Figure 1. Assessment of nanoparticle capture: n = 5; error bars represent standard deviations Sodium Chloride (TSI 3160); Silver (custom-built); Flow rate 85 L/min
Introduction
Each day millions of workers in the United States use National Institute for Occupational Safety and Health (NIOSH) certified respirators to reduce exposure to harmful gases, vapors, and particulate hazards. NIOSH has certification, quality assurance, and auditing procedures in place (42 CFR Part 84) that assure purchasers and users that the products they are buying/using have been tested and manufactured to strict standards. When selected, maintained and used in the context of an Occupational Safety and Health Administration (OSHA)-compliant respiratory protection program, in which personal protective technology is part of the hierarchy of controls to protect the worker, respirator users can expect that their respirator is working and reducing the amount of hazards that they could potentially breathe. However, as new hazards emerge, the applicability of the science that NIOSH uses to base respirator test methods, performance requirements, and use recommendations needs to be continually reaffirmed, updated and improved to assure the expected level of protection is provided.
One such emerging hazard is engineered nanoparticles. Engineered nanoparticles are materials with at least one dimension between 1 and 100 nanometers. Because of their distinctive physical and chemical properties, little is known about what possible health effects these properties may have on workers. Previous NIOSH Science Blogs (see the list on the right) and the NIOSH website discussed the health concerns of engineered nanoparticles. Because of these concerns, NIOSH recommends limiting worker exposures to engineered nanoparticles through standard industrial hygiene practices, including respiratory protection (when needed). Although the NIOSH recommended exposure limits (REL) for some types of nanoparticles are based on mass, particle count (number) may be a more significant concern because of their distinctive physical and chemical properties (e.g., increased surface area and reactivity).
The purpose of this blog is to provide an update on the science and rationale behind NIOSH’s recommendations for the use and selection of respirators against engineered nanoparticles.
Respirator Performance Research
While NIOSH has been certifying and conducting research for decades to assure that particulate respirators provide predictable levels of exposure reduction, engineered nanoparticles present new challenges. For example, the current NIOSH certification test for filtration performance (to determine whether a respirator is at least 95, 99, or 99.97% efficient) uses a broad range of particles, including a significant number of nanoparticles, but only measures particles larger than ~100 nm in size penetrating through the filter. In general, because of the challenges in generating and measuring nanoparticles, little research had been done to assess nanoparticle filter penetration and leakage around the face seal area of the respirator. In particular, concerns had been raised that due to their small size, engineered nanoparticles would penetrate through respirators at higher rates than larger particles (so called “thermal rebound” effects).
In 2005, NIOSH initiated a laboratory research program to better understand respirator performance against nanoparticles. As part of that project, NIOSH researchers constructed test systems to generate aerosol challenges as small as 4 nanometers to determine filtration properties of NIOSH-certified and EU marked respirators as well as non-certified dust masks. These experiments were conducted under aggressive conditions (e.g., high flow rates, charge neutralized particles, etc) to assess worst-case situations. Much of this work has been reviewed recently by Shaffer & Rengasamy 2009. In a subsequent study, respirators were donned on a manikin and various sized controlled leaks were created in the respirator face seal to assess whether nanoparticles preferentially leak compared to larger particles (Rengasamy, AOH 2011). In general, these studies found that:
- As predicted by single fiber filtration theory, 4 to 20 nanometer particles were captured very efficiently by respirator filter media, because these small particles are constantly bombarded by air molecules, which causes them to deviate from the airstream and come into contact with a filter fiber to become trapped by the filter;
- The most penetrating particle size (MPPS) range (shown in Figure 1) for electret filter media (the most common type of filter used in respirators on the market today) was between 30 and 100 nanometers, with 100-class respirators having higher levels of laboratory filtration performance compared to 95-class respirators; and
- Leak size was the largest factor affecting the number of nanoparticles inside the facepiece of the respirator worn on the manikin, although for small leaks nanoparticles were more likely than larger particles to be found inside the facepiece of disposable N95-class filtering facepiece respirators.
Respirator Selection
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. The respirator performance research discussed above suggests that NIOSH’s traditional respirator selection tools apply to nanoparticles. There are several types of NIOSH certified respirators (e.g., disposable filtering facepiece, half-mask elastomeric, full facepiece, powered, airline, self-contained, etc.) that can provide different levels of expected protection to airborne particulate when used in the context of a complete respirator program. In a survey to better understand health and safety practices in the carbonaceous nanomaterial industry, NIOSH found half-mask elastomeric particulate respirators to be the most commonly used respiratory protection followed by disposable filtering facepiece respirators. However, this application of respiratory protection appears based on subjective assessment of hazard and risk to high aspect ratio carbonaceous nanomaterials and was before the proposed NIOSH RELs for carbon nanotubes/carbon nanofibers were established.
