Effect of an Ionic Air Cleaner on Indoor/Outdoor Particle Ratios in a Residential Environment

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1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Effect of an Ionic Air Cleaner on Indoor/Outdoor Particle Ratios in a Residential Environment David Berry, Gediminas Mainelis & Donna Fennell To cite this article: David Berry, Gediminas Mainelis & Donna Fennell (2007) Effect of an Ionic Air Cleaner on Indoor/Outdoor Particle Ratios in a Residential Environment, Aerosol Science and Technology, 41:3, , DOI: / To link to this article: Published online: 02 Mar Submit your article to this journal Article views: 785 View related articles Citing articles: 4 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 22 November 2017, At: 06:48

2 Aerosol Science and Technology, 41: , 2007 Copyright c American Association for Aerosol Research ISSN: print / online DOI: / Effect of an Ionic Air Cleaner on Indoor/Outdoor Particle Ratios in a Residential Environment David Berry, 1,2 Gediminas Mainelis, 1 and Donna Fennell 1 1 Department of Environmental Sciences, Rutgers, The State University of New Jersey, New Brunswick, New Jersey, USA 2 Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, Ann Arbor, Michigan, USA We tested a leading commercially available ionic air cleaner in a typical residential apartment to study the effect of the device on indoor/outdoor airborne particle number and mass concentration ratios. In addition, we also determined the indoor ozone and ion concentration levels. When measured during normal daily activity, the average indoor/outdoor mass concentration ratio was reduced from 1.03 to 0.73 and the number concentration ratios underwent reductions for most of the particle size fractions. However, due to a substantial inter- and intra-measurement variation in particle ratios, the observed average reductions were not statistically significant. When measurements were performed in a still room, the indoor/outdoor particle mass concentration ratio decreased from to in eight hours when the air cleaner was operating. Ambient ozone concentrations measured in the middle of the apartment were between ppb during normal daily activity and the ozone levels increased to 77 ppb when measured in front of the ionic cleaner during still conditions. We also found that that there was a limited vertical diffusion of ions. The highest ion concentrations were measured at a 0.5 m height from the floor and decreased substantially with increasing measurement height. This finding may have implications for effective particle removal from a person s breathing zone. Overall, we found that the tested brand of commercially available ionic air cleaners may have the capability to remove some airborne particulate matter in actual residential settings, but its cleaning effect is reduced under normal daily activity. INTRODUCTION Indoor air pollution has been associated with a variety of health problems such as asthma, allergies, fevers, headaches, rashes, and severe lung disease (USEPA 1994), and therefore Received 15 November 2005; accepted 5 January This study was supported in part through the Rutgers Undergraduate Research Fellowship Program. The authors are thankful for the support. Address correspondence to Gediminas Mainelis, Department of Environmental Sciences, Rutgers, The State University of New Jersey, 14 College Farm Road, New Brunswick, NJ mainelis@envsci.rutgers.edu is an important public health concern. Asthma symptoms have been correlated with the presence of ambient levels of fine particulate matter ( μm) (Klot et al. 2002) and long-term exposure to particulate matter has been associated with elevated rates of cardiopulmonary and lung cancer mortality (Pope et al. 2002). With people spending up to 90% of their time indoors (USEPA, 1989), it is clear that their exposure to indoor particulate matter contributes to negative health effects. Sources of indoor particulate matter have been characterized in several studies. Reported indoor sources of particulate matter include smoking (Jones et al. 2000; Chao et al. 1998; Clayton et al. 1993), candle burning (Matson, 2005), incense burning (Chao et al. 1998), cooking (Jones et al. 2000; Chao et al. 1998; Kamens et al. 1991), and house cleaning such as vacuum operation (Clayton et al. 1993; Kamens et al. 1991). It has also been shown that diffusion of particulate matter from outside through the building envelope is a significant source of indoor particulate matter, especially when there are no other indoor sources (Matson 2005; Ho et al. 2004). One way to quantify the relative indoor air quality is through the use of the indoor/outdoor particle number and mass concentration ratios. Such ratios can help to indicate whether the sources of indoor air pollutants are mostly from the outdoor or indoor environment. Indoor/outdoor particle number concentration ratios reported in the literature range from 0.38 to 4.29 (Matson 2005; Monkonnen et al. 2005) and indoor/outdoor mass concentration ratios range from 0.54 to 3.7 for typical residential environments (Monkonnen et al. 2005; Ho et al. 2004; Sawant et al. 2004; Jones et al. 2000; Long et al. 2000; Monn et al. 1997; Miguel et al. 1995; Li 1994). The indoor/outdoor ratios vary due to several factors including averaging periods (Ni Riain et al. 2003; Monkonnen et al. 2005), particle size (Li 1994), time of day (Long et al. 2000), and wind speed and direction (Ni Riain et al. 2003). For example, Monkonnen et al. (2004) found that one hour averaging periods yielded indoor/outdoor particle number concentration ratios of , while 24-hour averaging periods yield ratios of Although there is variation in the indoor/outdoor ratios, they seem to be an effective tool for comparison of data sets. 315

