








key: cord-340639-hdn85mik
authors: Uk Lee, Byung; Yermakov, Mikhail; Grinshpun, Sergey A.
title: Unipolar ion emission enhances respiratory protection against fine and ultrafine particles
date: 2004-11-30
journal: Journal of Aerosol Science
DOI: 10.1016/j.jaerosci.2004.05.006
sha: 
doc_id: 340639
cord_uid: hdn85mik

Abstract We developed a novel concept that allows to considerably improve the performance of conventionally used filtering-facepiece respirators against fine and ultrafine aerosols including airborne viral and bacterial agents. The concept is based on the continuous emission of unipolar ions. The effect was evaluated through the real-time monitoring of the concentration and size distribution of fine and ultrafine aerosol particles. The measurements were conducted inside and outside of a respiratory mask that was face sealed on a breathing manikin. A commonly used Type N95 respirator and surgical mask were utilized for the tests. The manikin was placed in a 24.3-m3 indoor test chamber and exposed to polydisperse surrogate aerosols simulating viral and bacterial particles with respect to the aerodynamic size. The particle penetration through the mask was found to decrease by one-to-two orders of magnitude as a result of continuous unipolar ion emission in the chamber. The flux of air ions migrated to the breathing zone and imparted electrical charges of the same polarity to the aerosol particles and the respirator filter surface. This created an electrostatic shield along the external surface of the filter, thus enhancing the protection characteristics provided by the respirator. The above performance enhancement effect is crucial for minimizing the infectious risk in the cases when the conventional filtering-facepiece respirators are not able to provide an adequate protection against airborne viruses and bacteria.

The outbreaks of emerging diseases (e.g., SARS) and the threat of bioterrorism have triggered an urgent demand for adequate respiratory protection against bioaerosol agents, including airborne viruses and bacteria. Particular interest has been directed towards increasing the e ciency of existing respiratory protection devices.

The ÿltering-facepiece masks, including Type N95 respirators, are frequently used in indoor air environments to prevent or considerably reduce inhalation of droplet nuclei that can potentially carry viable microorganisms. Millions of workers, including health-care personnel, routinely use respirators in their workplaces (United States Department of Labor, 1995) . In case of bioterrorist attack in a major urban area, there may be a need in millions readily available respiratory protection devices. The existing respirators have been extensively evaluated against ÿne particles (e.g., Brosseau, Evans, Ellenbecker, & Feldstein, 1989; Chen, Ruuskanen, Pilacinski, & Willeke, 1990; Chen & Willeke, 1992; Huang, Willeke, Qian, Grinshpun, & Ulevicius, 1998; Johnston, Myers, Colton, Birkner, & Campbell, 2001; Qian, Willeke, Grinshpun, Donnelly, & Co ey, 1998; Halvorsen, 1998) and microorganisms (Centers for Disease Control and Prevention, 1994; Lee, Slavcev, & Nicas, 2004a; Qian, Willeke, Grinshpun, & Donnelly, 1997; Qian et al., 1998; Reponen, Wang, Willeke, & Grinshpun, 1999; Willeke, Qian, Donnelly, Grinshpun, & Ulevicius, 1996) . At the same time, the protection e ciency of existing facepiece respirators have not been well characterized with respect to ultraÿne particles, i.e. those below 0:1 m (Hinds, 1999) .

The respirators di er from one another by their ÿltration e ciency, which is dependent on the ÿlter properties and the particle size. For example, a Type N95 respirator may allow up to 5% penetration in "a worst case scenario," when most-penetrating sodium chloride particles of 0:3 m mass median aerodynamic diameter are drawn through the ÿlter at a ow rate of 85 l min −1 (Federal Register, 1995) . The penetration e ciency of larger Mycobacterium tuberculosis bacteria (Mtb, 0:8 m) through a face-sealed N95 respirator at strenuous workload is as low as about 0.5% . The face-sealed ÿlter of a conventional health-care mask, which ensures relatively low pressure drop and consequently good comfort level, allows approximately 15% of airborne Mtb surrogate bacteria to penetrate, thus providing 85% protection against these bacteria (Willeke et al., 1996) .

