"Worst case" aerosol testing parameters: II. Efficiency dependence of commercial respirator filters on humidity pretreatment

Previous studies have shown that relative humidity has a degrading effect on the performance of commercially available particulate air-purifying respirator filters. That degradation results from a reduction of charge within the filter. This study was done to evaluate the time-dependent effects of relative humidity pretreatment and the reduction of charge on filter penetration against a most penetrating, "worst case" aerosol challenge. Filters of the dust and mist; dust, fume, and mist; paint, lacquer, and enamel mist; and high efficiency types were tested after being pretreated in an environment of 38 degrees C and 85% relative humidity for periods up to 42 days. After various intervals of pretreatment (1, 7, 14, 28, and 42 days), the filters were tested against neutralized worst-case sodium chloride (NaCl) and dioctyl phthalate (DOP) aerosols for percent penetration. The results showed a drop in filter efficiency of approximately 2%-6% depending on preconditioning time, except for the high efficiency filters tested which showed no detectable change.     

"Worst-case" aerosol testing parameters: III. Initial penetration of charged and neutralized lead fume and silica dust aerosols through clean, unloaded respirator filters

The National Institute for Occupational Safety and Health (NIOSH) tests and certifies respirator filter media according to Title 30, Code of Federal Regulations, Part 11 (30 CFR 11). Subpart K of those regulations specifies that a silica dust test, silica mist test, and/or lead fume test will be used to test and certify dust and mist; and dust, fume, and mist particulate air-purifying respirator filter media. NIOSH studies have shown that an aerosol particle of a certain size can be identified as the most penetrating particle ("worst case") size. Commercial filter media of various types have been studied and the filter's performance against a worst-case sodium chloride (NaCl) and dioctyl phthalate (DOP) aerosol evaluated. This investigation was done to complement those previous studies by determining how one manufacturer's particulate filters performed against the existing certification aerosol challenges as compared with the worst-case size DOP and NaCl aerosols. Only initial penetration values were determined, and no loading effects were considered. Both neutralized (Boltzman charge distribution) and unneutralized aerosols were used in order to assess the contribution of charging. The results show the dramatic effect of particle size on filter efficiency, and they show that the present methods are not as sensitive as the worst-case aerosol method.     

Comparison of pressure drop and filtration efficiency of particulate respirators using welding fumes and sodium chloride

Respirators are used to help reduce exposure to a variety of contaminants in workplaces. Test aerosols used for certification of particulate respirators (PRs) include sodium chloride (NaCl), dioctyl phthalate, and paraffin oil. These aerosols are generally assumed to be worst case surrogates for aerosols found in the workplace. No data have been published to date on the performance of PRs with welding fumes, a hazardous aerosol that exists in real workplace settings. The aim of this study was to compare the performance of respirators and filters against a NaCl aerosol and a welding fume aerosol and determine whether or not a correlation between the two could be made. Fifteen commercial PRs and filters (seven filtering facepiece, two replaceable single-type filters, and six replaceable dual-type filters) were chosen for investigation. Four of the filtering facepiece respirators, one of the single-type filters, and all of the dual-type filters contained carbon to help reduce exposure to ozone and other vapors generated during the welding process. For the NaCl test, a modified National Institute for Occupational Safety and Health protocol was adopted for use with the TSI Model 8130 automated filter tester. For the welding fume test, welding fumes from mild steel flux-cored arcs were generated and measured with a SIBATA filter tester (AP-634A, Japan) and a manometer in the upstream and downstream sections of the test chamber. Size distributions of the two aerosols were measured using a scanning mobility particle sizer. Penetration and pressure drop were measured over a period of aerosol loading onto the respirator or filter. Photos and scanning electron microscope images of clean and exposed respirators were taken. The count median diameter (CMD) and mass median diameter (MMD) for the NaCl aerosol were smaller than the welding fumes (CMD: 74 versus 216 nm; MMD: 198 versus 528 nm, respectively). Initial penetration and peak penetration were higher with the NaCl aerosol. However, pressure drop increased much more rapidly in the welding fume test than the NaCl aerosol test. The data and images clearly show differences in performance trends between respirator models. Therefore, general correlations between NaCl and weld fume data could not be made. These findings suggest that respirators certified with a surrogate test aerosol such as NaCl are appropriate for filtering welding fume (based on penetration). However, some respirators may have a more rapid increase in pressure drop from the welding fume accumulating on the filter. Therefore, welders will need to choose which models are easier to breathe through for the duration of their use and replace respirators or filters according to the user instructions and local regulations.     

