Evaluation on Air Purifier's Performance in Reducing The

International Journal of Environmental Research and Public Health

Article Evaluation on Air Puriﬁer’s Performance in Reducing the Concentration of Fine Particulate Matter for Occupants according to its Operation Methods

Hyungyu Park 1, Seonghyun Park 2 and Janghoo Seo 3,* 1 Department of Architecture, Graduated School, Kookmin University, Seoul 02707, Korea; [email protected] 2 Department of Industry-Academic Cooperation Foundation, Kookmin University, Seoul 02707, Korea; [email protected] 3 School of Architecture, Kookmin University, Seoul 02707, Korea * Correspondence: [email protected]; Tel.: +82-02-910-4593

Received: 29 April 2020; Accepted: 29 July 2020; Published: 1 August 2020

Abstract: Fine particulate matter entering the body through breathing cause serious damage to humans. In South Korea, filter-type air purifiers are used to eliminate indoor fine particulate matter, and there has been a broad range of studies on the spread of fine particulate matter and air purifiers. However, earlier studies have not evaluated an operating method of air purifiers considering the inflow of fine particulate matter into the body or reduction performance of the concentration of fine particulate matter. There is a limit to controlling the concentration of fine particulate matter of the overall space where an air purifier is fixed in one spot as the source of indoor fine particulate matter is varied. Accordingly, this study analyzed changes in the concentration of indoor fine particulate matter through an experiment according to the discharging method and location of a fixed air purifier considering the inflow route of fine particulate matter into the body and their harmfulness. The study evaluated the purifiers’ performance in reducing the concentration of fine particulate matter in the occupants’ breathing zone according to the operation method in which a movable air purifier responds to the movement of occupants. The results showed the concentration of fine particulate matter around the breathing zone of the occupants had decreased by about 51 µg/m3 compared to the surrounding concentration in terms of the operating method in which an air purifier tracks occupants in real-time, and a decrease of about 68 µg/m3 in terms of the operating method in which an air purifier controls the zone. On the other hand, a real-time occupant tracking method may face a threshold due to the moving path of an air purifier and changes in the number of occupants. A zone controlling method is deemed suitable as an operating method of a movable air purifier to reduce the concentration of fine particulate matter in the breathing zone of occupants.

Keywords: particulate matters (PM); air puriﬁer; experiment; real-time monitoring unit; transfer unit; occupant; breathing zone

1. Introduction Recently, high levels of fine particulate matter are observed in the atmosphere of the East Asian regions, including South Korea, regardless of the season due to rapid economic growth and industrialization, causing adverse effects on the body [1,2]. The inflow of outdoor particulate matter into the indoor space is causing deterioration of the indoor air quality. According to the World Health Organization, 3.8 million people a year die prematurely from illness attributable to household air pollution [3]. Particulate matter is classified into PM10, which has particulate matter less than 10 µm and PM2.5, which has particulate matter less than 2.5 µm [4]. Particles, in general, are filtered