If the bacterial concentration in the air is 1000 m −3 , an unprotected individual breathing at 30 l min −1 inhales 1800 microorganisms per hour, whereas the one wearing a perfectly ÿt N95 respirator inhales up to 90 microorganisms per hour. If the infectious dose of a bioaerosol agent of interest is less than 90, the N95 respirator may not provide an adequate respiratory protection once the exposure time exceeds one hour. The use of an improperly ÿt-tested tight-ÿtting respirator may further decrease the respiratory protection level because of the additional particle penetration that occurs through the face-seal leaks (Chen et al., 1990; Chen & Willeke, 1992; Oestenstad, Dillion, & Perkins, 1990a; Oestenstad, Perkins, & Rose, 1990b) . Based on the above considerations, it seems very useful if the ÿltration e ciency of existing respirators can be increased while the comfort level provided by these devices would remain the same.

With respect to the respiratory exposure and protection, the particle aerodynamic diameter range of d a ∼ 0:04-2 m is of special public interest because of its health relevance. Many bioaerosol agents, including viruses and bacteria that cause emerging diseases as well as those that can be used for biological warfare or in the event of bioterrorism, belong to this size range. For example, according to the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov), the dimension of the coronavirus virion (the etiological agent of the SARS) are (60-120) × (160-200) nm, which corresponds to d a ∼ 0:1 m. For Bacillus anthracis (bacteria causing anthrax), d a ∼ 1 m. As the above range is broad and includes both the ÿne and ultraÿne particle fractions (Baron & Willeke, 2001; Hinds, 1999) , the particle penetration e ciency through the ÿlter media can be a ected by several mechanisms and is generally characterized by the particle aerodynamic size. This allows testing the performance of respirator ÿlters against pathogenic agents using non-pathogenic aerosol surrogates that simulate the aerodynamic characteristics of the particles of interest.

In this study, we developed and evaluated a novel concept that drastically enhances the performance of conventional ÿltering-facepiece respirators against ÿne and ultraÿne aerosol particles. The concept is based on the continuous emission of unipolar ions into the air in the vicinity of the respirator. The aerosol particles are unipolarly charged by air ions primarily due to the di usion charging mechanism (Adachi, Kousaka, & Okuyama, 1985; Frank, Cederfelt, & Martinsson, 2004; Hernandez-Sierra, Alguacil, & Alonso, 2003; Wiedensohler et al., 1994) . The ion-ÿlter interaction and the deposition of unipolarly charged particles on the external surface of the ÿlter impose signiÿcant unipolar charge on the ÿlter. This creates a shield for the incoming particles (as they carry charges of the same polarity), which decreases the penetration e ciency through the ÿlter.

The new concept was experimentally evaluated in a non-occupied, unventilated indoor test chamber (L × W × H = 3:78 m × 2:44 m × 2:64 m = 24:3 m 3 ). This facility, developed in the Center for Health-Related Aerosol Studies at the University of Cincinnati, has been used in our previous studies (Choe et al., 2000; Grinshpun et al., 2002 Grinshpun et al., , 2004 .

The experimental setup is schematically shown in Fig. 1 . A breathing manikin with a face-sealed respiratory mask was exposed to the airborne polydisperse surrogate aerosols that simulated viral and bacterial particles with respect to their aerodynamic size. The leakage tests were conducted between the mask and the face of the manikin with a bubble-producing liquid (Trubble Bubble, New Jersey Meter Co., Paterson, NJ, USA). The manikin operated at a breathing ow rate, 30 l min −1 , representing human breathing during light workloads (Mineral Resources, 1994; Johnson, Weiss, & Grove, 1992) .

The electrical low pressure impactor (ELPI, TSI Inc./Dekati Ltd., St. Paul, MN, USA) was used to determine the concentration and aerodynamic particle size distribution in real-time. This instrument utilizes the cascade impaction principle and-in addition-has a direct-reading capability. The aerosol particles are charged by the corona charger downstream of the ELPI inlet and subsequently detected by the electrometers inside the cascade impactor.