Electrostatic respirator filter media: filter efficiency and most penetrating particle size effects

New electrostatic filter media has been developed for use in 42 CFR 84 negative pressure particulate respirator filters. This respirator filter media was not available for evaluation prior to the change from 30 CFR 11 to 42 CFR 84. Thus, characterization of this filter media is warranted. In this study, the new 42 CFR 84 electrostatic respirator filters were investigated with respect to filter penetration and most penetrating particle size. Three different models of N95 filters, along with one model each of the N99, R95, and P100 class filters were used in this study. First, three of each filter were loaded with a sodium chloride (NaCl) aerosol, and three of each filter were loaded with a dioctyl phthalate (DOP) aerosol to obtain normal background penetration results for each filter. Then, two new filters of each type were dipped in isopropanol for 15 seconds and allowed to dry. This isopropanol dip should reduce or eliminate any electrostatic charge on the fibers of each filter, as reported in the technical literature. These dipped filters, along with controls of each filter type, were tested on a TSI 8160 filter tester to determine the most penetrating particle size. These same filters were then tested against a NaCl aerosol to get final penetration values. Electret filters rely heavily on their electrostatic charge to provide adequate filter efficiencies, and correlations between penetration and a filter's electrostatic characteristics are found in the technical literature. In all six of the filter models tested, filter penetration values increased considerably and the most penetrating particle size noticeably shifted toward larger particles. These results are important in better understanding how these new filter materials perform under various conditions, and they indicate the need for additional research to define environmental conditions that may affect electrostatic filter efficiency.     

Penetration of respirator filters by an asbestos aerosol

Five models of respirator filters were challenged with a chrysotile aerosol, and the asbestos fiber penetration of the media was measured in five experimental series simulating different environmental conditions. Penetration of the facial seal was not evaluated. The penetration of each filter was determined from the ratio of the downstream to upstream concentrations of airborne asbestos fibers as measured by the standard National Institute for Occupational Safety and Health (NIOSH) asbestos fiber counting method P&CAM 239. Penetrations measured in this project and presented are indicative of penetrations of filters used a) fresh from the package, b) after exposure to an organic oil mist, c) after storage at high humidity at room and elevated temperatures, and d) after preexposure to water mist. Penetrations varied between less than 0.1% and 2.7% for filters tested fresh from the package and up to 18% after exposure to simulated field/environmental conditions. The data indicate that each of the filter models tested, except for the high efficiency filter, was affected adversely to some degree by one or more of the simulated field/environmental conditions.     

