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Aerosol Science and Technology

ISSN: 0278-6826 (Print) 1521-7388 (Online) Journal homepage: https://www.tandfonline.com/loi/uast20

Effect of an Ionic Air Cleaner on Indoor/OutdoorParticle 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 IonicAir Cleaner on Indoor/Outdoor Particle Ratios in a Residential Environment, Aerosol Science andTechnology, 41:3, 315-328, DOI: 10.1080/02786820701199702

To link to this article: https://doi.org/10.1080/02786820701199702

Published online: 02 Mar 2007.

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Aerosol Science and Technology, 41:315–328, 2007Copyright c© American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/02786820701199702

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

David Berry,1,2 Gediminas Mainelis,1 and Donna Fennell1

1Department of Environmental Sciences, Rutgers, The State University of New Jersey,New Brunswick, New Jersey, USA2Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor,Ann Arbor, Michigan, USA

We tested a leading commercially available ionic air cleaner ina typical residential apartment to study the effect of the device onindoor/outdoor airborne particle number and mass concentrationratios. In addition, we also determined the indoor ozone and ionconcentration levels. When measured during normal daily activity,the average indoor/outdoor mass concentration ratio was reducedfrom 1.03 to 0.73 and the number concentration ratios underwentreductions for most of the particle size fractions. However, due toa substantial inter- and intra-measurement variation in particleratios, the observed average reductions were not statistically sig-nificant. When measurements were performed in a still room, theindoor/outdoor particle mass concentration ratio decreased from0.9–1.4 to 0.3–0.4 in eight hours when the air cleaner was operat-ing. Ambient ozone concentrations measured in the middle of theapartment were between 13–19 ppb during normal daily activityand the ozone levels increased to 77 ppb when measured in frontof the ionic cleaner during still conditions. We also found that thatthere was a limited vertical diffusion of ions. The highest ion con-centrations were measured at a 0.5 m height from the floor anddecreased substantially with increasing measurement height. Thisfinding may have implications for effective particle removal froma person’s breathing zone. Overall, we found that the tested brandof commercially available ionic air cleaners may have the capabil-ity to remove some airborne particulate matter in actual residen-tial settings, but its cleaning effect is reduced under normal dailyactivity.

INTRODUCTIONIndoor 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 2007.This study was supported in part through the Rutgers Undergrad-

uate Research Fellowship Program. The authors are thankful for thesupport.

Address correspondence to Gediminas Mainelis, Department ofEnvironmental Sciences, Rutgers, The State University of New Jer-sey, 14 College Farm Road, New Brunswick, NJ 08901. E-mail:mainelis@envsci.rutgers.edu

is an important public health concern. Asthma symptoms havebeen correlated with the presence of ambient levels of fine par-ticulate matter (0.01–2.5 μm) (Klot et al. 2002) and long-termexposure to particulate matter has been associated with elevatedrates 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 particu-late matter contributes to negative health effects.

Sources of indoor particulate matter have been characterizedin several studies. Reported indoor sources of particulate matterinclude smoking (Jones et al. 2000; Chao et al. 1998; Claytonet 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 oper-ation (Clayton et al. 1993; Kamens et al. 1991). It has also beenshown that diffusion of particulate matter from outside throughthe building envelope is a significant source of indoor particu-late matter, especially when there are no other indoor sources(Matson 2005; Ho et al. 2004). One way to quantify the rela-tive indoor air quality is through the use of the indoor/outdoorparticle number and mass concentration ratios. Such ratios canhelp to indicate whether the sources of indoor air pollutants aremostly from the outdoor or indoor environment. Indoor/outdoorparticle number concentration ratios reported in the literaturerange from 0.38 to 4.29 (Matson 2005; Monkonnen et al. 2005)and indoor/outdoor mass concentration ratios range from 0.54to 3.7 for typical residential environments (Monkonnen et al.2005; Ho et al. 2004; Sawant et al. 2004; Jones et al. 2000; Longet al. 2000; Monn et al. 1997; Miguel et al. 1995; Li 1994).The indoor/outdoor ratios vary due to several factors includ-ing averaging periods (Ni Riain et al. 2003; Monkonnen et al.2005), particle size (Li 1994), time of day (Long et al. 2000), andwind speed and direction (Ni Riain et al. 2003). For example,Monkonnen et al. (2004) found that one hour averaging periodsyielded indoor/outdoor particle number concentration ratios of0.38–4.29, while 24-hour averaging periods yield ratios of 0.7–1.17. Although there is variation in the indoor/outdoor ratios,they seem to be an effective tool for comparison of data sets.

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316 D. BERRY ET AL.

