe shoes, full-length pants without rolling them up, long-sleeved shirts, and lab coats [42,43,44,45]. Furthermore, several guidelines have emphasized the importance of utilizing disposable coverings such as headbands, wristbands, leggings, and one-piece work clothes when working with nanomaterials [8, 9]. Eye protection, while crucial, has received comparatively less emphasis in the guidelines. The majority of them recommend the use of goggles for adequate protection against nanomaterials, considering glasses and face shields to be insufficient [42,43,44,45].

Conclusion

Based on the discussed information, it is advisable to implement control measures in environments associated with nanomaterials by starting from the apex of the hierarchy of controls pyramid and progressing toward its base (Fig. 2). This approach entails initiating the provision of suitable personal protective equipment recommended for working with nanomaterials to employees as the first step. Concurrently, it is essential to commence comprehensive training addressing the safety, health, and environmental aspects of nanotechnology, with a particular emphasis on the proper utilization, maintenance, and disposal of personal protective equipment. In addition to these measures, it is imperative not to overlook the endeavors aimed at developing and implementing appropriate engineering controls for handling nanomaterials.

Fig. 2
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Proposed hierarchy of controls for nanomaterials work and laboratory environments

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