Filter quality of pleated filter cartridges

The performance of dust cartridge filters commonly used in dust masks and in room ventilation depends both on the collection efficiency of the filter material and the pressure drop across the filter. Currently, the optimization of filter design is based only on minimizing the pressure drop at a set velocity chosen by the manufacturer. The collection efficiency, an equally important factor, is rarely considered in the optimization process. In this work, a filter quality factor, which combines the collection efficiency and the pressure drop, is used as the optimization criterion for filter evaluation. Most respirator manufacturers pleat the filter to various extents to increase the filtration area in the limit space within the dust cartridge. Six sizes of filter holders were fabricated to hold just one pleat of filter, simulating six different pleat counts, ranging from 0.5 to 3.33 pleats cm(-1). The possible electrostatic charges on the filter were removed by dipping in isopropyl alcohol, and the air velocity is fixed at 100 cm s(-1). Liquid dicotylphthalate particles generated by a constant output atomizer were used as challenge aerosols to minimize particle loading effects. A scanning mobility particle sizer was used to measure the challenge aerosol number concentrations and size distributions upstream and downstream of the pleated filter. The pressure drop across the filter was monitored by using a calibrated pressure transducer. The results showed that the performance of pleated filters depend not only on the size of the particle but also on the pleat count of the pleated filter. Based on filter quality factor, the optimal pleat count (OPC) is always higher than that based on pressure drop by about 0.3-0.5 pleats cm(-1). For example, the OPC is 2.15 pleats cm(-1) from the standpoint of pressure drop, but for the highest filter quality factor, the pleated filter needed to have a pleat count of 2.65 pleats cm(-1) at particle diameter of 122 nm. From the aspect of filter quality factor, this study suggests that the respirator manufacturers should add approximately 0.5 pleats cm(-1) to the OPC derived from the generalized correlation curve for pleated filter design based on minimum pressure drop.     

Performance of dust respirators with facial seal leaks: II. Predictive model

A performance model for half-mask and single-use respirators is presented. It represents a possible alternative to field measurements of respirator performance. Experimental data on filter and leak performance given in Part I were used to develop a model that allows one to predict 1) the overall respirator penetration as a function of particle size for any work rate and 2) overall total mass penetration for any work rate and exposure aerosol-size distribution for a known respirator filter and facial seal leak condition. A simplified method based on general regression equations is presented that allows one to estimate these quantities based on QNFT (quantitative fit testing) measurements and a knowledge of the exposure aerosol-size distribution. Example calculations are given for a situation in which QNFT gives a fit factor of 50 for a half-mask with dust, fume and mist filter cartridges, but predicted protection factors for various use conditions range from 20 to 81 depending on exposure particle-size distribution and work rate of the wearer.     

Performance of respirator filters using quality factor in Korea

A respirator filter of good quality has not only high aerosol collection efficiency but also low air resistance. "Quality factor", which is expressed with aerosol penetration and pressure drop, can be used to rank the performance of respirator filters within the same category. This study focuses on evaluating several respirator filters which are widely used in Korea using quality factor. Two mechanical filters and three filtering facepieces made by different manufacturers were measured aerosol penetrations and pressure drops by an automatic filter tester (CertiTest Model 8110, TSI Inc., St. Paul, USA) at four flow rates of 10, 32, 64 and 85 L/min. NaCl aerosols used were reported to be mean size of 0.1 microm and geometric standard deviation of <1.9. The penetrations and pressure drops of all filters have strong flow rate dependency. The filter quality factors decrease sharply as flow rates are increased. The mechanical filter S and filtering facepiece M are shown better filter quality than others in the same category. Since some certified filters are found to be inappropriate in the workplace exposed to fume, this result suggests that the current certified filter test protocol for respirators should be changed for the new protocol using smaller aerosols.     

Aerosol penetration through filtering facepieces and respirator cartridges.

Air-purifying respirators must be certified following the National Institute for Occupational Safety and Health (NIOSH) filter test criteria (30 CFR 11). The criteria specify a range for the mean particle size and the measure of spread permissible for the test aerosol. The authors' experiments have shown that aerosol penetration as a function of particle size differs considerably among certified respirators of the same type. Filtering facepieces (disposable respirators) and cartridges of the dust-mist, dust-mist-fume, and high-efficiency particulate air type were tested. The respirators were sealed to mannequins in a test chamber. The aerosol concentrations inside and outside the respirator were measured by an aerodynamic particle sizer and a laser aerosol spectrometer over a particle size range of 0.1 to 15 microns. Five flow rates ranging from 5 to 100 L/min were used to study flow dependency. The aerosol penetration through the filters is presented as a function of particle size. Aerosol penetration and pressure drop are combined to express the performance of each filter in terms of "quality factor." Under the same test conditions, the quality factor of one respirator may be as much as 6.6 times more than that of another respirator of the same type. The filter quality factor has a greater aerosol size dependency as airflow and aerosol size increase. In general, cartridges have a larger surface area than filtering facepieces but not necessarily lower filter penetration or higher filter quality.(ABSTRACT TRUNCATED AT 250 WORDS)     