A variety of air cleaners are used to improve indoor air qual-ity. These include High Efficiency Particulate Air (HEPA) fil-ters (Antonicelli et al. 1991), ozone generators (Boeniger 1995),electrostatic precipitators (Zhuang et al. 2000), and negative iongenerators (Daniell et al. 1991). There has recently been an in-crease in the popularity and use of ionic air cleaners. The op-erating principle of an ionic air cleaner is ion emission throughcorona discharge, which electrically charges airborne particleswith unipolar ions causing them either to repel each other ordeposit on nearby surfaces due to image charges or static elec-trification. In addition, some ionic cleaners act as electrostaticprecipitators: directional ionic wind creates air and particle flowover the precipitation plates and particles are charged and de-posited onto those plates. Several studies have been conductedto test the ability of ionic cleaners to remove particles from theair (Niu et al. 2001; Grabarczyk 2001; Grinshpun et al. 2001,2004, 2005; Lee et al. 2004). These studies have detected andquantified a large reduction in airborne particulate matter due tothe presence of unipolar ions. Grabarczyk (2001) found that thenumber concentration of particles between 0.4 and 2.5 μm un-derwent a 20-fold reduction after one hour of ion production inan unoccupied chamber of 50 m3. A study examining wearableionizers (Grinshpun et al. 2001) found that the particle removalefficiency of the ionizer was 80% after 30 minutes and 100%after 1.5 hours in a 2 m3chamber. A later study by Grinshpunet al. (2005) tested commercially available ionic air cleaners ina 2.6 m3 chamber and found that the unit which produced themost ions demonstrated 100% particulate matter removal within10 to 12 minutes for particle sizes between 0.3 and 3.0 μm. Leeet al. (2004) tested commercially available ionic cleaners in a24.3 m3 test chamber and found that a 30 minute operation of thedevice, which produced the most ions, resulted in the removalof about 95% of 1.0 μm particles from the air above and beyondthe decay rate due to particle settling. The studies clearly indi-cate that ions can facilitate a reduction in airborne particulatematter. The limitation of the aforementioned studies, however,is that they utilized uninhabited chamber environments to studythe effect of ions on air quality, and thereby have not challengedthe ionic air cleaners with real-life environments. Unlike in thetest chambers, the airborne particles in real indoor living spacescan be generated continuously by various activities of the in-habitants and the infiltration of outdoor particles through thebuilding envelope.

Therefore, the goal of our study was to further the understand-ing of the effect of ionic air cleaners on the concentrations ofairborne indoor particles by testing an ionic cleaner in a charac-teristic residential environment with residents engaging in theirusual daily activities.

One of the concerns for the application of ionic air cleaners isproduction of ozone, a harmful respiratory irritant. The coronadischarge can cause ozone formation through oxidation of thedischarge wires and back-charging on the plates, a phenomenonthat increases as the device is operated for longer time peri-ods and becomes dusty (Dorsey and Davidson 1994). Niu et al.

(2001) measured the ozone emissions from 27 ionic air cleanersand found that 5 devices generated ozone, with generation ratesranging from 56 to 2,757μg/h. None of the other aforementionedpapers investigating the efficiency of ionic air cleaners presenteddata on ozone levels concurrent to air cleaner operation. Thus,the primary goal of our investigation was to determine the ef-fect of a leading commercially available ionic air cleaner on thepresence of airborne indoor particles and to measure the ozonelevels produced by the cleaner in a typical residential setting.Since an actual residential setting features a constant generationof airborne particles by the occupants of the residency as wellas infiltration of particles from the outdoors, we measured notthe absolute indoor particle concentrations but rather ratios ofindoor/outdoor particle number and mass concentrations.

All of the experiments were conducted in a typical one bed-room apartment inhabited by two adults and a dog. The experi-ments tested the effect of the air cleaner on ambient air qualityduring normal daily activities and also in a still room, simulatingnighttime conditions. Ion concentration, ozone production, andindoor/outdoor particle mass and number concentration ratioswere measured to characterize the temporal and spatial effect ofthe ionic air cleaner. Additionally, an analysis of the horizontaland vertical distribution of ions was conducted to better under-stand the ion diffusion process in ionic air cleaner operation.

MEASUREMENT METHODS

Ionic Air CleanerThe ionic air cleaner used in this study is a popular com-

mercially available unit produced by a leading manufacturer ofionic air cleaners. It features a tower design and, according to themanufacturer, is to be used in larger rooms. The cleaner has threesettings for ion production rate and it was used at the highestsetting during all experiments.

The ionic air cleaner’s air flow rate was estimated by usingthe flowhood approach, in which an aluminum hood was usedto channel the cleaner’s entire air flow into a well defined area,where the flow became rather uniform. By measuring the aver-age air flow velocity (Traceable Hotwire Anemometer, ControlCompany, Friendswood, TX) across the plain (about a dozenmeasurements were taken) we determined the air flow to be0.012 m3/sec (25.5 ft3/min). The average ion and ozone con-centrations were measured across the same plane using Air IonCounter (AlphaLab Inc., West Salt Lake City, UT) and UV Pho-tometric O3 Analyzer (Thermo Environmental Instruments, Inc.,Waltham, MA), respectively. Based on these measurements, weestimated the ozone production rate to be 0.8 μg/sec (∼3.0mg/hr) and the ion production rate to be 5.3 × 109 ions/sec.