The 2009 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. With the establishment of the proposed RELs for TiO2 and carbon nanotubes, respirators should be selected according to the NIOSH Respirator Selection Logic (RSL 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 99 or 100-class filters can be selected for workplaces with high concentrations of nanoparticles near their MPPS (50 to 100 nanometers). 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.
Next Steps
The research done by NIOSH to date has been done in laboratory settings using filtration test systems and manikins under aggressive test conditions. To further validate that its recommendations on respirator use against nanoparticles are optimal, further research is needed in field settings and using human test subjects. Well-designed studies on face seal leakage of nanoparticles, especially workplace protection factor (WPF) studies that validate assigned protection factor (APF) levels for respirators against nanoparticles will be important. Such studies are currently underway.
Ziqing Zhuang, PhD, is the Respiratory Protection Research Team Leader in the Technology Research Branch in the NIOSH National Personal Protective Technology Laboratory (NPPTL).
Mr. Viscusi is a physical scientist in the Technology Research Branch in the NIOSH National Personal Protective Technology Laboratory (NPPTL).
Nano on the website
Approaches to safe nanotechnology
TiO2: Current Intelligence Bulletin
Works Cited
- Dahm, M. M., M. S. Yencken, et al. (2011). “Exposure Control Strategies in the Carbonaceous Nanomaterial Industry.” Journal of Occupational and Environmental Medicine 53(6): S68-S73.
- NIOSH (1995) Respiratory protective devices; final rule and notices. Fed Regist 60(110):30335–30404. Accessed on Sept. 7, 2011.
- NIOSH (2004) NIOSH respirator selection logic. DHHS (NIOSH) Publication No. 2005-100. Cincinnati, OH. http://www.cdc.gov/niosh/docs/2005-100/. Accessed on Sept. 7, 2011.
- NIOSH (2009). “Approaches to Safe Nanotechnology: Managing the Health and Safety Concerns Associated with Engineered Nanomaterials.” NIOSH Publication No. 2009-125. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health. Accessed on Sept. 7, 2011.
- NIOSH. NIOSH Current Intelligence Bulletin: Occupational Exposure to Carbon Nanotubes and Nanofibers. November 2010 draft. DHHS (NIOSH) Publication, in draft.
- NIOSH. NIOSH Current Intelligence Bulletin 63: Occupational Exposure to Titanium Dioxide. DHHS (NIOSH) Publication No. 2011160.
- OSHA (1998) Respiratory protection—OSHA. Fed Regist 63:1152–1300.
- Rengasamy, S. and B. C. Eimer (2011). “Total Inward Leakage of Nanoparticles Through Filtering Facepiece Respirators.” Annals of Occupational Hygiene 55(3): 253-263.
- Shaffer, R. E. and A. Rengasamy (2009). “Respiratory protection against airborne nanoparticles:a review.” J Nanopart Res 11:1661–1672.
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Public Comments
Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».
December 7, 2011 at 2:46 pm ET - Mark Levin
What confidence do we have that these filtering devices will remove nanoparticles which are much much smaller than 0.3 microns?
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AUTHOR COMMENT December 14, 2011 at 11:03 pm ET - Ziqing Zhuang and Dennis Viscusi
Thanks for the question, Mark. We are confident that nanoscale particles (much smaller than 0.3 micrometers) will be trapped by these filters, based on our scientific understanding of filtration performance of particulate filters and respirators for a wide range of particle sizes. Particles larger than 0.3 micrometers are collected by being intercepted, impacted on or settled on the filter material. Particles smaller than 0.3 micrometers are efficiently captured by diffusion mechanisms and trapped by their electrostatic properties. (See: “How do filters collect particles?.”) In addition to this theory-based answer, NIOSH did experiments and showed that particles down to 4 nanometers are captured with an efficiency greater than the rated efficiency of N95 and P100 filters on which they were tested; for example see Figure 1 in this blog (Shaffer and Rengasamy,2009).
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December 15, 2011 at 8:10 am ET - James Allen
I’m lost on the scale of nanoparticles.
one micrometer or micron is 1,000 nanometer. Is that correct?
A TB bacillus can be as small as 3 microns or 3,000 nanometer. The upper respriatory tract, including the turbinates in the nose can capture 50% of particles greater than 10 microns or 10,000 nanoparticles.
Do I have my sizes correct?
