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NIOSH Safety and Health Topic:Nanotechnology |
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Approaches to Safe Nanotechnology:
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Given the limited information about the health risks associated with occupational exposure to engineered nanoparticles, work practices and engineering controls should be tailored to the processes and job tasks in which exposure might occur. For most processes and job tasks, the control of airborne exposure to nanoparticles can most likely be accomplished using a wide variety of engineering control techniques similar to those used in reducing exposures to general aerosols [Ratherman 1996; Burton 1997]. To ensure that the appropriate steps are taken to minimize the risk of exposure, a risk management program should be implemented. Elements of such a program should include the establishment of guidelines for installing and evaluating engineering controls (e.g., exhaust ventilation), the education and training of workers in the proper handling of nanomaterials (e.g., good work practices), and the development of procedures for selecting and using personal protective equipment (e.g., clothing, gloves, respirators).
In general, control techniques such as source enclosure (i.e., isolating the generation source from the worker) and local exhaust ventilation systems should be effective for capturing airborne nanoparticles, based on what is known of nanoparticle motion and behavior in air. The use of ventilation systems should be designed, tested, and maintained using approaches recommended by the American Conference of Governmental Industrial Hygienists [ACGIH 2001]. In light of current scientific knowledge about the generation, transport, and capture of aerosols, these control techniques should be effective for controlling airborne exposures to nanometer-scale particles [Seinfeld and Pandis 1998; Hinds 1999].
Current knowledge indicates that a well-designed exhaust ventilation system with a high- efficiency particulate air (HEPA) filter should effectively remove nanoparticles [Hinds 1999]. Filters are tested using particles that have the lowest probability of being captured (typically around 300 nm in diameter). It is expected that the collection efficiencies for smaller particles should exceed the measured collection efficiency at this particle diameter [Lee and Liu 1982]. NIOSH is conducting research to validate the efficiency of HEPA filter media used in environmental control systems and in respirators in removing nanoparticles. As results of this research become available, they will be posted on the NIOSH Web site.
If HEPA filters are used in the dust collection system, they should be coupled to a well-designed filter housing. If the filter is improperly seated, nanoparticles have the potential to bypass the filter, leading to filter efficiencies much less than predicted [NIOSH 2003].
The incorporation of good work practices in a risk management program can help to minimize worker exposure to nanomaterials. Examples of good practices include the following:
Currently, no guidelines are available on the selection of clothing or other apparel for the prevention of dermal exposure to nanoparticles. Published research has shown that penetration efficiencies for 8 widely different fabrics (including woven, non-woven, and laminated fabrics) against 0.477 µm particles range from 0.0 % to 31%, with an average of 12% [Shalev et al. 2000]. Penetration efficiencies for nanoparticles have not been studied. However, even for powders in the macro scale, it is recognized that skin protective equipment (i.e. suits, gloves and other items of protective clothing) is very limited in its effectiveness to reduce or control dermal exposure [Schneider et al. 2000]. In any case, although nanoparticles may penetrate the epidermis, there has been little work to suggest that penetration leads to disease; and no dermal exposure standards have been proposed.
Some existing clothing standards already incorporate testing with nanometer-sized particles, and therefore provide some indication of the effectiveness of protective clothing to nanoparticles. For instance, ASTM standard F1671–03 specifies the use of a 27-nm bacteriophage to evaluate the resistance of materials used in protective clothing from the penetration of bloodborne pathogens [ASTM Subcommittee F23.40 2003].
NIOSH plans to conduct laboratory research on test methods to determine particle penetration through fabrics used into protective clothing and ensembles. As results from this research become available, they will be posted to the NIOSH website.
The use of respirators is often required when engineering and administrative controls do not adequately keep worker exposures to an airborne contaminant below a regulatory limit or an internal control target. Currently, there are no specific exposure limits for airborne exposures to engineered nanoparticles although occupational exposure limits and guidelines (e.g., OSHA, NIOSH, ACGIH) exist for larger particles of similar chemical composition. Current scientific evidence indicates that nanoparticles may be more biologically reactive than larger particles of similar chemical composition and thus may pose a greater health risk when inhaled. In determining the effectiveness of controls or the need for respirators, it would therefore be prudent to consider both the current exposure limits or guidelines (e.g., PELs, RELs, TLVs) and the increase in surface area of the nanoparticles relative to that of particles for which the exposure limits or guides were developed.