Testing EnvironmentThe study environment was a furnished one bedroom apart-

ment with a total volume of 151.3 m3 (the floor area of the entireapartment was approximately 55 m2) (Figure 1). The measure-ments were performed in a living room, where residents spend

EFFICIENCY OF IONIC AIR CLEANER 317

FIG. 1. 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 theroom with ion emission directed towards the middle of the room.The indoor monitoring station was placed approximately in themiddle of the room. During the measurements, the occupants ofthe apartment (two adults and a dog, a 65-pound Labrador Re-triever) engaged in their normal activities. The apartment wasnot mechanically ventilated and windows and doors remainedclosed for the experiments with the exception of occasional andbrief door openings and closings in the portion of the study chal-lenging the air cleaner to normal daily activity. The entryway ofthe apartment opened directly to outside and not to a commu-nal hallway, so the apartment was isolated from the effects of airmovement in other apartments. During the testing period a stove-top cooking range operating on natural gas was used for mealpreparation at times common for breakfast, lunch, and dinner.

Ionic Cleaner Effect on Indoor/Outdoor Particle RatiosDuring Normal Daily Activities

In its most common use, an ionic air cleaner is put directlyon the floor and turned on. Thus, in our first set of experiments,

we used such a setup and investigated how the ionic air cleaneraffects indoor/outdoor particle number and mass concentrationratios during normal daily activity in the apartment.

The measurements were performed from 9 AM Saturdays to5 PM Sundays (32 hours) and were repeated for three weekendswith the ionic air cleaner turned ON, and for three weekendswith the ionic air cleaner turned OFF, with the ON/OFF patternalternated each week. We chose to perform the measurementsduring the same time period so that the activity pattern of theresidents would be similar during the measurements with theionic cleaner turned ON and OFF. In addition, the outdoor ve-hicular traffic activity, which may influence particle penetrationthough the building envelope, was expected to be similar duringthe measurements with the ionic cleaner turned ON and OFF.

Two optical particle counters (model 1.108, Grimm Tech-nologies Inc., Douglasville, GA) were used to measure particlenumber concentrations indoors and outdoors for particles from0.3 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 out-doors. The agreement between the two optical counters and the

318 D. BERRY ET AL.

two mass monitors was tested in an indoor environment prior tothe experiments. The observed differences, if any, were used tocalculate the correction factor which was applied in our calcula-tions. The hourly particle number and mass concentrations pro-vided by these devices were used to calculate the indoor/outdoornumber concentration ratio, I/ONC , and indoor/outdoor massconcentration ratio, I/OMC , for a particular hour. The hourlydata were also used to average particle number and mass con-centrations over the entire 32 hour measurement period. Thehourly averaging times have been used by other indoor pollu-tion studies as well (Johnson et al. 2004; Hussein et al. 2005; andNi Riain et al. 2003).The airborne ion concentration, CION, wasmeasured using an ion detector (Air Ion Counter, AlphaLab Inc.)and was determined every fifteen minutes between the hours of 9AM and 5 PM, and hourly between 6 PM and 12 AM. A portableozone meter (Z-1200 Ozone Meter, Environmental Sensors Co,Boca Raton, FL) was used to determine ozone concentration,COZONE, concurrent to the ion readings.

A measurement station equipped with instruments for indoormeasurements was placed in the middle of the living room andaxially to the ionic air cleaner at a distance, dAC , of 1.8 m (Fig-ure 1). The indoor measurement height, h, was about 1.0 m abovethe floor, consistent with the US Environmental ProtectionAgency’s (EPA) practice for measuring indoor particle concen-trations. The outdoor monitoring station was placed at the sameabsolute elevation as the indoor measurement station (1.5 mabove ground) and approximately 0.5 m from the nearest wall.

Temporal Investigation of the Ionic Cleaner OperationMany people use the ionic air cleaners in their bedrooms

during night hours. If an ionic cleaner is positioned next to abed during rest hours, a person’s breathing zone would be atapproximately the middle of the ionic cleaner’s height. Thus, inour second set of experiments we investigated the effect of theionic air cleaner on indoor/outdoor particle ratios as a function oftime and axial distance from the mid-height of the ionic cleaner.We also measured the output of ions and ozone. Experimentduration was 8 hours to represent the recommended time forsleep. Measurement stations were located axially to the ionic aircleaner at dAC = 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 everyhour for eight hours. The room was undisturbed for the durationof the experiment and only one person, the observer, was presentduring the measurements.

Measurements using the indoor optical particle counter andparticle mass monitor were taken hourly for five minutes at eachmeasurement station from the start of the air cleaner operationusing one minute averages. The measurements from the outdooroptical particle counter and passive mass monitor were used tocalculate hourly averages and then combined with the indoormeasurements to calculate the indoor/outdoor number concen-tration ratio, I/ONC , and indoor/outdoor mass concentrationratio, I/OMC for a particular hour. The measurements of ionconcentration, CION, and the ozone concentration, COZONE, were

taken at the start of the air cleaner operation and successivelyevery hour at each measurement station. The entire experimentwas repeated on three different days and the data presented in theResults section show averages and standard deviations resultingfrom those repeats.

