# Current Strategies for Engineering Controls in Nanomaterial Production and Downstream Handling Processes/Exposure Control Strategies and the Hierarchy of Controls

​CHAPTER 2
Exposure Control Strategies and the Hierarchy of Controls
Controlling exposures to occupational hazards is the fundamental method of protecting workers. Traditionally, a hierarchy of controls has been used as a means of determining how to implement feasible and effective controls. Figure 4 shows one representation of this hierarchy. The idea behind the hierarchy of controls is that the control methods at the top of the triangle are generally more effective in reducing the risk associated with a hazard than those at the bottom. Following the hierarchy normally leads to the implementation of inherently safer systems, ones where the risk of illness or injury has been substantially reduced. Designing out hazards early in the design process is a basic tenet of PtD. When PtD is implemented, the control hierarchy is applied by designers and owners/managers to include safety into the process. The following sections discuss each element of the hierarchy of controls—elimination, substitution, engineering controls, administrative controls, and PPE—and how it may relate to nanotechnology.
2.1 Elimination
Elimination and substitution are generally most cost effective if implemented when a process is in the design or development stage. If done early enough, implementation is simple and, in the long run, can result in substantial savings (e.g., cost of protective equipment, first cost and operational cost for ventilation system). For an existing process, elimination or substitution may require major changes in equipment and/or procedures in order to reduce a hazard.
Figure 4. Graphical representation of the hierarchy of controls
​Elimination is the most desirable approach in the hierarchy of controls. As its name implies, the idea behind elimination is to remove the hazard. Eliminating hazards is generally easiest to accomplish at the design stage, while the material, process, and/or facility is being developed. An example of elimination in a process step might be the removal of an incoming inspection step for nanomaterials. An incoming inspection that requires opening a package containing nanomaterials leads to the potential of aerosolization of those materials and therefore a potential hazard to the inspector. Eliminating the inspection step removes the hazard, thus creating an inherently safer process.
2.2 Substitution
Within the hierarchy of controls, the purpose of substitution is to replace one set of conditions having a high hazard level with a different set of conditions having a lower hazard level. Examples of substitution could include replacing a solvent-based (i.e., flammable) material with a water-based material, substituting a highly toxic material for one of lower toxicity, or changing a process’s operating conditions so they are less severe (e.g., reduced pressure). Substitution of a nanomaterial may be difficult since it was likely introduced for its particular properties; however, some substitution may be possible. Substituting a nanomaterial slurry for a dry powder version will reduce aerosolization and provide a level of protection for workers handling the material. The specific nanomaterial should also be assessed because in some cases a less hazardous nanomaterial may provide the desired performance.
2.3 Engineering Controls
Engineering controls protect workers by removing hazardous conditions (e.g., local exhaust ventilation that captures and removes airborne emissions) or placing a barrier between the worker and the hazard (e.g., isolators and machine guards). Well-designed engineering controls can be highly effective in protecting workers and will typically be passive, that is, independent of worker interactions. It is important to design engineering controls that do not interfere with the productivity and ease of processing for the worker. If engineering controls make the operation more difficult, there will be a strong motivation by the operator to circumvent these controls. Ideally, engineering controls should make the operation easier to perform rather than more difficult. A good mantra in designing engineering controls is to “make it easier to do it the safe way.” This also applies to administrative controls that are discussed later. The initial cost of engineering controls can be higher than administrative controls or personal protective equipment (PPE); however, over the long term, operating costs are frequently lower and, in some instances, can provide a cost savings in other areas of the process. The major benefit of engineering controls over administrative controls or PPE is, however, the inherent safety of the worker under a variety of conditions and stress levels. The use of engineering controls reduces the potential for worker behavior to impact exposure levels. Thus, when elimination and substitution are not viable options, the most desirable alternative for mitigating occupational hazards is to employ engineering controls. Engineering controls are likely the most effective and applicable control strategy for most nanomaterial processes.
​In most cases, they should be more feasible than elimination or substitution and, given the potential toxicity of many nanomaterials, should prove to be more protective than administrative controls and PPE.