3 316 D. BERRY ET AL. A variety of air cleaners are used to improve indoor air quality. These include High Efficiency Particulate Air (HEPA) filters (Antonicelli et al. 1991), ozone generators (Boeniger 1995), electrostatic precipitators (Zhuang et al. 2000), and negative ion generators (Daniell et al. 1991). There has recently been an increase in the popularity and use of ionic air cleaners. The operating principle of an ionic air cleaner is ion emission through corona discharge, which electrically charges airborne particles with unipolar ions causing them either to repel each other or deposit on nearby surfaces due to image charges or static electrification. In addition, some ionic cleaners act as electrostatic precipitators: directional ionic wind creates air and particle flow over the precipitation plates and particles are charged and deposited onto those plates. Several studies have been conducted to test the ability of ionic cleaners to remove particles from the air (Niu et al. 2001; Grabarczyk 2001; Grinshpun et al. 2001, 2004, 2005; Lee et al. 2004). These studies have detected and quantified a large reduction in airborne particulate matter due to the presence of unipolar ions. Grabarczyk (2001) found that the number concentration of particles between 0.4 and 2.5 μm underwent a 20-fold reduction after one hour of ion production in an unoccupied chamber of 50 m 3. A study examining wearable ionizers (Grinshpun et al. 2001) found that the particle removal efficiency of the ionizer was 80% after 30 minutes and 100% after 1.5 hours in a 2 m 3 chamber. A later study by Grinshpun et al. (2005) tested commercially available ionic air cleaners in a 2.6 m 3 chamber and found that the unit which produced the most ions demonstrated 100% particulate matter removal within 10 to 12 minutes for particle sizes between 0.3 and 3.0 μm. Lee et al. (2004) tested commercially available ionic cleaners in a 24.3 m 3 test chamber and found that a 30 minute operation of the device, which produced the most ions, resulted in the removal of about 95% of 1.0 μm particles from the air above and beyond the decay rate due to particle settling. The studies clearly indicate that ions can facilitate a reduction in airborne particulate matter. The limitation of the aforementioned studies, however, is that they utilized uninhabited chamber environments to study the effect of ions on air quality, and thereby have not challenged the ionic air cleaners with real-life environments. Unlike in the test chambers, the airborne particles in real indoor living spaces can be generated continuously by various activities of the inhabitants and the infiltration of outdoor particles through the building envelope. Therefore, the goal of our study was to further the understanding of the effect of ionic air cleaners on the concentrations of airborne indoor particles by testing an ionic cleaner in a characteristic residential environment with residents engaging in their usual daily activities. One of the concerns for the application of ionic air cleaners is production of ozone, a harmful respiratory irritant. The corona discharge can cause ozone formation through oxidation of the discharge wires and back-charging on the plates, a phenomenon that increases as the device is operated for longer time periods and becomes dusty (Dorsey and Davidson 1994). Niu et al. (2001) measured the ozone emissions from 27 ionic air cleaners and found that 5 devices generated ozone, with generation rates ranging from 56 to 2,757 μg/h. None of the other aforementioned papers investigating the efficiency of ionic air cleaners presented data on ozone levels concurrent to air cleaner operation. Thus, the primary goal of our investigation was to determine the effect of a leading commercially available ionic air cleaner on the presence of airborne indoor particles and to measure the ozone levels produced by the cleaner in a typical residential setting. Since an actual residential setting features a constant generation of airborne particles by the occupants of the residency as well as infiltration of particles from the outdoors, we measured not the absolute indoor particle concentrations but rather ratios of indoor/outdoor particle number and mass concentrations. All of the experiments were conducted in a typical one bedroom apartment inhabited by two adults and a dog. The experiments tested the effect of the air cleaner on ambient air quality during normal daily activities and also in a still room, simulating nighttime conditions. Ion concentration, ozone production, and indoor/outdoor particle mass and number concentration ratios were measured to characterize the temporal and spatial effect of the ionic air cleaner. Additionally, an analysis of the horizontal and vertical distribution of ions was conducted to better understand the ion diffusion process in ionic air cleaner operation. MEASUREMENT METHODS Ionic Air Cleaner The ionic air cleaner used in this study is a popular commercially available unit produced by a leading manufacturer of ionic air cleaners. It features a tower design and, according to the manufacturer, is to be used in larger rooms. The cleaner has three settings for ion production rate and it was used at the highest setting during all experiments. The ionic air cleaner s air flow rate was estimated by using the flowhood approach, in which an aluminum hood was used to channel the cleaner s entire air flow into a well defined area, where the flow became rather uniform. By measuring the average air flow velocity (Traceable Hotwire Anemometer, Control Company, Friendswood, TX) across the plain (about a dozen measurements were taken) we determined the air flow to be m 3 /sec (25.5 ft 3 /min). The average ion and ozone concentrations were measured across the same plane using Air Ion Counter (AlphaLab Inc., West Salt Lake City, UT) and UV Photometric O 3 Analyzer (Thermo Environmental Instruments, Inc., Waltham, MA), respectively. Based on these measurements, we estimated the ozone production rate to be 0.8 μg/sec ( 3.0 mg/hr) and the ion production rate to be ions/sec. Testing Environment The study environment was a furnished one bedroom apartment with a total volume of m 3 (the floor area of the entire apartment was approximately 55 m 2 ) (Figure 1). The measurements were performed in a living room, where residents spend