Spatial Investigation of the Ionic Cleaner OperationDuring normal daily activities in the apartment, when an ionic

air cleaner is placed on the floor, the operator’s breathing zoneis above the cleaner’s height. Thus, we investigated how ionconcentration depended on vertical distance h from the floor. Toaccomplish this, we measured CION at six stations at the startof the ionic air cleaner’s operation and hourly for four succes-sive hours. Measurements were taken at three different heights,h, from the floor with h = 0.5 m (the mid-height of the aircleaner), 1 m, and 2 m from the floor and at dAC = 0.6 and1.2 m for each of the three heights. These measurements helpedus determine whether the vertical distance from the cleaner af-fects the concentration of ions, which, in turn, may influence theremoval of airborne particles, especially in the breathing zone.

RESULTS AND DISCUSSION

Ionic Cleaner Effect on Indoor/Outdoor Particle RatiosDuring Normal Daily Activities

In our first test, the ionic air cleaner was challenged to a 32hour operation during typical weekend activities at an apartment.The average concentration of airborne ions, CION, increased ap-proximately linearly (r2 = 0.91) during all three 32 hour exper-iments, resulting in an average CION = 5.5 × 104 e−/cm3 at theend of the experiments (Figure 2). The ion concentration did not

FIG. 2. Ion concentration over a 32-hour period when measurements wereperformed during normal daily activities. The graph displays values from threerepeats for both the Ionic cleaner ON and OFF.

EFFICIENCY OF IONIC AIR CLEANER 319

FIG. 3. Ozone concentration over a 32-hour period when measurements wereperformed during normal daily activities. The graph displays values from threerepeats for both the ionic cleaner ON and OFF.

achieve steady state during this experiment because of the effectof continuous air mixing due to normal indoor activity. If theair cleaner was operated for a longer period we would expectto see the development of a steady state ion concentration, aswas observed in later experiments (Figures 7 and 11). The ionlevels were below detection limit during the weekends when theair cleaner was not operated. During the air cleaner’s operation,the measured ozone levels rose quickly in the first hour and thenremained relatively steady with COZONE ranging from 13 to 19ppb (Figure 3). Ozone was not detected during the weekendswhen the air cleaner was not operated.

Figure 4 shows the indoor/outdoor particle mass concentra-tion ratios,I/OMC , for three repeats with the ionic cleaner turnedOFF and ON. With the ionic cleaner turned OFF the ratiosranged from 0.3 to 5.0. The peaks of the mass ratios seem tocoincide with peaks of daily activity and meal preparation usingstove: breakfast time 9–12 AM, dinner time 5–9 PM. I/OMC

was frequently above unity during the experiment, an indicationof indoor sources of particulate matter, such as walking or foodpreparation. With the ionic cleaner turned ON, the mass ratiosranged from 0.2 to 3.6. Visually it seems that with the ioniccleaner turned ON the peaks were fewer and less pronounced.However, Chi-square analysis of readings above certain I/OMC

ratios (1.0, 1.5, 2.0, and 2.5) when the cleaner was ON and OFFindicated that the difference was not statistically significant: p >

0.05. Also, during all six trials a valley of particle mass concen-tration ratios (0.2–0.6) was measured during night-time hours:12–7 AM. During this rest period, there was minimal activity inthe apartment and the I/OMC was less than one.

Unlike the studies described in the Introduction, the datapresented in Figure 4 do not show unambiguous reduction inairborne particle mass concentration over the 32 hour measure-ment period. Therefore, we calculated average hourly particle

FIG. 4. Indoor/outdoor particle mass concentration ratios with ionic cleanerturned OFF and ON as measured over a 32-hour period during normal indooractivities.

mass concentrations indoors and outdoors with the ionic cleanerturned OFF and ON as well as indoor/outdoor ratios based onthese average particle mass concentrations. As shown in Table 1,based on three repeats, the 32 hour average I/OMC was 1.03when the air cleaner was turned OFF. This value is withinthe range established in the literature. When the ionic cleanerwas operating, the average I/OMC was 0.73. Thus, the averageI/OMC was 30% lower during the tests when the air cleanerwas operating; however, given a substantial variation in indoorand outdoor particle mass concentrations, a t-test comparison ofthese two sets (32 hour averages with cleaner ON and OFF) didnot indicate statistical significance (p = 0.87). ANOVA com-parison of all hourly I/OMC data with the ionic cleaner ON andOFF also did not yield a statistical significance (p = 0.08). On theother hand, an F-test for variance of all hourly indoor/outdoormass ratios with ionic cleaner OFF and ON indicated that thedifference in variance was significant (p = 0.006) with the vari-ance for ON set being lower. This could be an indication that thepresence of ions suppressed the generation of airborne particlesby daily activities such as walking, cooking, cleaning, and so on.