Engineering controls are divided into two broad categories for discussion below: ventilation and nonventilation controls.
2.3.1 Ventilation
The general concept behind ventilation for controlling occupational exposures to air contaminants, including nanomaterials, is to remove contaminated air from the work environment. The efficiency of the ventilation system can be affected by its configuration and flow volumes of both the air supplied to and the air exhausted from the work space. Effective ventilation applies to a wide range of applications including office heating, ventilating, and air conditioning (HVAC); infection control in healthcare; and control of emissions in industrial processes. Ventilation for occupant comfort, HVAC, is a specialized application of dilution ventilation and is not within the scope of this document. Filtration is a topic directly affecting ventilation; exhaust air laden with nanomaterials may need to be cleaned before being released into the environment.
General ventilation can be used to achieve several goals for workplace contaminant control. A properly designed supply air ventilation system can provide plant ventilation, building pressurization, and exhaust air replacement. As new local exhaust hoods are installed in the production area, it is important to consider the need for replacement air, the location of the hood installation, and the need to rebalance the ventilation system. In general, it is necessary to balance the amount of exhausted air with a nearly equal amount of supply air. Without this replacement air, uncontrolled drafts will occur at doors, windows, and other openings; doors will become difficult to open due to the high pressure difference, and exhaust fan performance may degrade. In addition, turbulence created through high pressure differentials can defeat the design intent of the ventilation. Placement of the air supply registers in relation to other exhaust ventilation systems is important so that they do not negatively impact the desired performance. The use of general ventilation for dilution of contaminants being generated in the space should be restricted in its use depending on several factors discussed below.
General ventilation used for dilution of contaminants by its nature is inefficient. One of two methods, recirculated air or single-pass air, may be used for this purpose. As the terms imply, recirculated air involves the treatment of exhaust air prior to its being returned to the area from which it was exhausted. Single-pass air is exhausted to the outside and may or may not require treatment prior to discharge. Both of these methods are expensive—the treatment of the recirculated air involves both first-cost and operating-cost penalties, while makeup-air treatment for single-pass air is inherently costly.
According to the American Conference of Governmental Industrial Hygienists (ACGIH) Industrial Ventilation: A Manual of Recommended Practice for Design (hereafter referred to as the Industrial Ventilation Manual), dilution ventilation (i.e., air changes) to control exposure should be used only under specific conditions. Dilution ventilation for controlling health hazards is restricted by four limiting factors: (1) the quantity of contaminant generated must ​not be too great or the airflow rate necessary for dilution will be impractical, (2) workers must be far enough away from the contaminant source or the evolution of contaminant must be in sufficiently low concentrations so that workers will not have an exposure in excess of the established threshold limit values (TLV®), (3) the toxicity of the contaminant must be low, and (4) the evolution of contaminants must be reasonably uniform [ACGIH 2013]. There are several issues with using dilution ventilation to control nanomaterial concentrations, including (1) there are no occupational exposure limits (TLVs mentioned above) or health effects data for many of the nanomaterials, (2) the toxicology data from some nanomaterials indicate that they may be associated with adverse health effects, and (3) it is difficult or impossible to calculate proper air change rates for contaminant control due to the variability in most operations. Therefore, local exhaust ventilation and good work practices should be used for controlling exposure, and air change rates should be based on the heat load requirements, general air movement, and comfort needs.
The use of supply air for maintaining proper pressurization between production and nonproduction areas is a reasonable approach to reducing the exposure to nanomaterials outside of the immediate work zone. The fugitive emissions from nanomaterial production and processing may result in high background concentrations in the production area. When adjacent plant areas are nonproduction areas (e.g., office, quality assurance/control labs) or production areas where nanomaterials are not used, infiltration of nanoparticles may occur and result in the exposure of workers in those areas. Therefore, a negative air pressure differential should be maintained in the nanomaterial production area with respect to adjacent rooms/areas. This will help reduce the potential migration of airborne nanomaterials and exposure to other workers in adjacent rooms or areas. To maintain a slight negative pressure, the room supply air volume should be slightly less than the exhaust air. A general guide is to set a 5% flow difference between supply and exhaust flow rates but no less than 50 cfm [ACGIH 2013]. As with any good engineering control, a real-time monitor of differential pressure between areas should be employed, preferably with the control capability to modify airflows to maintain the required pressure differential.