4 EFFICIENCY OF IONIC AIR CLEANER FIG Schematic of the one bedroom apartment used for this study. Windows were closed and there was no mechanical ventilation during the experiments. most of their time. The ionic cleaner was placed in a corner of the room with ion emission directed towards the middle of the room. The indoor monitoring station was placed approximately in the middle of the room. During the measurements, the occupants of the apartment (two adults and a dog, a 65-pound Labrador Retriever) engaged in their normal activities. The apartment was not mechanically ventilated and windows and doors remained closed for the experiments with the exception of occasional and brief door openings and closings in the portion of the study challenging the air cleaner to normal daily activity. The entryway of the apartment opened directly to outside and not to a communal hallway, so the apartment was isolated from the effects of air movement in other apartments. During the testing period a stovetop cooking range operating on natural gas was used for meal preparation at times common for breakfast, lunch, and dinner. Ionic Cleaner Effect on Indoor/Outdoor Particle Ratios During Normal Daily Activities In its most common use, an ionic air cleaner is put directly on the floor and turned on. Thus, in our first set of experiments, we used such a setup and investigated how the ionic air cleaner affects indoor/outdoor particle number and mass concentration ratios during normal daily activity in the apartment. The measurements were performed from 9 AM Saturdays to 5 PM Sundays (32 hours) and were repeated for three weekends with the ionic air cleaner turned ON, and for three weekends with the ionic air cleaner turned OFF, with the ON/OFF pattern alternated each week. We chose to perform the measurements during the same time period so that the activity pattern of the residents would be similar during the measurements with the ionic cleaner turned ON and OFF. In addition, the outdoor vehicular traffic activity, which may influence particle penetration though the building envelope, was expected to be similar during the measurements with the ionic cleaner turned ON and OFF. Two optical particle counters (model 1.108, Grimm Technologies Inc., Douglasville, GA) were used to measure particle number concentrations indoors and outdoors for particles from 0.3 to 3 μm in 8 size channels. Two real-time passive samplers (pdr-model 1000AN, Thermo Electron Corporation, Franklin, MA) measured total particle mass concentration indoors and outdoors. The agreement between the two optical counters and the

5 318 D. BERRY ET AL. two mass monitors was tested in an indoor environment prior to the experiments. The observed differences, if any, were used to calculate the correction factor which was applied in our calculations. The hourly particle number and mass concentrations provided by these devices were used to calculate the indoor/outdoor number concentration ratio, I/O NC, and indoor/outdoor mass concentration ratio, I/O MC, for a particular hour. The hourly data were also used to average particle number and mass concentrations over the entire 32 hour measurement period. The hourly averaging times have been used by other indoor pollution studies as well (Johnson et al. 2004; Hussein et al. 2005; and Ni Riain et al. 2003).The airborne ion concentration, C ION,was measured using an ion detector (Air Ion Counter, AlphaLab Inc.) and was determined every fifteen minutes between the hours of 9 AM and 5 PM, and hourly between 6 PM and 12 AM. A portable ozone meter (Z-1200 Ozone Meter, Environmental Sensors Co, Boca Raton, FL) was used to determine ozone concentration, C OZONE, concurrent to the ion readings. A measurement station equipped with instruments for indoor measurements was placed in the middle of the living room and axially to the ionic air cleaner at a distance, d AC, of 1.8 m (Figure 1). The indoor measurement height, h, was about 1.0 m above the floor, consistent with the US Environmental Protection Agency s (EPA) practice for measuring indoor particle concentrations. The outdoor monitoring station was placed at the same absolute elevation as the indoor measurement station (1.5 m above ground) and approximately 0.5 m from the nearest wall. Temporal Investigation of the Ionic Cleaner Operation Many people use the ionic air cleaners in their bedrooms during night hours. If an ionic cleaner is positioned next to a bed during rest hours, a person s breathing zone would be at approximately the middle of the ionic cleaner s height. Thus, in our second set of experiments we investigated the effect of the ionic air cleaner on indoor/outdoor particle ratios as a function of time and axial distance from the mid-height of the ionic cleaner. We also measured the output of ions and ozone. Experiment duration was 8 hours to represent the recommended time for sleep. Measurement stations were located axially to the ionic air cleaner at d AC = 0, 0.3, 0.6, 1.2, 1.8, 2.4, and 3.0 m (Figure 1). At each measurement station the measurements were taken every hour for eight hours. The room was undisturbed for the duration of the experiment and only one person, the observer, was present during the measurements. Measurements using the indoor optical particle counter and particle mass monitor were taken hourly for five minutes at each measurement station from the start of the air cleaner operation using one minute averages. The measurements from the outdoor optical particle counter and passive mass monitor were used to calculate hourly averages and then combined with the indoor measurements to calculate the indoor/outdoor number concentration ratio, I/O NC, and indoor/outdoor mass concentration ratio, I/O MC for a particular hour. The measurements of ion concentration, C ION, and the ozone concentration, C OZONE, were taken at the start of the air cleaner operation and successively every hour at each measurement