Similar observations could be made about indoor/outdoorparticle number concentration ratios, I/ONC,for different parti-cle sizes. An example of such ratios is shown in Figure 5. De-pending on particle size, the ratios exhibit substantial variationranging from less than unity to close to 10.0 (cleaner ON) andabove 10.0 (cleaner OFF). For both, ionic cleaner turned OFFand ON, larger particles exhibited higher I/ONC compared tosmaller particles. Most likely this was caused by the generationof larger particles by regular indoor activities. The other tworepeats (not shown) yielded similar profiles of particle concen-tration ratios.

320 D. BERRY ET AL.

TABLE 1Mean 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 concentrationover 32 hour measurement period, mg/m3

Indoor/Outdoor Ratio,Indoors Outdoors I/OMC

Ionic Air Cleaner OFFRepeat 1 0.028 ± 0.025 0.026 ± 0.014 1.10 ± 1.24Repeat 2 0.080 ± 0.180 0.083 ± 0.088 0.96 ± 2.29Repeat 3 0.062 ± 0.045 0.060 ± 0.022 1.03 ± 0.86

Average ratio from three repeats 1.03 ± 2.83Ionic Air Cleaner ON

Repeat 1 0.040 ± 0.022 0.079 ± 0.058 0.50 ± 0.23Repeat 2 0.030 ± 0.016 0.036 ± 0.016 0.82 ± 0.47Repeat 3 0.029 ± 0.013 0.033 ± 0.008 0.89 ± 0.48

Average ratio from three repeats 0.73 ± 0.39

Similar to our calculations with particle mass ratios, we usedthe data set shown in Figure 5 as well as data from two otherrepeats and calculated the average I/ONC for different particlesize fractions (Table 2). There was substantial intra- and inter-measurement variability in I/ONC , so observed differences inindoor/outdoor particle number concentration ratios was not sta-tistically significant: p > 0.25 for averages of all size fractionsaccording to ANOVA. While not statistically significant, themost substantial decrease (40–70%) in the average I/ONC whenthe ionic cleaner was turned ON compared to when it was turnedOFF was observed for particles smaller than 0.8 μm. For parti-cles between 0.8 and 2.0 μm the decrease was smaller (0–14%),and for the largest investigated particle size fraction of 2.0–3.0

TABLE 2Average indoor/outdoor particle number concentration ratios for different particle size fractions when measured with ioniccleaner turned OFF and ON. The measurements were performed using optical particle counters and the concentration ratioswere calculated by averaging indoor and outdoor hourly particle number concentrations over 32-hour measurement period

Average hourly indoor/outdoor particle number concentrationratios for different particle size fractions (μm)

0.3–0.4 0.4–0.5 0.5–0.65 0.65–0.8 0.8–1.0 1.0–1.6 1.6–2.0 2.0–3.0

Ionic Air Cleaner turned OFFRepeat 1 0.77 0.44 0.91 1.10 1.38 4.27 2.70 1.87Repeat 2 2.42 3.18 5.40 2.68 1.54 2.27 0.87 0.53Repeat 3 1.22 0.63 0.90 0.39 0.31 0.59 0.41 1.11

Average ratio from three repeats 1.47 1.41 2.40 1.39 1.08 2.38 1.39 1.17Ionic Air Cleaner turned ON

Repeat 1 0.52 0.22 0.50 0.68 0.88 2.68 1.21 1.67Repeat 2 0.61 0.28 0.22 0.12 0.19 0.26 0.17 0.51Repeat 3 1.14 0.80 1.96 1.81 1.71 4.16 2.21 2.27

Average ratio from three repeats 0.75 0.43 0.90 0.87 0.93 2.37 1.19 1.48

μm, there was actually an increase in the average I/ONC by26% when the ionic cleaner was operating.

Our data presented above show certain reductions in averageI/OMC and I/ONC , however such reductions were not statis-tically significant. This result is different from chamber studiesdescribed in the Introduction that showed an unambiguous andmarked decrease in particle concentrations due to the presence ofan ionic air cleaner. For example, Grinshpun et al. (2004), foundthat 10–12 minutes of ionic cleaner operation yields 100% par-ticle removal. We believe that the difference is a result of thedifferent experimental setup used. In previous studies, the ex-periments were performed in chambers with controlled particlegeneration before starting the ionic air cleaner’s operation and

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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 duringnormal indoor activities.

absence of particle generation during the ionic air cleaner’s op-eration. Our measurements were performed in an actual apart-ment where particles were constantly generated by daily activi-ties such as walking and cooking. In addition, there was particleinfiltration though a building envelope. It seems that in the pres-ence of a constant particle generation, the ionic cleaner is lesseffective in particle removal compared with chamber studies. Infact, the only time during our measurements resembling chamberstudies was the rest period from 12 AM to 7 AM, when therewas no activity in the apartment and particle generation wasminimal. The average indoor/outdoor particle number concen-tration ratios from that time period are shown in Figure 6. Under

sleeping conditions, when the ionic cleaner was turned OFF, theI/ONC ranged from 0.3 to 4.0 across various particle size rangesand the inter-measurement variation (three different weekends)was clearly noticeable. Also, there was no noticeable particleclearance due to the gravitational settling, most likely due topenetration of additional particles from outdoors. On the otherhand, when the ionic air cleaner was turned ON, the I/ONC

range for different particle sizes was tighter (0.4-1.0) and theF-test for the equality of variances of two sets indicated a statis-tical significance: p < 0.001. The analysis of all hourly I/ONC

data presented in Figure 6 using General Linear Model (GLM)indicated that the ratio with the ionic cleaner turned ON was