Collection efficiency of respirator filters challenged with monodisperse latex aerosols

Collection efficiency was evaluated for 10 respirator manufacturers' electrostatically-charged dust/mist filters challenged with eight sizes of latex spheres in a range representative of the silica aerosol used in National Institute for Occupational Safety and Health (NIOSH) certification tests. Minimum efficiency occurred at or below the smallest size of 0.102 microns. Appreciable differences were found in the performances of filters distributed by the 10 manufacturers. Filters produced by 1 manufacturer exhibited significantly lower collection efficiency than other filters tested; those produced by 2 manufacturers performed somewhat better than the others tested. Statistical evaluation of Weibull-transformed data using analyses of covariance and Tukey's significant difference test allowed classification of the 10 manufacturers into three performance categories.     

Comparison of a sodium chloride aerosol filter test method to silica-dust and silica-mist filter test methods.

A LASL-developed sodium chloride aerosol filter penetration test has been compared to the silica-dust and silica-mist tests used by NIOSH for respirator approval testing. Filter discs were made from two types of resin impregnated electrostatic felt filter materials and tested at LASL for resistance to air flow and sodium chloride aerosol penetration. Sets of these filters were sent to four respirator manufacturing companies, where silica-dust and silica-mist penetration tests were performed. One set of filters was sent to NIOSH Division of Physical Sciences and Engineering in Cincinnati where sodium chloride aerosol penetration tests were performed by NIOSH personnel.     

Advanced testing method to evaluate the performance of respirator filter media

Filter media for respirator applications are typically exposed to the cyclic flow condition, which is different from the constant flow condition adopted in filter testing standards. To understand the real performance of respirator filter media in the field it is required to investigate the penetration of particles through respirator filters under cyclic flow conditions representing breathing flow patterns of human beings. This article reports a new testing method for studying the individual effect of breathing frequency (BF) and peak inhalation flow rate (PIFR) on the particle penetration through respirator filter media. The new method includes the use of DMA (Differential Mobility Analyzer)-classified particles having the most penetrating particle size, MPPS (at the constant flowrate of equivalent mean inhalation flow rate, MIFR) as test aerosol. Two condensation particle counters (CPCs) are applied to measure the particle concentrations at the upstream and downstream of test filter media at the same time. Given the 10 Hz sampling time of CPCs, close-to-instantaneous particle penetration could be measured. A pilot study was performed to demonstrate the new testing method. It is found that the effect of BF on the particle penetration of test respirator filter media is of importance at all the tested peak inhalation flow rates (PIFRs), which is different from those reported in the previous work.     

What does respirator certification tell us about filtration of ultrafine particles?

Recent interest in exposures to ultrafine particles (less than 100 nm) in both environmental and occupational settings led the authors to question whether the protocols used to certify respirator filters provide adequate attention to ultrafine aerosols. The authors reviewed the particle size distribution of challenge aerosols and evaluated the aerosol measurement method currently employed in the National Institute for Occupational Safety and Health (NIOSH) particulate respirator certification protocol for its ability to measure the contribution of ultrafine particles to filter penetration. Also considered were the differences between mechanical and electrically charged (electret) filters in light of the most penetrating particle size. It was found that the sodium chloride (NaCl) and dioctylphthalate (DOP) aerosols currently used in respirator certification tests contain a significant fraction of particles in the ultrafine region. However, the photometric method deployed in the certification test is not capable of adequately measuring light scatter of particles below approximately 100 nm in diameter. Specifically, 68% (by count) and 8% (by mass) of the challenge NaCl aerosol particles and 10% (by count) and 0.3% (by mass) of the DOP particles below 100 nm do not significantly contribute to the filter penetration measurement. In addition, the most penetrating particle size for electret filters likely occurs at 100 nm or less under test conditions similar to those used in filter certification. The authors conclude, therefore, that the existing NIOSH certification protocol may not represent a worst-case assessment for electret filters because it has limited ability to determine the contribution of ultrafine aerosols, which include the most penetrating particle size for electret filters. Possible strategies to assess ultrafine particle penetration in the certification protocol are discussed.     