2.3.1.1 Local Exhaust Ventilation
Local exhaust ventilation (LEV) is the application of an exhaust system at or near the source of contamination. If properly designed, it will be much more efficient at removing contaminants than dilution ventilation, requiring lower exhaust volumes, less make-up air, and, in many cases, lower costs. By applying exhaust at the source, contaminants are removed before they get into the general work environment. When designing a local exhaust ventilation system, it is important to understand the transport mechanisms of the contaminants that are to be removed. This will allow the design to use optimal flow rates and capture locations, maximizing the contaminant capture while minimizing impact on the process and reducing operating costs. LEV typically involves five components [Washington State L & I, no date]:
* Exhaust hood. Examples include an enclosing hood to contain the contaminant, a receiving hood to capture or receive a contaminant that is released at a high velocity (e.g., grinding swarf), or simply an open duct. ​
* Duct. Transports the contaminant through the exhaust ventilation system.
* Air cleaner. Reduces the concentration of the contaminant in the exhaust air stream; may or may not be required.
* Fan. Moves the air through the exhaust system.
* Exhaust stack. Installed where the exhaust system discharges the air.
The exhaust hood captures the contaminant released by the process. It should be designed for the specific process being controlled, an important consideration for hot processes and those processes generating contaminants at high velocities. In either case, induced air flow (from high velocity air streams or rising air from a hot process) can overwhelm an insufficiently designed hood and allow contaminants to escape into the work environment. An important hood design factor is the capture velocity. This is the velocity of air needed to overcome contaminant velocity as well as room air currents. ACGIH Industrial Ventilation Manual contains a large collection of industrial ventilation hood designs for a wide selection of industrial processes [ACGIH 2013]. Though many of these designs have not been tested with nanomaterials, most are expected to perform effectively with these materials. An important consideration in hood design with nanomaterials is to provide the appropriate flow rates to prevent fugitive emissions without causing conditions that will remove nanomaterials from the process stream. Because of their very low mass, entrainment of nanomaterials in airflow streams occurs much more readily than with higher-mass particles.
Duct systems transport air between the various components of the LEV system. Designing duct systems requires balancing several factors. Duct losses caused by friction will increase with higher duct velocities, resulting in increased fan requirements and higher energy consumption; however, using larger ducts (in an effort to reduce duct velocity) results in increased duct purchase costs. A detailed method for designing and sizing LEV duct systems is provided by ACGIH [ACGIH 2013]. The choice of duct materials and sealing methods is particularly important when dealing with nanomaterials. The duct material needs to be impervious to the nanomaterials and suitable for use with nanomaterials having increased reactivity. The joints in the ducts should be sealed in such a way as to contain the nanomaterials.
Fans move air throughout the LEV system. Fans need to be sized to ensure adequate air flow while overcoming the system pressure drop (i.e., resistance to flow). Pressure drop is encountered when air is accelerated, such as within a hood; through ductwork due to frictional losses, particularly in fittings such as elbows; and through filters and other air-cleaning devices. Fan selection affects not only the control effectiveness of the LEV system but also its energy consumption. The fan system and the make-up air conditioning are typically the two greatest energy-consuming components of an LEV system. Proper fan selection needs to balance both control performance and operating efficiency [ACGIH 2013]. The same leakage and reactivity factors mentioned in the section on ductwork apply to fan selection.
Air cleaning is an important component of the LEV system, particularly if the exhaust air is returned to the building environment. Air cleaning involves the removal of gases and vapors, often with scrubbers and sorbent systems; however, in the case of nanomaterials, particulate removal systems will be required to eliminate them from the air stream. Cyclones, ​scrubbers, and other similar systems can be used to remove larger-sized particles, but smaller, nanoparticles will most likely be collected by filtration (see next section, Air Filtration).