station. The entire experiment was repeated on three different days and the data presented in the Results section show averages and standard deviations resulting from those repeats. Spatial Investigation of the Ionic Cleaner Operation During normal daily activities in the apartment, when an ionic air cleaner is placed on the floor, the operator s breathing zone is above the cleaner s height. Thus, we investigated how ion concentration depended on vertical distance h from the floor. To accomplish this, we measured C ION at six stations at the start of the ionic air cleaner s operation and hourly for four successive hours. Measurements were taken at three different heights, h, from the floor with h = 0.5 m (the mid-height of the air cleaner), 1 m, and 2 m from the floor and at d AC = 0.6 and 1.2 m for each of the three heights. These measurements helped us determine whether the vertical distance from the cleaner affects the concentration of ions, which, in turn, may influence the removal of airborne particles, especially in the breathing zone. RESULTS AND DISCUSSION Ionic Cleaner Effect on Indoor/Outdoor Particle Ratios During Normal Daily Activities In our first test, the ionic air cleaner was challenged to a 32 hour operation during typical weekend activities at an apartment. The average concentration of airborne ions, C ION, increased approximately linearly (r 2 = 0.91) during all three 32 hour experiments, resulting in an average C ION = e /cm 3 at the end of the experiments (Figure 2). The ion concentration did not FIG. 2. Ion concentration over a 32-hour period when measurements were performed during normal daily activities. The graph displays values from three repeats for both the Ionic cleaner ON and OFF.

6 EFFICIENCY OF IONIC AIR CLEANER 319 FIG. 3. Ozone concentration over a 32-hour period when measurements were performed during normal daily activities. The graph displays values from three repeats for both the ionic cleaner ON and OFF. achieve steady state during this experiment because of the effect of continuous air mixing due to normal indoor activity. If the air cleaner was operated for a longer period we would expect to see the development of a steady state ion concentration, as was observed in later experiments (Figures 7 and 11). The ion levels were below detection limit during the weekends when the air cleaner was not operated. During the air cleaner s operation, the measured ozone levels rose quickly in the first hour and then remained relatively steady with C OZONE ranging from 13 to 19 ppb (Figure 3). Ozone was not detected during the weekends when the air cleaner was not operated. Figure 4 shows the indoor/outdoor particle mass concentration ratios,i/o MC, for three repeats with the ionic cleaner turned OFF and ON. With the ionic cleaner turned OFF the ratios ranged from 0.3 to 5.0. The peaks of the mass ratios seem to coincide with peaks of daily activity and meal preparation using stove: breakfast time 9 12 AM, dinner time 5 9 PM. I/O MC was frequently above unity during the experiment, an indication of indoor sources of particulate matter, such as walking or food preparation. With the ionic cleaner turned ON, the mass ratios ranged from 0.2 to 3.6. Visually it seems that with the ionic cleaner turned ON the peaks were fewer and less pronounced. However, Chi-square analysis of readings above certain I/O MC ratios (1.0, 1.5, 2.0, and 2.5) when the cleaner was ON and OFF indicated that the difference was not statistically significant: p > Also, during all six trials a valley of particle mass concentration ratios ( ) was measured during night-time hours: 12 7 AM. During this rest period, there was minimal activity in the apartment and the I/O MC was less than one. Unlike the studies described in the Introduction, the data presented in Figure 4 do not show unambiguous reduction in airborne particle mass concentration over the 32 hour measurement period. Therefore, we calculated average hourly particle FIG. 4. Indoor/outdoor particle mass concentration ratios with ionic cleaner turned OFF and ON as measured over a 32-hour period during normal indoor activities. mass concentrations indoors and outdoors with the ionic cleaner turned OFF and ON as well as indoor/outdoor ratios based on these average particle mass concentrations. As shown in Table 1, based on three repeats, the 32 hour average I/O MC was 1.03 when the air cleaner was turned OFF. This value is within the range established in the literature. When the ionic cleaner was operating, the average I/O MC was Thus, the average I/O MC was 30% lower during the tests when the air cleaner was operating; however, given a substantial variation in indoor and outdoor particle mass concentrations, a t-test comparison of these two sets (32 hour averages with cleaner ON and OFF) did not indicate statistical significance (p = 0.87). ANOVA comparison of all hourly I/O MC data with the ionic cleaner ON and OFF also did not yield a statistical significance (p = 0.08). On the other hand, an F-test for variance of all hourly indoor/outdoor mass ratios with ionic cleaner OFF and ON indicated that the difference in variance was significant (p = 0.006) with the variance for ON set being lower. This could be an indication that the presence of ions suppressed the generation of airborne particles by daily activities such as walking, cooking, cleaning, and so on. Similar observations could be made about indoor/outdoor particle number concentration ratios, I/O NC, for different particle sizes. An example of such ratios is shown in Figure 5. Depending on particle size, the ratios exhibit substantial variation ranging from less than unity to close to 10.0 (cleaner ON) and above 10.0 (cleaner OFF). For both, ionic cleaner turned OFF and ON, larger particles exhibited higher I/O NC compared to smaller particles. Most likely this was caused by the generation of larger particles by regular indoor activities. The other two repeats (not shown) yielded similar profiles of particle concentration ratios.

