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
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.
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.
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, and Dennis Viscusi
Dr. Zhuang 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).