The decision to institute respiratory protection should be based on a combination of professional judgment and the results of the hazard assessment and risk management practices recommended in this document. The effectiveness of administrative, work practice, and engineering controls can be evaluated using the measurement techniques described in Exposure Assessment and Characterization. If worker airborne exposure to nanoparticles remains a concern after instituting measures to control exposure, the use of respirators can further reduce worker exposures. Several classes of respirators exist that can provide different levels of protection when properly fit tested on the worker. Table 1 lists various types of particulate respirators that can be used; information is also provided on the level of exposure reduction that can be expected from each and the advantages and disadvantages of each respirator type. To assist respirator users, NIOSH has published the document NIOSH Respirator Selection Logic (RSL) that provides a process that respirator program administrators can use to select appropriate respirators (see www.cdc.gov/niosh/docs/2005-100/default.html). As new toxicity data for individual nanomaterials become available, NIOSH will review the data and make recommendations for respirator protection.
When respirators are required to be used in the workplace, the Occupational Safety and Health Administration (OSHA) respiratory protection standard [29 CFR 1910.134] requires that a respiratory program be established that includes the following program elements: (1) an evaluation of the worker’s ability to perform the work while wearing a respirator, (2) regular training of personnel, (3) periodic environmental monitoring, (4) respirator fit testing, and (5) respirator maintenance, inspection, cleaning, and storage. The standard also requires that the selection of respirators be made by a person knowledgeable about the workplace and the limitations associated with each type of respirator. OSHA has also issued guidelines for employers who choose to establish the voluntary use of respirators [29 CFR 1910.134 Appendix D].
Table 1 lists the NIOSH assigned protection factors (APF) for various classes of respirators. The APF is defined as the minimum anticipated protection provided by a properly functioning respirator or class of respirators to a given percentage of properly fitted and trained users. The APF values developed by NIOSH are based in part on laboratory studies and take into consideration a variety of factors including the inward leakage caused by penetration through the filter and leakage around the face seal of the respirator. NIOSH is not aware of any data specific to the face seal leakage of nanoparticles. Numerous studies have been conducted on larger particles and on gases/vapors. For example, work done by researchers at the U.S. Army RDECOM on a head-form showed that mask leakage (i.e., simulated respirator fit factor) measured using submicron aerosol challenges (0.72 µm polystyrene latex spheres) was representative of vapor challenges such as sulfur hexafluoride (SF6) and isoamyl acetate (IAA) [Gardner et al, 2004]. NIOSH plans to conduct a laboratory study to determine whether nanoparticle face seal leakage is consistent with the leakage seen by larger particles and gases/vapors. As results from this research become available, they will be posted to the NIOSH website.
NIOSH tests and certifies the filtration performance of air purifying respirators. One NIOSH certification test uses a polydisperse distribution of NaCl particles with a count median diameter (CMD) of 0.075 +/- 0.020 µm and a geometric standard deviation (GSD) of less than 1.86 for N- designated respirators [NIOSH, 2005a]. For R- and P- designated respirators, NIOSH tests using a polydisperse distribution of dioctyl phthalate (DOP) particles with a CMD of 0.185 +/- 0.020 µm and a GSD of less than 1.60 [NIOSH, 2005b]. For the lognormal distribution of NaCl aerosols used in the certification test, a broad range of particle sizes (e.g., 95% of the particles lie in the range of 22 nm – 259 nm) with a mass median diameter (MMD) of about 0.24 µm (or 240 nm) is used to determine whether the respirator filter performance is at least 95%, 99%, or 99.97% efficient. All of the particles penetrating through the filter are measured simultaneously using a forward light scattering photometer. According to single fiber filtration theory, particles larger than 0.3 µm are collected most efficiently by impaction, interception, and gravitational settling, while particles smaller than 0.3 µm are collected most efficiently by diffusion or electrostatic attraction [Hinds 1999]. Penetration of approximately 0.3-µm particles represents the worst case because these particles are considered to be in the range of the most penetrating particle size [Stevens and Moyer 1989, TSI 2005; NIOSH 1996]. However, the most penetrating particle size range for a given respirator can vary based on the type of filter media employed and the condition of the respirator. For example, the most penetrating particle size for N95 respirators containing electrostatically charged filter media can range from 50-100 nm [Martin and Moyer, 2000; Richardson et al, 2005] to 30-70 nm [Balazy et al, 2006].