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 andstandard deviations from three repeats.

statistically different from the I/ONC ratio with the cleanerturned OFF (p < 0.005). In addition, linear regression of thedata indicated that there was a statistically significant (p =0.004), albeit small, temporal decrease in I/ONC when the ioniccleaner was ON. No such decrease was observed for data withionic cleaner turned OFF. Thus, the profile of the data observedhere seem to resemble the chamber studies.

Another factor that could have an impact on the performanceof air cleaners in indoor environments is the Air ExchangeRate (AER). In the absence of indoor sources, fine particle in-door/outdoor mass ratios are usually lower when the AER isreduced by closing windows and stopping mechanical ventila-

tion (Cyrys et al. 2004). The present study was conducted withclosed windows and without mechanical ventilation, thus op-timizing the conditions in the residence for observing changesin indoor/outdoor particle ratios when the air cleaner was op-erating. An increase in AER would likely reduce the observedperformance of the air cleaner by increasing the transport ofoutdoor particles indoors. AER was not determined in this study,but a survey of 349 residences found that the AER had an aver-age value of 1.06 hr−1 (Meng et al. 2005). Studies of individualresidences with windows closed and no mechanical ventilationhave 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).

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FIG. 7. Airborne ion concentration as a function of time for several distances from the ionic air cleaner as measured during still conditions. The measurementswere 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 increasein AER (Wallace et al. 2002).

Temporal Investigation of the Ionic Cleaner OperationThe results above discussed the ionic cleaner’s operation dur-

ing regular activity in the apartment with indoor measurementsperformed at a horizontal distance dAC = 1.8 meters from thecleaner and at a height h ≈ 1.0 meter. One of the common appli-cations of air cleaners is their use in bedrooms with the cleanerplaced close to a person’s bed. During such application, a per-son’s breathing zone is at approximately the mid-height of theair cleaner. To test the air cleaner in this scenario we measuredion concentration, CION, ozone concentration, COZONE, and in-door/outdoor particle mass ratios, I/OMC , and number concen-tration ratios, I/ONC , at several distances (dAC = 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 mea-surements and the only person present was the observer. In thestill room, the ion levels quickly increased over the first hourof air cleaner operation, and a saturation concentration of ionswas achieved within one hour for every dAC (Figure 7). Thisbehavior is in agreement with the findings of Grinshpun et al.(2004), who determined that ion levels quickly stabilized whenmeasured directly in line with an ionizer.

The ion saturation levels decreased rapidly with increasingdistance from the air cleaner. The maximum saturation CION

was about 1.0 × 106 e−/cm3, observed at dAC = 0 m (measuredat the air cleaner’s grid). At dAC = 0.3 and 0.6 m the satura-tion concentration dropped to approximately half and less thanone quarter of the concentration at dAC = 0 m, respectively.Overall, it was observed that the saturation concentration de-creased 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 samehorizontal distance, dAC = 1.8 m, were almost twice as high.We believe that this difference in the ion concentration is dueto the difference in measurement height and the limited verticaldiffusion of ions. This issue was analyzed further in experimentsdescribed under the section “Spatial Investigation of the IonicCleaner Operation.”

Ozone levels were found to be highly dependent upon thedistance from the ionic cleaner in the still room (Figure 8).At dAC = 0 m the COZONE rose for the first three hours andthen stabilized between 70 and 77 ppb. At other measurementdistances, the increase in COZONE was slower and stabilizationwas not apparent at dAC = 0.3 m and 0.6 m, where the finalCOZONE was 53 and 33 ppb, respectively. At further distancesthe ozone concentrations were below 20 ppb and remained rel-atively stable during the last six hours of measurement. Therewas no ozone detected throughout the experiment at dAC =3.0 m. While there are no regulations governing ozone lev-els in residences, the EPA has established a health-based Na-tional Ambient Air Quality Standard for ozone of a maximum8 hour average outdoor concentration of 80 ppb. This is slightlymore stringent than the Occupational Safety and Health Admin-istration’s maximum 8 hour average of 100 ppb for work en-vironments (http://www.epa.gov/iaq/pubs/ozonegen.html). TheAmerican Conference of Governmental Industrial Hygienistshas an occupational threshold limit (ACGIH TLV-TWA) of 50ppb for heavy work, 80 ppb for moderate work, and 100 ppbfor light work (ACGIH 2001). Ozone levels measured in frontof the ionic air cleaner were very close to the EPA’s standardfor outdoor ozone levels. It is also important to remember thatindoor ozone levels may be a result of several sources, includingthose from indoors and outdoors; therefore the combined ozonefrom several unrelated sources could lead to a level of indoor

324 D. BERRY ET AL.

FIG. 8. Ozone concentration as a function of time for several distances fromthe ionic air cleaner as measured during still conditions. The measurements wereperformed at a height of h = 0.5 m from the floor. The data represent averagesand standard deviations from three repeats.

ozone exceeding the EPA guidelines. For example, in summer,high outdoor ozone levels could contribute to indoor ozone lev-els and together with ozone generated indoors could exceed EPAguidelines. Our experiments took place during spring when theweather was still cool so that ozone contribution from outdoorswas unlikely to be significant. No background ozone was mea-sured in any of the experiments when the ionic air cleaner wasturned off.