2.3.1.2 Air Filtration
Air filtration removes unwanted particulate from an air stream. Particulate air filters are classified as either mechanical or electrostatic filters. Although the two types of filters have important performance differences between them, both are fibrous media or membranes and are used extensively in HVAC and industrial applications. Efficiency is dependent on several factors including fiber diameters, packing density, and material used. A fibrous filter is an assembly of fibers that are randomly laid perpendicular to the airflow. The fibers may range in size from less than 1 μm to greater than 50 μm in diameter. Filter packing density ranges from 1%–30%. Fibers are made from cotton, fiberglass, polyester, polypropylene, or a number of other materials [Davies 1977].
Fibrous filters of different designs are used for various applications. Three types are used for capturing particulate:
* Flat-panel filters contain all the media in the same plane. This design keeps the filter face velocity and the filter media velocity roughly the same.
* Pleated filters have additional filter media added to reduce the air velocity through the filter. This allows for an increased collection efficiency for a given pressure drop. Alternatively, pleated filters can be used to reduce the pressure drop for a given airflow velocity because of the larger filter area.
* Pocket or baghouse filters allow the flow of exhaust air through small pockets or bags consisting of filter media. As with pleated filters, the increased surface area of the pocket filter reduces the velocity of the airflow through the filter media, allowing increased collection efficiency for small particles at a given pressure drop.
Figure 5 presents four different collection mechanisms that govern particulate air filter performance:
* Diffusion is the result of the random (Brownian) motion of a particle. The particle may contact a fiber on its path through the filter.
* Interception occurs when the radius of a particle moving along an air streamline is greater than the distance from the streamline to the surface, thus causing the particle surface to contact the surface of the fiber. The particle adheres to the fiber due to intermolecular forces.
* Inertial impaction occurs when an air stream bends around a fiber, and a particle traveling in that air stream continues in a straight path due to particle inertia. The particle collides with the fiber and adheres to it due to intermolecular forces.
* Electrostatic attraction occurs when the particle and the fiber are oppositely charged. As the force of this attraction is governed by the charge-to-mass ratio of the particle, it becomes more effective as particle size decreases.
These mechanisms apply mainly to mechanical filters and are influenced by particle size. Impaction and interception are the dominant collection mechanisms for particles greater than 0.2 μm, and diffusion and electrostatic attraction are dominant for particles less than 0.2 μm, including nanomaterials. The combined effect of these collection mechanisms results in the classic collection efficiency curve, shown in Figure 6.
​Research on common air filter materials has shown that fractional efficiency for collection of particles of different sizes is consistent with the single fiber theory [Heim et al. 2005; Kim et al. 2007; Shin et al. 2008]. Kim et al. [2006] found that humidity has little effect on particle collection efficiency. Huang et al. [2007b] determined that the use of electrostatic filters (commonly used for respirators) improves particle collection in the 0.1–1-µm particle size range. Testing of respirator filters showed that the most penetrating particle size (MPPS) shifted from 30–60 nm to 200–300 nm following treatment of respirators by liquid isopropanol, which removes electrostatic charges on the filter materials [Rengasamy et al. 2009]. This result suggests that capture by electrostatic forces is important for particles in the 250–300-µm range. Overall, filters appear to behave in a manner consistent with theoretical predictions that common filter materials allow for efficient collection through diffusion of nanoparticles less than about 10 nm [Heim et al. 2005; Huang et al. 2007b; Kim et al. 2007; Shin et al. 2008].