According to single fiber filtration theory, below the most penetrating particle size, filtration efficiency will increase as particle size decreases. This trend will continue until the particles are so small that they behave like vapor molecules. As particles approach molecular size, they may be subject to thermal rebound theory, in which particles literally bounce through a filter. As a result, particle penetration will increase. The exact size at which thermal rebound will occur has not been reported in the literature. However, a recent study by Heim et al [2005] found that there was no discernable deviation from classical single-fiber theory for particles as small as 2.5 nm diameter. NIOSH recently funded a contract with the University of Minnesota to study the collection efficiency of respirator filter media for particles in the 3-100 nm range. In this study, the researchers observed that penetration of nanoparticles through filter media decreased down to 3 nm as expected by traditional filtration theory [Pui and Kim, 2006]. No evidence for thermal rebound of nanoparticles in the size ranges studied was found. Based on these preliminary findings, NIOSH certified respirators should provide the expected levels of protection. NIOSH plans to continue studying the nanoparticle collection efficiency of NIOSH certified respirators to validate these findings. As results from this research become available, they will be posted to the NIOSH website.
Respirator type |
NIOSH |
Advantages |
Disadvantages |
Cost |
Filtering facepiece (disposable) |
10 |
– Lightweight |
|
$0.70 to $10 |
Elastomeric half-facepiece |
10 |
– Low maintenance |
– Provides no eye protection |
Facepiece: $12 to $35 |
Powered with loose-fitting facepiece |
25 |
– Provides eye protection |
– Added weight of battery and blower |
Unit: $400 to $1,000 |
Elastomeric full-facepiece with N-100, R-100, or P-100 filters |
50 |
– Provides eye protection |
– Can add to heat burden |
Facepiece: $90 to $240 |
Powered with tight-fitting half-facepiece or full-facepiece |
50 |
–Provides eye protection with full-facepiece |
–Added weight of battery and blower |
Unit: $500 to $1,000 |
Note: The assigned protection factors in this table are from the NIOSH Respirator Selection Logic [NIOSH 2004]. When the table was prepared, OSHA had proposed amending the respiratory protection standard to incorporate assigned protection factors. The Internet sites of NIOSH (www.cdc.gov/niosh) and OSHA (www.osha.gov) should be periodically checked for the current assigned protection factor values. |
No specific guidance is currently available on cleaning up nanomaterial spills or contaminated surfaces. Until relevant information is available, it would be prudent to base strategies for dealing with spills and contaminated surfaces on current good practices, together with available information on exposure risks and the relative importance of different exposure routes. Standard approaches to cleaning up powder and liquid spills include the use of HEPA-filtered vacuum cleaners, wetting powders down, using dampened cloths to wipe up powders and applying absorbent materials/liquid traps.
Damp cleaning methods with soaps or cleaning oils is preferred. Cleaning cloths should be properly disposed. Drying and reuse of contaminated cloths can result in re-dispersion of particles. Use of commercially available wet or electrostatic microfiber cleaning cloths may also be effective in removing particles from surfaces with minimal dispersion into the air.
Energetic cleaning methods such as dry sweeping or the use of compressed air should be avoided or only be used with precautions that assure that particles suspended by the cleaning action are trapped by HEPA filters. If vacuum cleaning is employed, care should be taken that HEPA filters are installed properly and bags and filters changed according to manufacturer’s recommendations.
While vacuum cleaning may prove to be effective for many applications, the following issues should be considered. Forces of attraction may make it difficult to entrain particles off surfaces with a vacuum cleaner. The electrostatic charge on particles will cause them to be attracted to oppositely charge surfaces and repelled by similarly charged surfaces. An oppositely charged vacuum brush or tool may repel particles, making it difficult to capture the aerosol or even causing it to be further dispersed. Vigorous scrubbing with a vacuum brush or tool or even the friction from high flow rates of material or air on the vacuum hose can generate a charge. The vacuum cleaners recommended for cleaning copier and printer toners have electrostatic-charge-neutralization features to address these issues.
When developing procedures for cleaning up nanomaterial spills or contaminated surfaces, consideration should be given to the potential for exposure during cleanup. Inhalation exposure and dermal exposure will likely present the greatest risks. Consideration will therefore need to be given to appropriate levels of personal protective equipment. Inhalation exposure in particular will be influenced by the likelihood of material re-aerosolization. In this context, it is likely that a hierarchy of potential exposures will exist, with dusts presenting a greater inhalation exposure potential than liquids, and liquids in turn presenting a greater potential risk than encapsulated or immobilized nanomaterials and structures.
As in the case of any material spill or cleaning of contaminated surfaces, handling and disposal of the waste material should follow exiting Federal, State, or local regulations.
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