The operation of the ionic air cleaner in the still room hadan effect on the presence of indoor particulate matter, as can beseen in the drop in indoor/outdoor particle mass ratio over time(Figure 9). On average, the I/OMC was between 0.9 and 1.4 atall measurement stations at the start of the air cleaner operation.Over the eight hour experiment, the I/OMC decreased for everymeasurement station (every distance from the ionic air cleaner)to a range of about 0.3 to 0.4. Commensurate with the decline inI/OMC there was a reduction in the amount of variation withinthe repeats. This implies that the air cleaner was able to repro-duce a stable and similar I/OMC after eight hours of operationeven though the starting particle mass concentration ratios weredifferent. Previous studies have highlighted the propensity forvariation in indoor/outdoor particles ratios (Jones et al. 2000), soconsistent reduction in variation also indicates an effect broughtabout by the ionic cleaner. One may also notice that there is noapparent correlation between the decline in indoor/outdoor ratioand the distance from the air cleaner between 0 and 3 meters.This was unexpected because we observed a clear decrease inion concentration with increasing distance. This result may beat least partially explained by the fact that this particular ioniccleaner also acts as an electrostatic precipitator. The ionic wind

FIG. 9. Effect of ionic air cleaner on indoor/outdoor particle mass concentra-tion ratios over time for several horizontal distances from the ionic air cleaneras measured during still conditions. The measurements were performed at h =0.5 m from the floor. The data represent the averages and standard deviationsfrom three repeats.

created by the corona discharge creates the flow of air and par-ticles across the precipitation plates where a certain fraction ofparticles is charged and deposited. This way, the ionic wind hasfewer particles than surrounding air. We speculate that the airflow created by the ionic wind mixes with the surrounding airand smoothes differences in particle concentration along its flowaxis. Since our measurements were performed along this axis,the particle concentration differences as a function of distancewere obscured. It is important to note that the spatial variationin ion, ozone, and particle concentrations is highly dependentupon air mixing and indoor air flow, which are building specific.

Figure 10 shows changes in indoor/outdoor particle numberconcentration ratios for different particle size fractions when theionic air cleaner was operated in a still room. Each subplot inFigure 10 shows the effect of the ionic cleaner on I/ONC ofa particular size fraction. Among all size fractions, the startingI/ONC ranged from 1.0 to 6.0 and every size fraction experi-enced a decline of I/ONC to below unity over the course of eighthour measurements at every dAC . The only exception was thelargest measured size fraction of 2.0–3.0 μm, which had an aver-age final I/ONC of 1.4. It was anticipated that ions would haveless effect on larger particles as a result of the limited charg-ing that could occur because of their relatively small specificsurface area. The smallest measured fraction, 0.3–0.4 μm, alsohad a high final I/ONC relative to the other fractions, with anaverage I/ONC of 0.9. This is surprising because small parti-cles have a larger specific surface area and should thus be pref-erentially affected by ions. However, the small particles mayhave been continuously replaced by particles diffusing throughthe building envelope from the outside, a process that also

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 horizontaldistance 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 averagesand standard variations from three repeats.

326 D. BERRY ET AL.

preferentially selects small particles. Final average ratios forparticles between 0.4 and 2.0 μm ranged between I/ONC = 0.5and 0.7. As with the indoor/outdoor mass ratios, the deviationamong measurements tended to decrease somewhat as the ex-periment progressed. The smallest particle size fraction (0.3–0.4μm) was an exception in this case. As with the particle I/OMC ,the data with particle number concentration ratios did not showa clear correlation between indoor/outdoor ratios and distancefrom the ionic cleaner within the investigated range.

Overall, during the measurements in the still room and atthe mid-height of the air cleaner, we observed a decrease inindoor/outdoor particle number and mass concentration ratios.The decrease was apparent from 0 to 3.0 meters from the de-vice. We did not observe a removal of particles to the degreethat ionic air cleaners have been shown to achieve in previouschamber studies, but there was an observable effect. It mustbe reiterated, however, that coinciding with this reduction inindoor/outdoor particle ratios there was an increase in ozonelevels as a byproduct of ion production. The ozone concentra-tion next to the cleaner was about 70–77 ppb, and though thelevels dropped off quickly with increasing distance, there is stilla risk of exposure to ozone that consumers should be aware ofwhen using indoor ionic cleaners. While there are few studies ofthe health effects on long-term exposure to low levels of ozone,there is some indication that exposure might be associated withasthma development. McDonell et al. (1999) observed that foradult males there was a statistically significant relationship be-tween asthma (report of doctor diagnosis) and 20-year mean 8-haverage ambient ozone concentration. The study reported rela-tive risk of 2.09 for a 27 ppb increase in ozone concentration(95% CI = 1.03 to 4.16).