7 320 D. BERRY ET AL. TABLE 1 Mean hourly averages ± standard deviations for indoor and outdoor particle mass concentrations and their ratios (indoor/outdoor) as measured with ionic cleaner turned OFF and ON Average hourly airborne particle mass concentration over 32 hour measurement period, mg/m 3 Indoor/Outdoor Ratio, Indoors Outdoors I/O MC Ionic Air Cleaner OFF Repeat ± ± ± 1.24 Repeat ± ± ± 2.29 Repeat ± ± ± 0.86 Average ratio from three repeats 1.03 ± 2.83 Ionic Air Cleaner ON Repeat ± ± ± 0.23 Repeat ± ± ± 0.47 Repeat ± ± ± 0.48 Average ratio from three repeats 0.73 ± 0.39 Similar to our calculations with particle mass ratios, we used the data set shown in Figure 5 as well as data from two other repeats and calculated the average I/O NC for different particle size fractions (Table 2). There was substantial intra- and intermeasurement variability in I/O NC, so observed differences in indoor/outdoor particle number concentration ratios was not statistically significant: p > 0.25 for averages of all size fractions according to ANOVA. While not statistically significant, the most substantial decrease (40 70%) in the average I/O NC when the ionic cleaner was turned ON compared to when it was turned OFF was observed for particles smaller than 0.8 μm. For particles between 0.8 and 2.0 μm the decrease was smaller (0 14%), and for the largest investigated particle size fraction of μm, there was actually an increase in the average I/O NC by 26% when the ionic cleaner was operating. Our data presented above show certain reductions in average I/O MC and I/O NC, however such reductions were not statistically significant. This result is different from chamber studies described in the Introduction that showed an unambiguous and marked decrease in particle concentrations due to the presence of an ionic air cleaner. For example, Grinshpun et al. (2004), found that minutes of ionic cleaner operation yields 100% particle removal. We believe that the difference is a result of the different experimental setup used. In previous studies, the experiments were performed in chambers with controlled particle generation before starting the ionic air cleaner s operation and TABLE 2 Average indoor/outdoor particle number concentration ratios for different particle size fractions when measured with ionic cleaner turned OFF and ON. The measurements were performed using optical particle counters and the concentration ratios were calculated by averaging indoor and outdoor hourly particle number concentrations over 32-hour measurement period Average hourly indoor/outdoor particle number concentration ratios for different particle size fractions (μm) Ionic Air Cleaner turned OFF Repeat Repeat Repeat Average ratio from three repeats Ionic Air Cleaner turned ON Repeat Repeat Repeat Average ratio from three repeats

8 EFFICIENCY OF IONIC AIR CLEANER 321 FIG. 5. An example of indoor/outdoor particle number concentration ratios with ionic air cleaner turned OFF and ON as measured over a 32-hour period during normal indoor activities. absence of particle generation during the ionic air cleaner s operation. Our measurements were performed in an actual apartment where particles were constantly generated by daily activities such as walking and cooking. In addition, there was particle infiltration though a building envelope. It seems that in the presence of a constant particle generation, the ionic cleaner is less effective in particle removal compared with chamber studies. In fact, the only time during our measurements resembling chamber studies was the rest period from 12 AM to 7 AM, when there was no activity in the apartment and particle generation was minimal. The average indoor/outdoor particle number concentration ratios from that time period are shown in Figure 6. Under sleeping conditions, when the ionic cleaner was turned OFF, the I/O NC ranged from 0.3 to 4.0 across various particle size ranges and the inter-measurement variation (three different weekends) was clearly noticeable. Also, there was no noticeable particle clearance due to the gravitational settling, most likely due to penetration of additional particles from outdoors. On the other hand, when the ionic air cleaner was turned ON, the I/O NC range for different particle sizes was tighter ( ) and the F-test for the equality of variances of two sets indicated a statistical significance: p < The analysis of all hourly I/O NC data presented in Figure 6 using General Linear Model (GLM) indicated that the ratio with the ionic cleaner turned ON was