Characterization of paint dust aerosol generated from mechanical abrasion of TiO2-containing paints

The purpose of the study was to determine the potential for release of titanium dioxide nanoparticles in paint dust. The coatings aerosol resuspension system was developed and used for testing the generation and physical, chemical, and morphological properties of paint dust particles from mechanical abrasion (i.e., sanding) of coated wood surfaces. The paint dust emissions from bare and coated wood surfaces with multiple coatings using variable sandpaper grits were evaluated. Substantially higher particle number concentrations were measured for paint dust containing particles in the nano range (particles with aerodynamic diameter less than 100?nm) than those measured for wood dust. The variability of particle number concentration and size distribution of paint dust derived under different conditions indicated that considerable quantities of nanoparticles might be released from mechanical abrasion of painted surfaces that may induce unhealthy exposure conditions. Moreover, spectroscopic and microscopic analysis identified the presence of paint and wood components in paint dust, including titanium dioxide agglomerates that were originally embedded in the paint. The agglomerates were mostly attached to particles with sizes <100?nm, enabling them to potentially penetrate into the lower respiratory tract. These results demonstrated that the paint dust exposure generation system can provide qualitative and quantitative information on particle emissions and the abundance of nanoparticles from paint sanding in realistic conditions and they may be used to assess occupational and environmental exposures and risks. Furthermore, the prevalence of titanium dioxide nanoparticles in paint dust highlights the potential for exposures of painters and other occupational groups to hazardous paint dust and the need for protective devices and strategies aiming to reduce exposures to nanoparticles.     

The effect of aerosol size distribution and measurement method on respirator fit

The particle size-dependent leakage into a respirator was examined by measuring the leakage of particle sizes between 0.07 to 4.4 microns through three hole sizes in a negative-pressure half-mask respirator worn by a human subject. This investigation showed that the size distribution of an aerosol test agent and the measurement method have an effect on the leakage measured in a quantitative fit test. For instance, the ratio of percent leakage measured by light scattering between test aerosols with count median diameters of 2.2 and 0.28 microns can be as large at 5:1. Likewise, the ratio of the percent leakage measured by a particle count method vs. a mass method of detection of the same polydisperse aerosol with a count median diameter equal to 2.2 microns can be as high as 4:1. The mass leakage into a mask with a leak is also greater for an exposure aerosol with a count median diameter between 0.15 to 0.30 micron compared to exposure aerosols with larger count median diameters for aerosols with the same mass concentration.     

Filter performance of n99 and n95 facepiece respirators against viruses and ultrafine particles