Some researchers have found evidence of thermal rebound, which increases particle penetration through filters for nanoparticles in the size range of 1–10 nm [Bałazy et al. 2004; Kim et al. 2006]; however, several other filter testing studies did not reveal this effect, even at higher temperatures [Heim et al. 2005; Huang et al. 2007b; Kim et al. 2007; Shin et al. 2008]. The thermal rebound effect is a result of the thermal velocity of the particle exceeding the critical sticking velocity for a particle on a filter, allowing the particle to move past the filter fiber and penetrate the filter. The critical sticking velocity of an incident particle is defined as the maximum impact speed at which the particle will stick to a surface; above this velocity, the particle will bounce and not stick to the filter. The primary adhesive forces for nanomater-sized particles are the London-van der Waals forces. These forces are caused by random movement of electrons creating complementary dipoles between particle and filter material [Hinds 1999]. As the particle gets smaller, it is more difficult to remove the particle from surfaces.
High efficiency particulate air (HEPA) filtration is commonly used for applications requiring reliably high filtration. By definition, HEPA filters are 99.97% efficient at the most penetrating particle size of 0.3 microns (Figure 6). These filters are disposable and are usually replaced when the pressure drop exceeds a predetermined number, typically 100 mm water gauge (wg). When properly sized and installed, HEPA filtration is appropriate for nanomaterial applications both for ventilation systems and respiratory protection.
2.3.2 Nonventilation Engineering Controls
Nonventilation engineering controls cover a range of control measures (e.g., guards and barricades, material treatment, or additives). Nonventilation controls can be used in conjunction with ventilation measures to provide an enhanced level of protection for nanomaterial workers.
A variety of dust control methods have been used and evaluated in many industries and may be applicable to the processes used in the manufacturing and processing of nanomaterials [Smandych et al. 1998]. These methods include the enclosure of material-conveying equipment, such as belt and screw conveyers, as well as the use of pneumatic conveyance systems. Other work practices have been used to reduce the aerosolization of dust during bag ​filling, including minimizing leak paths by securing the bag to the outlet spout and wetting the outside of the bag to prevent surface dust from becoming airborne. Research over the years in a variety of industrial settings has shown that water spray application is effective in lowering respirable dust levels [Mukherjee et al. 1986]. The use of atomization nozzles was shown to be one of the most effective water-spray delivery systems in dust knockdown performance tests. Water sprays lower respirable dust concentrations by knocking down the dust, fibers, and particles, and they also can induce airflow to direct the remaining dust away from the workers.
Other nonventilation engineering controls include many devices developed for the pharmaceutical industry, including isolation containment systems [Hirst et al. 2002]. One of the most common flexible isolation systems is glove box containment, which can be used as an enclosure around small-scale powder processes, such as mixing and drying. Rigid glove box isolation units also provide a method for isolating the worker from the process and are often used for medium-scale operations involving transfer of powders. Glove bags are similar to rigid glove boxes, but they are flexible and disposable. They are used for small operations for containment or protection from contamination. Another nonventilation control used in this industry is the continuous liner system, which allows the filling of product containers while enclosing the material in a polypropylene bag. This system is often used for off-loading materials when the powders are to be packed into drums.
2.4 Administrative Controls
Administrative controls and PPE are frequently used with existing processes where hazards are not well controlled. This could occur when engineering control measures are not feasible or do not reduce exposures to an acceptable level. Administrative controls (which include training, job rotation, work scheduling, and other strategies to reduce exposure) and PPE programs may be less expensive to establish but, over the long term, can be very costly to sustain. These methods for protecting workers have also proven to be less effective than other measures and often require significant effort by the affected workers [ACGIH 2013; DiNardi 2003]. A valuable application of administrative controls is as a redundancy to engineering controls. While the engineering controls provide the primary protection for the worker, the administrative controls serve as back-up should the engineering control fail.