The particles were collected by the ELPI inside and outside the respirator using identical sampling lines. For these measurements, a 10-mCi Kr 85 charge equilibrator (3M Company, St. Paul, MN, USA) was installed upstream of the ELPI inlet to neutralize the particles to Boltzmann charge equilibrium. This allowed us to avoid the in uence of high electric charges, imparted by the particles as a result of their interaction with air ions, on the ELPI performance. The time resolution of the instrument was adjusted to 10 s. The data were recorded in 12 ELPI channels (each channel = impaction stage), from 0.04 to 8:4 m. The latter sizes represent the midpoint diameters of the ÿrst and the 12th impaction stages (the midpoint = the geometric mean of the stage's boundaries). The natural aerosol concentration in the indoor test chamber was not su cient, particularly for the measurement inside the mask, because the ÿlter removed considerable number of ambient airborne particles. To increase the initial background aerosol concentration, we used a smoke generator. The smoke particles covered primarily the submicrometer aerodynamic size range (Cheng, Bechtold, Yu, & Hung, 1995) with a sharp decrease in the particle number at d a ¿ 1:5-2 m. Thus, the data recorded in the ÿrst 8 measurement channels of the ELPI (d a = 0:04-1:3 m) were used for the analysis.

The measured aerosol concentrations inside (C IN ) and outside (C OUT ) the mask were incorporated into the equation for the respirator penetration e ciency, E p :

which was determined as a function of the particle aerodynamic diameter. E P is actually the inversed protection factor that is frequently used as a respirator performance index. Initially, the background tests were conducted by measuring the penetration e ciency of the mask with no ion emission. Then, a unipolar ion emitter was turned on at a distance of 20 cm from the mask, and E p (d a ) was determined in 3-min time intervals during 12 min. The continuous air ion emission in the chamber decreased C OUT as the particles, charged by ions to the same polarity, repelled and subsequently migrated toward the chamber's walls and deposited on these walls (Grinshpun et al., 2004) . The change in the C OUT -value that occurred during each 3-min time interval due to ionic air puriÿcation in the chamber was taken into account through the linear interpolation of C OUT (t).

In this study, we used a negative ion emitter (VI-2500, Wein, Inc., Los Angeles, CA, USA) producing an air ion concentration of N i ∼ 1:3 × 10 6 elementary charges per cm 3 as determined at a distance of 1 m from the source. The ion concentration was measured by the Air Ion Counter (AlphaLab Inc., Salt Lake city, UT, USA) that operates within the range of 10 -2×10 6 ions per cm 3 .

Two types of ÿltering-facepiece respiratory masks commercially available from a major manufacturer were tested in this study. One was the NIOSH (US National Institute for Occupational Safety and Health) certiÿed N95 respirator and the other one was a conventional disposable surgical mask. The N95 respirator consists of inner and outer cover webs made of rayon. Its ÿlter made of polyester and polypropylene with the electrostatically charged microÿbers providing relatively high ÿltering e ciency. In the surgical mask, the polypropylene ÿlter is sandwiched between inner and outer webs made of rayon. The ÿlter of a surgical mask has lower ÿltration e ciency as compared to the one of an N95 respirator. Thus, the e ect of ion emission on the respirator ÿlter e ciency was tested for the masks having two distinctly di erent original performance characteristics.

The average values and the standard deviations of the penetration e ciency were calculated for each set of conditions as a result of at least three replicates. The data were statistically analyzed using the Microsoft Excel software package (Microsoft Co., Redmond, WA, USA). Fig. 2 shows the normalized initial aerosol concentration measured outside the respirator mask. Each data point represents an average of six replicates. It is seen that the aerosol particles were primarily within a range of d a ≈ 0:04-0:5 m (the concentration N= log d a was between ¿ 10 4 and ¿ 10 5 cm −3 ), while fewer micron-size particles were detected ( N= log d a ∼ 10 3 cm −3 ). For each measured particle size, the initial aerosol concentration was reproducible with the variability (the coe cient of variation) not exceeding about 50% for six replicates. Fig. 3 presents the data obtained with two types of face-sealed respirators: N95 respirator and a surgical mask. When no air ion emission was introduced, the E p -value, averaged over the test range of d a , was about 1.8% for the N95 respirator. The particle size did not considerably a ect the penetration through the N95 respirator ÿlter, although some decrease of E p with increasing d a was observed for d a ≈ 0:04-0:5 m. Once the negative ion emission began, the penetration decreased to 0.27% during the ÿrst 3 min. At t = 12 min, it further decreased to about 0.11%, enhancing the N95 respirator ÿlter performance approximately by a factor of 17 (Fig. 3a) . For the surgical mask, the initial penetration e ciency ranged from 18.7% (d a = 0:04 m) to 11.1% (d a = 1:3 m). Resulting from the emission of negative ions, the average penetration e ciency through the surgical mask dropped from 15.4% (t = 0) to 0.19% (t = 12 min), demonstrating a 80-fold enhancement (Fig. 3b) . The data in Fig. 3 show that the most pronounced e ect occurred within the ÿrst 3-min interval. 