9 322 D. BERRY ET AL. FIG. 6. Indoor/outdoor particle number concentration ratios during night-time hours with ionic air cleaner turned ON and OFF. The data represent averages and standard deviations from three repeats. statistically different from the I/O NC ratio with the cleaner turned OFF (p < 0.005). In addition, linear regression of the data indicated that there was a statistically significant ( p = 0.004), albeit small, temporal decrease in I/O NC when the ionic cleaner was ON. No such decrease was observed for data with ionic cleaner turned OFF. Thus, the profile of the data observed here seem to resemble the chamber studies. Another factor that could have an impact on the performance of air cleaners in indoor environments is the Air Exchange Rate (AER). In the absence of indoor sources, fine particle indoor/outdoor mass ratios are usually lower when the AER is reduced by closing windows and stopping mechanical ventilation (Cyrys et al. 2004). The present study was conducted with closed windows and without mechanical ventilation, thus optimizing the conditions in the residence for observing changes in indoor/outdoor particle ratios when the air cleaner was operating. An increase in AER would likely reduce the observed performance of the air cleaner by increasing the transport of outdoor particles indoors. AER was not determined in this study, but a survey of 349 residences found that the AER had an average value of 1.06 hr 1 (Meng et al. 2005). Studies of individual residences with windows closed and no mechanical ventilation have found that AER can range between 0.36 and 2.29 hr 1 (Cyrys et al. 2004; Wallace et al. 2002; Johnson et al. 2004).

10 EFFICIENCY OF IONIC AIR CLEANER 323 FIG. 7. Airborne ion concentration as a function of time for several distances from the ionic air cleaner as measured during still conditions. The measurements were performed at a height of h = 0.5 m from the floor. The data represent averages and standard deviations from three repeats. Opening windows and operating fans can yield a large increase in AER (Wallace et al. 2002). Temporal Investigation of the Ionic Cleaner Operation The results above discussed the ionic cleaner s operation during regular activity in the apartment with indoor measurements performed at a horizontal distance d AC = 1.8 meters from the cleaner and at a height h 1.0 meter. One of the common applications of air cleaners is their use in bedrooms with the cleaner placed close to a person s bed. During such application, a person s breathing zone is at approximately the mid-height of the air cleaner. To test the air cleaner in this scenario we measured ion concentration, C ION, ozone concentration, C OZONE, and indoor/outdoor particle mass ratios, I/O MC, and number concentration ratios, I/O NC, at several distances (d AC = 0, 0.3, 0.6, 1.2, 1.8, 2.4, and 3.0 m) at the mid-height of the air cleaner (h = 0.5 m). There was no activity in the apartment for these measurements and the only person present was the observer. In the still room, the ion levels quickly increased over the first hour of air cleaner operation, and a saturation concentration of ions was achieved within one hour for every d AC (Figure 7). This behavior is in agreement with the findings of Grinshpun et al. (2004), who determined that ion levels quickly stabilized when measured directly in line with an ionizer. The ion saturation levels decreased rapidly with increasing distance from the air cleaner. The maximum saturation C ION was about e /cm 3, observed at d AC = 0 m (measured at the air cleaner s grid). At d AC = 0.3 and 0.6 m the saturation concentration dropped to approximately half and less than one quarter of the concentration at d AC = 0 m, respectively. Overall, it was observed that the saturation concentration decreased exponentially with increasing distance from the ionizer. Compared to the ion concentrations presented in Figure 2, the ion concentrations measured during this experiment at the same horizontal distance, d AC = 1.8 m, were almost twice as high. We believe that this difference in the ion concentration is due to the difference in measurement height and the limited vertical diffusion of ions. This issue was analyzed further in experiments described under the section Spatial Investigation of the Ionic Cleaner Operation. Ozone levels were found to be highly dependent upon the distance from the ionic cleaner in the still room (Figure 8). At d AC = 0 m the C OZONE rose for the first three hours and then stabilized between 70 and 77 ppb. At other measurement distances, the increase in C OZONE was slower and stabilization was not apparent at d AC = 0.3 m and 0.6 m, where the final C OZONE was 53 and 33 ppb, respectively. At further distances the ozone concentrations were below 20 ppb and remained relatively stable during the last six hours of measurement. There was no ozone detected throughout the experiment at d AC = 3.0 m. While there are no regulations governing ozone levels in residences, the EPA has established a health-based National Ambient Air Quality Standard for ozone of a maximum 8 hour average outdoor concentration of 80 ppb. This is slightly more stringent than the Occupational Safety and Health Administration s maximum 8 hour average of 100 ppb for work environments ( The American Conference of Governmental Industrial Hygienists has an occupational threshold limit (ACGIH TLV-TWA) of 50 ppb for heavy work, 80 ppb for moderate work, and 100 ppb for light work (ACGIH 2001). Ozone levels measured in front of the ionic air cleaner were very close to the EPA s standard for outdoor ozone levels. It is also important to remember that indoor ozone levels may be a result of several sources, including those from indoors and outdoors; therefore the combined ozone from several unrelated sources could lead to a level of indoor