NIOSH recommends that facilities implement the following work practices as part of an overall strategy to control worker exposure to nanomaterials: (1) Educate workers on the safe handling of engineered nanomaterials to minimize the likelihood of inhalation exposure and skin contact. (2) Provide information on the hazardous properties of the materials being handled with instructions on how to prevent exposure. (3) Encourage workers to use handwashing facilities before eating, smoking, or leaving the worksite. (4) Provide additional control measures (e.g., use of a buffer area, decontamination facilities for workers if warranted by the hazard) to ensure that engineered nanomaterials are not transported outside of the work area. (5) Where there is the potential for area or personnel contamination, provide facilities for showering and changing clothes to prevent the inadvertent contamination of other areas (including take-home) caused by the transfer of nanomaterials on clothing and skin. (6) Avoid handling nanomaterials in the open air in a “free particle” state. (7)
​Store dispersible nanomaterials, whether suspended in liquids or in a dry particle form, in closed (tightly sealed) containers whenever possible. (8) Ensure work areas and designated equipment (e.g., balance) are cleaned at the end of each work shift, at a minimum, using either a HEPA-filtered vacuum cleaner or wet wiping methods (where the use of liquid does not create additional safety hazards). Dry sweeping (i.e., using a broom) or compressed air should not be used to clean work areas. Cleanup should be conducted in a manner that prevents worker contact with wastes. (9) Dispose of all waste material in compliance with all applicable federal, state, and local regulations. (10) Avoid storing and consuming food or beverages in workplaces where nanomaterials are handled [NIOSH 2009a].
2.5 Personal Protective Equipment (PPE)
PPE (e.g., respirators, gloves, protective clothing) is the least desired option for controlling worker exposures to hazardous substances. PPE is used when engineering and administrative controls are not feasible or effective in reducing exposures to acceptable levels or while controls are being implemented. It is the last line of defense after engineering controls, work practices, and administrative controls. A program that addresses the hazards present, employee training, and PPE selection, use, and maintenance should be in place when PPE is used.
2.5.1 Skin Protection
Nanomaterials have been shown to accumulate in hair follicles, and quantum dots have been shown to penetrate the skin into the dermis [Smijs and Bouwstra 2010]. Flexing the skin may enhance skin penetration [Smijs and Bouwstra 2010; Tinkle et al. 2003]. Woskie [2010] recommends wearing gloves, gauntlets, and laboratory clothing or coats when working with nanoparticles. Other studies of specifically engineered nanomaterials have resulted in the material not penetrating beyond the stratum corneum. Of importance is to establish a barrier between the potentially hazardous material and the skin.
Air-tight polyethylene was found to be more resistant to nanoparticle penetration by diffusion than cotton or polyester; gloves made of latex, neoprene, or nitrile resisted nanoparticle penetration “during exposure of a few minutes” [Woskie 2010]. Proper selection of gloves should take into account the resistance of the glove to the nanomaterial and any other chemicals or liquids with which the hands may come into contact. Gloves should be changed whenever they show visible signs of wear or contamination. Gao et al. [2011] studied nano- and submicron-size (30–500 nm) iron oxide particle penetration through some protective clothing materials. They found that particle penetration increased with increasing wind velocity and increasing particle size. Results from the study indicated that the MPPS for protective clothing materials tested was found to be about 300 to 500 nm, compared to an MPPS for N-95 respirators of 50 nm.
2.5.2 Respiratory Protection
Respiratory protection is used to reduce worker exposures to acceptable levels in the absence of effective engineering controls, during the installation or maintenance of engineering ​controls, for short-duration tasks that make engineering controls impractical, and during emergencies. The decision to use respiratory protection should be based upon professional judgment, hazard assessment, and risk management practices to keep worker inhalation exposures below an internal control or an exposure limit. Several types of NIOSH-certified respirators (e.g., disposable filtering facepiece, half-mask elastomeric, full facepiece, powered, airline, self-contained) can provide different levels of expected protection to airborne particulate when used in the context of a complete respirator program [60 Fed. Reg. 30336 (1995); NIOSH 2004]. In a survey designed to better understand health and safety practices in the carbonaceous nanomaterial industry, NIOSH found half-mask elastomeric particulate respirators fitted with HEPA filtration media to be the most commonly used respiratory protection, followed by disposable filtering facepiece respirators [Dahm et al. 2011].