The performance of three filtering facepiece respirators (two models of N99 and one N95) challenged with an inert aerosol (NaCl) and three virus aerosols (enterobacteriophages MS2 and T4 and Bacillus subtilis phage)-all with significant ultrafine components-was examined using a manikin-based protocol with respirators sealed on manikins. Three inhalation flow rates, 30, 85, and 150 l min(-1), were tested. The filter penetration and the quality factor were determined. Between-respirator and within-respirator comparisons of penetration values were performed. At the most penetrating particle size (MPPS), >3% of MS2 virions penetrated through filters of both N99 models at an inhalation flow rate of 85 l min(-1). Inhalation airflow had a significant effect upon particle penetration through the tested respirator filters. The filter quality factor was found suitable for making relative performance comparisons. The MPPS for challenge aerosols was <0.1 mum in electrical mobility diameter for all tested respirators. Mean particle penetration (by count) was significantly increased when the size fraction of <0.1 mum was included as compared to particles >0.1 mum. The filtration performance of the N95 respirator approached that of the two models of N99 over the range of particle sizes tested ( approximately 0.02 to 0.5 mum). Filter penetration of the tested biological aerosols did not exceed that of inert NaCl aerosol. The results suggest that inert NaCl aerosols may generally be appropriate for modeling filter penetration of similarly sized virions.     

Respiratory protection and the risk of Mycobacterium tuberculosis infection

Tuberculosis (TB) can be transmitted to susceptible healthcare workers via inhalation of droplet nuclei carrying viable Mycobacterium tuberculosis bacilli. Several types of respiratory protective devices are compared with respect to efficacy against droplet nuclei penetration: surgical masks, disposable dust/mist particulate respirators (PRs), elastomeric halfmask respirators with high-efficiency (HEPA) filters, and powered airpurifying respirators (PAPRs) with elastomeric halfmask facepieces and HEPA filters. It is estimated that these devices permit, respectively, 42%, 5.7%, 2%, and 0.39% penetration of droplet nuclei into the facepiece. More limited data for the disposable HEPA filtering-facepiece respirator suggest that it would allow droplet nuclei penetration of 3% or less, similar to the value estimated for the elastomeric halfmask HEPA filter respirator. Because a respirator wearer's cumulative infection risk depends on the extent of droplet nuclei penetration, the cumulative risk will differ, given use of these different respirators. Hypothetical but realistic “low-exposure” and “high-exposure” scenarios are posed that involve, respectively, a 1.6% and a 6.4% annual risk of infection for healthcare workers. For the low-exposure scenario, the 10-year cumulative risks given no respirators versus surgical masks versus disposable dust/mist PRs versus elastomeric halfmask HEPA filter respirators versus HEPA filter PAPRs are, respectively, 15%, 6.7%, 0.94%, 0.33%, and .064%. For the high-exposure scenario, the 10-year cumulative risks for no respirator use versus use of the same four types of respirators are, respectively, 48%, 24%, 3.7%, 1.3%, and 0.26%. The use of disposable HEPA filtering-facepiece respirator should permit cumulative risks close to those estimated for the elastomeric halfmask HEPA filter respirator. It is concluded that when an infectious TB patient undergoes a procedure that generates respiratory aerosols, and when droplet nuclei source control is inadequate, healthcare workers attending the patient may need to wear highly protective respirators, such as HEPA filter PAPRs.     

Powered, air-purifying particulate respirator filter penetration by a DOP aerosol

In 1995, new certification requirements for all nonpowered, air-purifying particulate filter respirators were put in place when 42 CFR 84 replaced 30 CFR 11. However, the certification requirements for all other classes of respirators, including powered air-purifying respirators (PAPRs), were transferred to 42 CFR 84 from 30 CFR 11 without major changes. Since the inception of 42 CFR 84, researchers have learned that the efficiency of electrostatic filter media, in contrast with mechanical filter media, can be rapidly degraded by oil aerosols. Further, confusion may exist among respirator users, since electrostatic PAPR filters have the same magenta color assigned to high-efficiency filters for nonpowered particulate respirators that have been tested and certified for use against oil aerosols (i.e., P100 filters). Users may expect that the magenta color of certified PAPR filters indicates suitability for use against oil aerosols. This may not be the case. To illustrate the potential degradation of electrostatic PAPR filters, new filters certified under 42 CFR 84 were tested using a TSI model 8122 Automated Respirator Tester against charged and neutralized DOP aerosols with intermittent loading schedules. The performance of a magenta-colored electrostatic PAPR filter--one for which the manufacturer's user instructions appropriately indicates is not suitable for use in oily environments--was compared with the performance of several mechanical PAPR filters. In tests against both DOP aerosols, the electrostatic PAPR filter showed a significant decrease in performance at DOP loadings exceeding 400 mg, whereas mechanical filters showed no significant change in the performance except at extremely high loadings. The decreased performance of the electrostatic PAPR filter was found to be significantly greater when tested against a neutralized DOP aerosol when compared with a charged DOP aerosol. While laboratory tests show that the filtration efficiency of this electrostatic PAPR filter degrades with exposure to DOP aerosol, the observed laboratory degradation may or may not affect workplace performance, as similar degradation has not been verified in workplace studies. Based on these laboratory results, a proposed method for evaluating high-efficiency PAPR filters is presented. This proposed method would ensure that high-efficiency PAPR filters (> or = 99.97% efficient and magenta in color) meet critical performance criteria when loaded.     