It was surprising to observe that the initial penetration e ciency (t=0) of ultraÿne particles through both masks slightly increased with decreasing particle size. In contrast, the available ÿltration models predict that the peak penetration is reached at d a between 0.1 and 0:3 m, and the particles below 0:1 m should be collected more e ciently as their size decreases (di usion regime) (Halvorsen, 1998; Hinds, 1999; Lee & Mukund, 2001) . The following considerations explain the results of our baseline test for the ultraÿne particles. These models (and the laboratory-generated experimental data that support them) characterize the particle penetration through a homogeneous perfectly sealed ÿbrous ÿlter material but not through a respirator mask. The design of a ÿltering-facepiece respirator does not assure a perfect peripheral connection of the assembly, so micro-leaks may be present between the core ÿlter material and the elastic peripheral support. These leaks can contribute to the penetration of the ultraÿne particles. In addition, although the mask was "glued" on the manikin, some very small, micrometer-or submicrometer-size leaks may still remain. Most of soap-bubble-based air leak detection methods are capable to identify micro-leaks greater than 1 m. If d a is much lower than the characteristic size of the micro-leaks (1 m), the particles may penetrate through these remaining submicrometer micro-leaks, thus a ecting the overall aerosol penetration e ciency through the ÿltering mask. One more possible factor is associated with the spatial variations in ÿber diameter, orientation, packing density, as well as initial ÿber electrostatic charge level (for those masks utilizing electret ÿlter media). These variations have been shown to signiÿcantly a ect the respirator performance increasing the penetration e ciency of particles of ∼ 0:1 m (Huang et al., 1998) .

The enhancement of the respirator performance, observed almost immediately after the ion emitter started operating, can be attributed to the electrostatic e ect. The emitted negative ions as well as the particles charged by these ions in the air, impose signiÿcant negative charge on the respirator ÿlter. This forms the "electrostatic shield" against the particles moving toward the mask. The repelling forces decrease the number of particles that can approach the ÿlter. The above-described e ect works outside of the respirator, as opposite to the aerosol ÿltration by di usion, impaction, interception, and electrostatic deposition (Lee & Mukund, 2001 ) that takes place inside the ÿlter. Therefore, the ion-induced decrease in the particle penetration e ciency does not cause the pressure drop increase through the ÿlter providing the enhanced performance with the same comfort level.

To quantitatively characterize the e ect, we calculated the velocity of particle migration induced by the electrostatic interaction in the vicinity of the ÿlter. It was then compared to the velocity of the air ow through the ÿlter caused by inhalation. In this calculation, we used information about the airborne particle electric charges and the air ion density, obtained by the ELPI and the Air Ion Counter, respectively. The particle size of d a = 0:1 m was chosen, as it represents the dominant size range used in our experiments. Furthermore, this value is at the boarder line between the ÿne and ultraÿne particle size ranges. In addition, many viral particles have an aerodynamic diameter of ∼ 0:1 m. Two main assumptions were made. First, the aerosol particles and air ions that interacted with the respirator were assumed to give all their electric charges to the respirator ÿlter. Second, the charged ÿlter was assumed to act as a point-charge located at the center of the respirator's surface. At the ion emission level produced in our experiment, a 0:1 m particle carries, on average, 10 elementary charges (Lee, Yermakov, & Grinshpun, 2004b) . The calculation showed that the total electric charge acquired by the respirator ÿlter, as a result of a 3-min continued emission from the VI-2500 ion source, is about 1:5 × 10 13 elementary charges, if the breathing ow rate is 30 l min −1 . The ion concentration at the center of the respirator was determined to be approximately 1:6 × 10 8 cm −3 (this point located 20 cm from the ion emission source). Thus, the particle migration velocity was found to exceed the air ow velocity, created by the inhalation in the breathing zone, approximately by a factor of 75. This suggests that the e ect of the repelling force between the unipolarly charged particles and ÿlter surface is much stronger than the aerodynamic force. Therefore, although our assumptions may not be su ciently conservative, the above assessment demonstrates that the enhancement of the respirator performance by the unipolar ion emission is governed by the electrostatic "shield" mechanism.