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AUTHOR COMMENT December 19, 2011 at 9:37 pm ET - Ziqing Zhuang
Yes, you are correct. One micrometer is 1,000 nanometers.
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December 17, 2011 at 5:43 am ET - Anthony Myers
The most important defence against respiratory hazards is to control the contamination at its source and prevent it from entering the air. This can be done by either substituting dangerous substances with less hazardous ones (particularly where chemicals are used), by isolating or enclosing hazardous operations, or by providing adequate local exhaust ventilation.
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December 20, 2011 at 2:33 pm ET - Paul Jordan
It is necessary to develop a complete respiratory protection program to make sure that the respirators will protect workers and that wearing them does not harm them.
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January 14, 2012 at 4:20 am ET - Richard Webb
Are nano particles as dangerous as asbestos fibres and can fire fighters be exposed via products of combustion normally encountered at structure or car fires?
Breathing apparatus will protect sensitive lung tissue during deployment however my concern is exposures post incident tasking. Is it possible that the numerous known toxic products of combustion may be present in PPE, in trucks and in living environments via aerosol and contamination routes?
If these toxic products were to be collected via station vacuum cleaners, could they be distributed around living areas when using the same vacuum cleaner to clean living areas? Is it appropriate to isolate truck cleaning machines from station cleaning machines?
Nano particles, asbestos and synthetic mineral fibres are all emerging as highly toxic products of combustion with fatal consequences. How should these risks be minimised?
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March 19, 2012 at 3:34 pm ET - Ziqing Zhuang
Are nanoparticles as dangerous as asbestos fibers?
Carbon nanotubes have fiber-like shapes and have been studied to see if they behave the same as asbestos. So far there is no direct evidence that carbon nanotubes present the same lung cancer health hazard as asbestos. However, adverse effects have been observed in recent animal studies; it would be prudent to limit exposure where possible.
Is it appropriate to isolate truck cleaning machines from station cleaning machines?
Yes. It is always a prudent practice to segregate cleaning equipment. NIOSH has recommended to nanoparticle manufacturing companies that they use a combination of wet wiping and vacuum cleaning to remove nanoparticles from surfaces. Canister-type vacuum cleaners equipped with HEPA filter cartridges of bags are efficient in collecting nanoparticles. Protective equipment, such as respirator and gloves, should be used when emptying the vacuum cleaner.
Nano particles, asbestos and synthetic mineral fibers are all emerging as highly toxic products of combustion with fatal consequences. How should these risks be minimized?
Using proper protective equipment and following decontamination/cleaning procedures, as described in NFPA 1851 should minimize the risk.
Chuck Geraci,Tom Hales, and Vladmir Murashov contributed to this response.
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February 22, 2012 at 12:37 am ET - Jon at www.imc.cc
Found this information…to collaborate and compare the results of this testing:
IRSST – Engineered Nanoparticles: Current Knowledge about Occupational Health and Safety Risks and
Prevention Measures
77
In most situations, unless contraindicated after a detailed analysis of the risks in the work environment, wearing an SAR respirator with P100 cartridge should offer adequate protection. If it is impossible to perform a quantitative risk analysis, the IRSST recommends considering NP dusts to be highly toxic and favours wearing an SAR respirator with very high-performance filters for all potential exposure situations. In situations where this level of protection is still insufficient or there
is an immediate risk to the worker’s health or life, supplied-air respirators or self-contained breathing apparatuses allow maximum protection.
•IRSST. (September 2010). Engineered Nanoparticles: Current Knowledge about OHS Risks and Prevention Measures, 2nd Edition. Report.
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March 12, 2012 at 8:49 am ET - Dwight Norris
The information about particular nanoparticles is very interesting. I have wanted to know the particulars between N95 and P100 respirators and cartridges when it comes to stopping certain particulates in the air. I own a respirator that I used HEPA Respirator), which is for just this type of application. It really comes into the process when individuals have to deal with hazardous gases and particulates in the air all day long. The life of the air filter is rapidly deteriorated and I think the breathing of that person changes over time.
I really have a hard time of approving anything that doesn’t give me 100% because a person’s life is worth more than a few hours on the job. I still don’t understand why people still work so haphazardly in mine shafts across the country. Not only is it dangerous by getting trapped, but you are forced to breathe sparse air that has many particulates and random gases running through it in high concentration.
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March 20, 2012 at 1:15 pm ET - OSHA Respiratory Protection Program
Workers should use respirators for protection from contaminants in the air only if other hazard control methods are not practical or possible under the circumstances. Respirators should not be the first choice for respiratory protection in workplaces.
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