Evaluation of respirator filters for asbestos fibers

Fiber aerosols are known to have different aerodynamic behaviors than spherical particles and usually carry higher electrostatic charges. We investigated the effects of flow rate and charge status of filter cartridges on the penetration of spherical and fiber aerosols. Four types of test respirator filters were selected: two for passive respirators, one for a powered respirator, and one disposable respirator. Surface charges on respirator filters were determined using a noncontact field electrostatic field meter. Penetration tests were performed for filter cartridges before and after charge neutralization. The surface charge measurements on the respirator filters showed that some filters, including those used in disposable face masks, are charged to enhance the collection efficiency. Only high-efficiency particulate air filters performed consistently for both spherical test aerosols and the three types of asbestos fibers. The surface charge potential of filter cartridges and flow rate did not appear to affect the performance of these filters. In contrast to the high-efficiency filters, the aerosol penetration performance of low-efficiency filters and face masks deteriorated when the charge potential on the filter was removed. Our data also showed that the surface charges decreased in a high-temperature, high-humidity environment and disappeared after 1 week. Deposition of spherical particles and fibers in the charged disposable facemask filter was enhanced. For charged-neutralized, low-efficiency filter cartridges, asbestos fibers may penetrate more than spherical particles with a mean particle size of 0.3 microm diameter.     

Electrostatic N-95 respirator filter media efficiency degradation resulting from intermittent sodium chloride aerosol exposure

The effects of intermittently loading small masses of sodium chloride aerosol on the filtration efficiency of N-95 filtering facepiece respirators was investigated. The National Institute for Occupational Safety and Health (NIOSH) certifies that N-95 respirators must provide at least 95 percent filtration efficiency against a sodium chloride aerosol challenge as per the respirator certification (42 CFR 84) test criteria. N-95 respirators are specified for protection against solid and water-based particulates (i.e., non-oil aerosols). New N-95 respirators from three different manufacturers were loaded with 5 +/- 1 mg of sodium chloride aerosol one day a week, over a period of weeks. Aerosol loading and penetration measurements were performed using the TSI 8130 Filter Tester. Respirators were stored uncovered on an office desktop outside the laboratory. To investigate environmental and temporal effects of filters being stored without sodium chloride exposure, control respirators were stored on the desk for various lengths of time before being initiated into weekly testing. For all manufacturers' respirators, the controls showed similar initial penetrations on their day of initiation (day zero) to those of the study samples on day zero. As the controls were tested weekly, they showed similar degradation rates to those of the study samples. Results show that some of the manufacturers' models had penetrations of greater than 5 percent when intermittently exposed to sodium chloride aerosol. It is concluded that intermittent, low-level sodium chloride aerosol loading of N-95 respirators has a degrading effect on filter efficiency. This reduction in filter efficiency was not accompanied by a significant increase in breathing resistance that would signal the user that the filter needs to be replaced. Furthermore, it was noted that the effect of room storage time prior to initial exposure was much less significant.