The drastic decrease of the particle penetration through the respirator ÿlter due to continuous unipolar ion emission may be critical in providing additional respiratory protection by existing masks against viral and bacterial particles. For example, an individual exposed to the in uenza virus concentration of 10; 000 m −3 inhales approximately 4500×0:18=810 viruses during 15 min when breathing through a conventional surgical mask at 30 l min −1 in the absence of ion emission (E p ≈ 18% for 0:1 m particles). The continuous emission of negative air ions (N i ∼ 10 6 e − cm −3 ) in a 25-m 3 room would reduce the indoor viral concentration by a factor of 9 during that 15-min interval (Lee et al., 2004a, b) . In addition, it would enhance the surgical mask protection reducing E p from 18% to at least 0.19%. Thus, only about (4500=9) × 0:0019 = 0:95 ≈ 1 virus would be inhaled in 15 min. Given that the infectious dose of in uenza A2 is 790 viruses (Lawrence Berkeley National Laboratory), the ion emission e ect would make an important di erence with respect to the health risk.

While this study is limited to the negative ion emission, we anticipate that the respirator performance enhancement e ect can be achieved also by generating positive air ions, as long as the ion concentration in the vicinity of the mask (breathing zone) is su ciently high. Future studies will address the e ects of polarity and the ion emission rate on the particle penetration e ciency through respirator ÿlters.

Generally, the mask protection factor depends not only on its ÿlter penetration e ciency but also on its face ÿt (in practice, the facepiece mask is not sealed to the human face allowing the particles to penetrate through the leak). This pathway may become especially apparent when the ÿlter material is highly e cient. Therefore, future tests involving human subjects, di erent ÿt factors, and other experimental conditions ( ow rates and di erent masks) are needed to better characterize the enhancement e ect, discovered in this study, and link it to the exposure.

Unipolar and bipolar di usion charging of ultraÿne aerosol particles

Aerosol measurement: Principles, techniques, and applications

Collection e ciency of respirator ÿlters challenged with monodisperse latex aerosols

Guidelines for preventing the transmission of Mycobacterium tuberculosis in health-care facilities

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Characteristics of face seal leakage in ÿltering facepieces

Incense smoke: Characterization and dynamics in indoor environments

Particle settling after lead-based paint abatement work and clearance waiting period

Respiratory protective devices: Final rules and notice

Characterisation of a unipolar charger for droplet aerosols of 0.1-20 m in diameter

Indoor air pollution control through ionization

E ciency of ÿnal cleaning for lead-based paint abatement in indoor environments

Respirator Fit Testing Application Notes

Unipolar charging of nanometer aerosol particles in a corona ionizer

Aerosol technology: Properties, Behavior, and Measurement of Airborne Particles

Method for measuring the spatial variability of aerosol penetration through respirator ÿlters

Review of respirator performance testing in the workplace: Issues and concerns

Respirator performance rating table for mask design

Filter collection

Respiratory protection against Mycobacterium tuberculosis: Quantitative ÿt test outcomes for ÿve type N95 ÿltering-facepiece respirators

Removal of ÿne and ultraÿne particles from indoor air environments by the unipolar ion emission

Distribution of faceseal leak sites on a half-mask respirator and their association with facial dimensions

Identiÿcation of faceseal leak sites on a half-mask respirator

Performance of N95 respirators: Reaerosolization of bacteria and solid particles

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Survival of Mycobacteria on N95 personal respirators

Respirator Use and Practice

A novel unipolar charger for ultraÿne aerosol particles with minimal particle losses

Penetration of airborne microorganisms through a surgical mask and a dust/mist respirator

The experimental evaluation part of this investigation was supported by the Wein Products Inc., Los Angeles, CA, USA. The participation of Dr. Lee in this study was partly due to the Post-doctoral Fellowship Program and the Advanced Environmental Monitoring Research Center (ADEMRC) Program of the Korea Science & Engineering Foundation (KOSEF). The authors are thankful for this support.

Reference to any companies or speciÿc commercial products does not constitute or imply their endorsement, recommendation, or favouring by the University of Cincinnati or the authors.