The 2009 NIOSH Approaches to Safe Nanotechnology document as well as the Current Intelligence Bulletins on titanium dioxide and carbon nanotubes contain recommendations on respirator use and selection when working with nanoparticles. Recommendations from other organizations and a discussion of the scientific rationale for respirator selection have been reviewed [Shaffer and Rengasamy 2009]. Current respirator performance research suggests that NIOSH’s traditional respirator selection tools apply to nanoparticles. NIOSH-certified respirators should provide the expected levels of protection, consistent with their assigned protection factor, and should be selected according to the NIOSH Respirator Selection Logic [NIOSH 2004] by the person who is in charge of the program and knowledgeable about the workplace and the limitations associated with each type of respirator. As part of the risk assessment process, respirators with 95-, 99-, or 100-class filters can be selected for workplaces with concentrations of nanoparticles near their MPPS (50 to 100 nm). Furthermore, NIOSH recommends that all elements of the OSHA Respiratory Protection Standard [29 CFR 1910.134] for both voluntary and required respirator use should be followed [63 Fed. Reg. 1152 (1998)].
Selection of respiratory protection for airborne particulate contaminants is typically done by dividing the measured or anticipated time-weighted average concentration of the airborne contaminant by the OEL and comparing that quotient to the respirator’s assigned protection factor (APF). Alternatively, the respirator’s APF can be multiplied by the OEL to find its maximum use concentration (MUC). The MUC is then compared to the TWA to select the appropriate respirator. In the absence of an OEL for nanoparticles, Woskie [2010] recommends that a health and safety professional “familiar with the workplace” choose the appropriate respirator based on goals for nanoparticle control, sampling results, and the capabilities of each type of respirator.
The NIOSH respirator selection logic recommends (and it is mandated by OSHA where the use of respirators is required) that respirators in the workplace be used as part of a comprehensive respiratory protection program. The program should include written standard operating procedures; workplace monitoring; hazard-based selection; fit-testing and training of the user; procedures for cleaning, disinfection, maintenance, and storage of reusable respirators; respirator inspection and program evaluation; medical qualification of the user; and the use of NIOSH-certified respirators [NIOSH 2004]. ​Several studies have been conducted of respirator media filtration performance against nanoparticles. Many employers provide filtering facepiece respirators (FFRs) due to their common availability and low cost. One study of N95 FFRs showed penetration levels by nanoparticles in the size range of ~30 to 70 nm, which exceeded the 5% level allowed by NIOSH [Balazy et al. 2006]. A later study used two test methods (challenges using a monodisperse aerosol and a polydisperse aerosol similar to the NIOSH certification test) and compared particle penetration of N-95 FFRs [Rengasamy et al. 2007]. Those authors found that a monodisperse aerosol challenge test using particles from 20 nm to 400 nm in diameter resulted in a MPPS near 40 nm. The monodisperse test found that two respirators exceeded the NIOSH 5% allowed penetration, but the exceedance was not statistically significant, while the polydisperse challenge produced penetration levels from 0.61% to 1.24%. The NIOSH-allowed penetration level of < 0.03% was not exceeded by P100 FFRs, but two nanoparticle test aerosols did exceed 1% penetration for two N99 FFRs [Eninger et al. 2008; Rengasamy et al. 2009]. Five models of N95 and two models of P100 FFRs challenged with 4–30-nm monodisperse aerosols provided approved levels of protection [Rengasamy et al. 2008]. Rengasamy et al. [2009] tested two models each of N95 and P100 respirators with monodisperse aerosols in the 4–30-nm range and the 20–400-nm range. The penetration levels were less than the NIOSH-allowed levels of < 5% and < 0.03% across all test methods used. The penetration was < 4.28% for the N95 respirators and < 0.009% for the P100 respirators at the MPPS range of 30–60 nm.
NIOSH-certified FFRs have been shown to provide “expected levels of filtration efficiency against polydisperse and monodisperse aerosols > 20 nm in size” [Rengasamy and Eimer 2011]. A study showed that eight commercially purchased models of NIOSH-approved N95 and P100 and CE-marked FFR models “provided expected levels of laboratory performance against nanoparticles” [Rengasamy et al. 2009].