11 324 D. BERRY ET AL. FIG. 8. Ozone concentration as a function of time for several distances from the ionic air cleaner as measured during still conditions. The measurements were performed at a height of h = 0.5 m from the floor. The data represent averages and standard deviations from three repeats. ozone exceeding the EPA guidelines. For example, in summer, high outdoor ozone levels could contribute to indoor ozone levels and together with ozone generated indoors could exceed EPA guidelines. Our experiments took place during spring when the weather was still cool so that ozone contribution from outdoors was unlikely to be significant. No background ozone was measured in any of the experiments when the ionic air cleaner was turned off. The operation of the ionic air cleaner in the still room had an effect on the presence of indoor particulate matter, as can be seen in the drop in indoor/outdoor particle mass ratio over time (Figure 9). On average, the I/O MC was between 0.9 and 1.4 at all measurement stations at the start of the air cleaner operation. Over the eight hour experiment, the I/O MC decreased for every measurement station (every distance from the ionic air cleaner) to a range of about 0.3 to 0.4. Commensurate with the decline in I/O MC there was a reduction in the amount of variation within the repeats. This implies that the air cleaner was able to reproduce a stable and similar I/O MC after eight hours of operation even though the starting particle mass concentration ratios were different. Previous studies have highlighted the propensity for variation in indoor/outdoor particles ratios (Jones et al. 2000), so consistent reduction in variation also indicates an effect brought about by the ionic cleaner. One may also notice that there is no apparent correlation between the decline in indoor/outdoor ratio and the distance from the air cleaner between 0 and 3 meters. This was unexpected because we observed a clear decrease in ion concentration with increasing distance. This result may be at least partially explained by the fact that this particular ionic cleaner also acts as an electrostatic precipitator. The ionic wind FIG. 9. Effect of ionic air cleaner on indoor/outdoor particle mass concentration ratios over time for several horizontal distances from the ionic air cleaner as measured during still conditions. The measurements were performed at h = 0.5 m from the floor. The data represent the averages and standard deviations from three repeats. created by the corona discharge creates the flow of air and particles across the precipitation plates where a certain fraction of particles is charged and deposited. This way, the ionic wind has fewer particles than surrounding air. We speculate that the air flow created by the ionic wind mixes with the surrounding air and smoothes differences in particle concentration along its flow axis. Since our measurements were performed along this axis, the particle concentration differences as a function of distance were obscured. It is important to note that the spatial variation in ion, ozone, and particle concentrations is highly dependent upon air mixing and indoor air flow, which are building specific. Figure 10 shows changes in indoor/outdoor particle number concentration ratios for different particle size fractions when the ionic air cleaner was operated in a still room. Each subplot in Figure 10 shows the effect of the ionic cleaner on I/O NC of a particular size fraction. Among all size fractions, the starting I/O NC ranged from 1.0 to 6.0 and every size fraction experienced a decline of I/O NC to below unity over the course of eight hour measurements at every d AC. The only exception was the largest measured size fraction of μm, which had an average final I/O NC of 1.4. It was anticipated that ions would have less effect on larger particles as a result of the limited charging that could occur because of their relatively small specific surface area. The smallest measured fraction, μm, also had a high final I/O NC relative to the other fractions, with an average I/O NC of 0.9. This is surprising because small particles have a larger specific surface area and should thus be preferentially affected by ions. However, the small particles may have been continuously replaced by particles diffusing through the building envelope from the outside, a process that also

12 EFFICIENCY OF IONIC AIR CLEANER 325 FIG. 10. Effect of ionic air cleaner on indoor/outdoor particle number concentration ratios for different particle size fractions as a function of time and horizontal distance from the ionic cleaner. The measurements were performed during still conditions at a height h = 0.5 m from the floor. The data represent the averages and standard variations from three repeats.