FIG. 11. The concentration of airborne ions as a function of measurement duration when measured at several different horizontal and vertical distances from theionic air cleaner during still conditions.

Spatial Investigation of the Ionic Cleaner OperationThe data presented in Figures 9 and 10, where the mea-

surements were performed in a still room, show a more pro-nounced effect on the indoor/outdoor particle ratios by the ionicair cleaner than was shown in Figures 4 and 5 and Tables 1 and2, where measurements were performed during regular daily ac-tivities and that were likely to contribute to the indoor particlelevels. Another difference was the ion concentration at the pointof measurement as shown in Figures 2 and 7. Since the axis ofthe ionic wind is parallel to the floor and the main stream of ionicwind is about 0.5 m above the floor, it could be expected thatmost of the emitted ions will be close to the floor and that limitednumbers of ions will diffuse upwards. The vertical distributionof ion concentration was determined by measuring CION at vary-ing height, h = 0.5, 1.0, and 2 m, and distance, dAC = 0.6 and1.8 m. (Figure 11). The data indicate that height plays a moreimportant role for CION value than horizontal distance. The ionlevels stabilized within an hour at h = 0.5 m as they did in Fig-ure 7, but at h = 1.0 and 2.0 m the ion levels continued to rise.Also, the largest average CION, 1.7 × 105 e−/cm3, was measuredat dAC = 0.6 m, h = 0.5 m. For h = 1.0 m, there was almost nodifference between dAC = 2.0 ft and 6.0 ft, with the two stationshaving final concentrations of CION = 7.0 × 104 and 6.0 × 104

e−/cm3, respectively. Likewise, the final concentrations at h =2.0 m for dAC = 0.6 and 1.8 m were similar with CION = 1.5× 104 and 1.4 × 104 e−/cm3, respectively. These measurementsindicate that the concentration of ions produced by the ioniccleaner substantially depends on the vertical distance from thecleaner, which has implications for the ability of the air cleanerto effectively reduce airborne particulates in the breathing zoneas was shown in our measurements reported in the first part of

EFFICIENCY OF IONIC AIR CLEANER 327

the manuscript. Also, based on the results of previous studies,any increase in natural or mechanical ventilation might furtherdecrease the observed performance of the ionic cleaner (Cyryset al. 2004; Wallace et al. 2004).

Comparison of Performance by Different Air CleanersOne of the ways to compare the performance of different air

cleaners in various environments is by using the Clean Air De-livery Rate (CADR) (Shaugnessy and Sextro 2006). The CADRis a measure of the contaminant removal as a result of operatingan air cleaning device. It can also be expressed as the productof the filtration efficiency and volumetric airflow through thedevice (Shaugnessy and Sextro, 2006). Thus, for this particularcleaner, even if all particles passing through it were collected,the CADR would not be higher than its flow rate estimated tobe 53.4 ft3/min. Our measurements of 0.5–3.0 μm particle con-centrations entering and leaving the operating ionic cleaner in-dicated the reduction in particle concentration of 25%, whichwould yield a CADR value of approximately 13 ft3/min. Thisestimate however, does not take into account the cleaning ef-fect by ions which would likely somewhat increase this value.In comparison, the Association of Home Appliance Manufac-turers (AHAM) measured 157 air cleaners and found that meanCADR values for three test aerosols (smoke, pollen, and dust)are essentially the same, approximately 160 ft3/min, and morethan 90% of the air cleaners tested have CADRs between 60 and300 ft3/min (AHAM 2004).

CONCLUSIONSOur data indicate that in actual residential environments ionic

air cleaners may not be able to reduce airborne particulate matteras effectively as has been demonstrated in uninhabited chamberstudies. When measured during normal daily activity, the aver-age I/OMC was reduced from 1.03 to 0.73 and I/ONC under-went reductions for most of the particle size fractions. However,due to a substantial inter- and intra-measurement variation inparticle ratios, the observed average reductions were not sta-tistically significant (p > 0.05). The effect of the ionic cleaneron particle ratios during quiescent conditions was more pro-nounced. However, ozone production was also observed withthe operation of the air cleaner. Indoor ozone concentrationswere measured at steady concentrations of 13–19 ppb duringnormal daily activity (measurement point was about 1.8 m fromthe device). The maximum measured ozone was 77 ppb, mea-sured after 8 hours of operation in front of the device (at theface plate). Any users of ionic air cleaners should be aware ofthe hazards associated with production of indoor ozone and theadditive effect of several ozone sources. Also, this study showsthat uninhabited chamber experiments may have limited utilitywhen analyzing performance of air cleaning devices for use inreal-life applications.

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