The protection level offered by filtering facepiece particulate respirators and face masks is defined by the percentage of ambient particles penetrating inside the protection device. There are two penetration pathways: (1) through the faceseal leakage, and the (2) filter medium. This study aimed at differentiating the contributions of these two pathways for particles in the size range of 0.03-1 microm under actual breathing conditions. One N95 filtering facepiece respirator and one surgical mask commonly used in health care environments were tested on 25 subjects (matching the latest National Institute for Occupational Safety and Health fit testing panel) as the subjects performed conventional fit test exercises. The respirator and the mask were also tested with breathing manikins that precisely mimicked the prerecorded breathing patterns of the tested subjects. The penetration data obtained in the human subject- and manikin-based tests were compared for different particle sizes and breathing patterns. Overall, 5250 particle size- and exercise-specific penetration values were determined. For each value, the faceseal leakage-to-filter ratio was calculated to quantify the relative contributions of the two penetration pathways. The number of particles penetrating through the faceseal leakage of the tested respirator/mask far exceeded the number of those penetrating through the filter medium. For the N95 respirator, the excess was (on average) by an order of magnitude and significantly increased with an increase in particle size (p < 0.001): approximately 7-fold greater for 0.04 microm, approximately 10-fold for 0.1 microm, and approximately 20-fold for 1 microm. For the surgical mask, the faceseal leakage-to-filter ratio ranged from 4.8 to 5.8 and was not significantly affected by the particle size for the tested submicrometer fraction. Facial/body movement had a pronounced effect on the relative contribution of the two penetration pathways. Breathing intensity and facial dimensions showed some (although limited) influence. Because most of the penetrated particles entered through the faceseal, the priority in respirator/mask development should be shifted from improving the efficiency of the filter medium to establishing a better fit that would eliminate or minimize faceseal leakage.

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.

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.

Aerosolization of bacteriophage MS2 virions by nebulization and charge-reduced electrospray were compared during testing of three filter media. Sample swatches were taken from a surgical mask, N95 filtering-facepiece respirator (FFR), and N100 FFR for use in repeated, short-duration (15 min) penetration tests with bacteriophage MS2 aerosolized by nebulizer and electrospray. Evaluated were (1) the virus suspension preparation protocol, (2) resulting particle size distribution, count stability, and count variability, and (3) the ability to generate culturable MS2 virions. While preparation of the electrospray bacteriophage suspension required additional purification and concentration steps and took more time than the nebulization protocol, it resulted in a much higher titer suspension. The nebulizer produced a polydisperse aerosol; conversely, the electrospray produced a relatively monodisperse aerosol with a count peak at the mobility size of the single virion. The nebulized aerosol particle count was 2.8 times as variable as the electrosprayed aerosol particle count although neither aerosolization method maintained a constant count over repeated 15-minute filter tests. No differences in filter penetration were observed between nebulized and electrosprayed MS2 aerosol particles. Electrosprayed dextrose particles, used as an inert aerosol particle comparator, exhibited higher penetration than MS2 particles in two of the three filter samples, which can be attributed, at least partially, to the difference in dielectric properties of dextrose and virus particles. Both aerosolization methods generated culturable MS2 virions with the electrospray producing an airborne concentration approximately 20-fold higher than the nebulizer. In general, the electrospray produced cleaner, more stable, and more viable bacteriophage aerosol particles compared to conventional nebulization. The findings of this study are expected to assist researchers in selecting appropriate generation methods when using viable virus-based challenge aerosol particles.

The feasibility of a novel testing protocol that allows differentiating between the physical (total) and viable bioaerosol penetrations through respirator filters was investigated. Three respirator models -two conventional N95 filtering-facepiece respirators (FFR) used as controls and one P95 iodinated polymer FFR with antimicrobial properties -were challenged with aerosolized MS2 bacteriophage virus. Physical (P physical ) and viable (P viable ) filter penetrations were simultaneously measured with the FFR sealed on a manikin at a constant inhalation flow rate of 85 L/min. Separate testing was performed on specially-manufactured P95 filter swatches with (i) no iodinated resin additive and (ii) "high" amount of the additive to determine whether it influenced filtration behavior of the P95 respirator. Bioaerosol collection on the N95 FFR filters fell in the range consistent with previous studies featuring about 2% penetration for MS2 and a peak around l5%. The P95 iodinated polymer respirator was found to be highly efficient, attributed in part to the iodinated resin powder which in separate swatch tests was found to increase the filter collection efficiency. No statistically significant differences were observed between penetration values obtained for total and culturable viruses for the two control respirators. Similarly, no difference was observed for the iodinated respirator, which suggested that the microbial inactivation effect was of insufficient magnitude to be detected or was not present for viral particles that penetrated the filter. Possible "long-term" inactivation effect of the iodine-based additive on the viable viruses, which were captured on the filter over time, was beyond the scope of this study. The novel testing protocol appears to be an adequate tool for evaluating respirators designed to protect against bioaerosol particles. Further improvement may be considered with respect to the aerosolization method for viable microorganisms.