13 326 D. BERRY ET AL. preferentially selects small particles. Final average ratios for particles between 0.4 and 2.0 μm ranged between I/O NC = 0.5 and 0.7. As with the indoor/outdoor mass ratios, the deviation among measurements tended to decrease somewhat as the experiment progressed. The smallest particle size fraction ( μm) was an exception in this case. As with the particle I/O MC, the data with particle number concentration ratios did not show a clear correlation between indoor/outdoor ratios and distance from the ionic cleaner within the investigated range. Overall, during the measurements in the still room and at the mid-height of the air cleaner, we observed a decrease in indoor/outdoor particle number and mass concentration ratios. The decrease was apparent from 0 to 3.0 meters from the device. We did not observe a removal of particles to the degree that ionic air cleaners have been shown to achieve in previous chamber studies, but there was an observable effect. It must be reiterated, however, that coinciding with this reduction in indoor/outdoor particle ratios there was an increase in ozone levels as a byproduct of ion production. The ozone concentration next to the cleaner was about ppb, and though the levels dropped off quickly with increasing distance, there is still a risk of exposure to ozone that consumers should be aware of when using indoor ionic cleaners. While there are few studies of the health effects on long-term exposure to low levels of ozone, there is some indication that exposure might be associated with asthma development. McDonell et al. (1999) observed that for adult males there was a statistically significant relationship between asthma (report of doctor diagnosis) and 20-year mean 8-h average ambient ozone concentration. The study reported relative risk of 2.09 for a 27 ppb increase in ozone concentration (95% CI = 1.03 to 4.16). Spatial Investigation of the Ionic Cleaner Operation The data presented in Figures 9 and 10, where the measurements were performed in a still room, show a more pronounced effect on the indoor/outdoor particle ratios by the ionic air cleaner than was shown in Figures 4 and 5 and Tables 1 and 2, where measurements were performed during regular daily activities and that were likely to contribute to the indoor particle levels. Another difference was the ion concentration at the point of measurement as shown in Figures 2 and 7. Since the axis of the ionic wind is parallel to the floor and the main stream of ionic wind is about 0.5 m above the floor, it could be expected that most of the emitted ions will be close to the floor and that limited numbers of ions will diffuse upwards. The vertical distribution of ion concentration was determined by measuring C ION at varying height, h = 0.5, 1.0, and 2 m, and distance, d AC = 0.6 and 1.8 m. (Figure 11). The data indicate that height plays a more important role for C ION value than horizontal distance. The ion levels stabilized within an hour at h = 0.5 m as they did in Figure 7, but at h = 1.0 and 2.0 m the ion levels continued to rise. Also, the largest average C ION, e /cm 3, was measured at d AC = 0.6 m, h = 0.5 m. For h = 1.0 m, there was almost no difference between d AC = 2.0 ft and 6.0 ft, with the two stations having final concentrations of C ION = and e /cm 3, respectively. Likewise, the final concentrations at h = 2.0 m for d AC = 0.6 and 1.8 m were similar with C ION = and e /cm 3, respectively. These measurements indicate that the concentration of ions produced by the ionic cleaner substantially depends on the vertical distance from the cleaner, which has implications for the ability of the air cleaner to effectively reduce airborne particulates in the breathing zone as was shown in our measurements reported in the first part of FIG. 11. The concentration of airborne ions as a function of measurement duration when measured at several different horizontal and vertical distances from the ionic air cleaner during still conditions.

14 EFFICIENCY OF IONIC AIR CLEANER 327 the manuscript. Also, based on the results of previous studies, any increase in natural or mechanical ventilation might further decrease the observed performance of the ionic cleaner (Cyrys et al. 2004; Wallace et al. 2004). Comparison of Performance by Different Air Cleaners One of the ways to compare the performance of different air cleaners in various environments is by using the Clean Air Delivery Rate (CADR) (Shaugnessy and Sextro 2006). The CADR is a measure of the contaminant removal as a result of operating an air cleaning device. It can also be expressed as the product of the filtration efficiency and volumetric airflow through the device (Shaugnessy and Sextro, 2006). Thus, for this particular cleaner, even if all particles passing through it were collected, the CADR would not be higher than its flow rate estimated to be 53.4 ft 3 /min. Our measurements of μm particle concentrations entering and leaving the operating ionic cleaner indicated the reduction in particle concentration of 25%, which would yield a CADR value of approximately 13 ft 3 /min. This estimate however, does not take into account the cleaning effect by ions which would likely somewhat increase this value. In comparison, the Association of Home Appliance Manufacturers (AHAM) measured 157 air cleaners and found that mean CADR values for three test aerosols (smoke, pollen, and dust) are essentially the same, approximately 160 ft 3 /min, and more than 90% of the air cleaners tested have CADRs between 60 and 300 ft 3 /min (AHAM 2004). CONCLUSIONS Our data indicate that in actual residential environments ionic air cleaners may not be able to reduce airborne particulate matter as effectively as has been demonstrated in uninhabited chamber studies. When measured during normal daily activity, the average I/O MC was reduced from 1.03 to 0.73 and I/O NC underwent reductions for most of the particle size fractions. However, due to a substantial inter- and intra-measurement variation in particle ratios, the observed average reductions were not statistically significant (p > 0.05). The effect of the ionic cleaner on particle ratios during quiescent conditions was more pronounced. However, ozone production was also observed with the operation of the air cleaner. Indoor ozone concentrations were measured at steady concentrations of ppb during normal daily activity (measurement point was about 1.8 m from the device). The maximum measured ozone was 77 ppb, measured after 8 hours of operation in front of the device (at the face plate). Any users of ionic air cleaners should be aware of the hazards associated with production of indoor ozone and the additive effect of several ozone sources. 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