Int. J. Environ. Res. Public Health 2020, 17, 5561; doi:10.3390/ijerph17155561 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 5561 2 of 14 using the cilia or respiratory tract and cannot enter the lung when they are under the body, but fine particles of small size can penetrate the lung and accumulate in the alveoli. Particles accumulated in the alveoli cause an inflammatory response and generate active oxygen, causing necrotized tissues. When particles accumulate in the bronchial tubes, they cause phlegm and cough as well as drying of the bronchial mucosa, which can easily allow penetration of germs and leave people with chronic lung conditions vulnerable to infectious diseases like pneumonia [5,6]. In particular, fine particulate matter less than 2.5 µm have been reported to cause diseases such as angina, stroke, and heart attack [7–10]. This indicates that fine and ultrafine particulate matter penetrating the body through breathing can cause serious damage to the body. Filter-type air purifiers are used to control indoor fine particulate matter in South Korea, and there has been a broad range of studies on air purifiers and fine particulate matter. However, previous studies focused on the purification effects caused by the airflow rate and filter grade of air purifiers and have not evaluated the operating method of air purifiers that considers the inflow path of fine particulate matter into the body and the breathing zone of occupants or the performance in reducing the concentration of particulate matter [11–19]. As various sources cause indoor fine particulate matter [20], operating a fixed air purifier can reduce the average concentration of indoor fine particulate matter, but the reduction effect against fine particulate matter in local areas such as the breathing zone of occupants is inadequate. Contrarily, we thought that delivering purified air discharged from the air purifier to the breathing zone of occupants by changing the discharge angle of the air purifier and attaching a transfer unit can reduce the concentration of fine particulate matter in the air that occupants breathe regardless of the average concentration of particulate matter in indoor spaces. Accordingly, this study considered the inflow of fine particulate matter into the body and their harmfulness and analyzed changes in the concentration of indoor fine particulate matter according to the discharge method of a fixed air purifier and changes in the concentration of indoor fine particulate matter through the experiment, and the study evaluated the reduction performance of fine particulate matter in the breathing zone of occupants according to the operation method in which a movable air purifier responds to the movement of occupants.

2. Materials and Methods

2.1. Experiment Equipment and Method The experiment was conducted indoor after sunset to minimize changes in indoor temperature caused by solar radiation. Figure1 shows the experimental space and installation locations of the experiment’s equipment. The size of the experiment space is 7.0 m (L) 10 m (W) 2.8 m (H), and the × × volume is 196 m3. The experiment space was divided into four artificial zones, and the particulate matter (PM) concentration measuring devices were installed in the center of each zone. Pollutants were set to be generated from a height of 1.2 m from the center of the hallway wall. Table1 shows the experiment devices used in this study. The target pollutant was a Polystyrene Latex (PSL) 2 µm standard solution, which was mixed with distilled water and sprayed in the space with the aerosol generator. The scanning electron microscope [21] was used to observe and analyze the particle shape of PSL, and the result showed the particle had an even spherical shape of 2.0 µm (SD 3%), as shown in ± Figure2. The TES-5322 model was used for concentrating the PM concentration, and the measurement accuracy of the PM concentration is below 5µg/m3 when less than 50 µg/m3, and below 10% 2.5 ± ± when greater than 50 µg/m3. The H13 grade filter that can filter out particles greater than 0.3 µm by 99.95% was applied. Int. J. Environ. Res. Public Health 2020, 17, x 3 of 14

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(a) (b)

Figure 1. Experimental space and location of experiment equipment. (a) Experimental space; (b) Location of experiment equipment.

(a) (b) Table 1. Equipment and materials for experiment. FigureFigure 1.1. ExperimentalExperimental space space and and location location of experiment of experiment equipment. equipment. (a) Experimental (a) Experimental space; ( bspace;) Location (b) Purpose Model Quantity Locationof experiment of experiment equipment. equipment. Air purifier Mi Air 2 1 Table 1. Equipment and materials for experiment. PM measuring deviceTable 1. Equipment and materials TES-5322 for experiment. 5 Thermo-hygrometerPurpose Purpose TR-72WFModel Model QuantityQuantity4 AnemometerAir purifierAir puriﬁer MiTSI Air 9565 Mi 2 Air 2 1 11 PM measuring device TES-5322 5 PMPM2.5 measuring generatingThermo-hygrometer device device Aerosol TES-5322 generator TR-72WF 4 5 1 AnemometerPolystyrene TSI Latex 9565 1 StandardThermo-hygrometer particle solution TR-72WF 43 PM2.5 generating device(Standard Aerosol particle generator 15 mL) 1 Anemometer TSIPolystyrene 9565 Latex 1 Standard particle solution 3 Air supply and flow control of (Standard particle 15 mL) PM2.5 generating device Compressor Aerosol generator (AM 400D) 1 1 Air supplyaerosol and generator ﬂow control of aerosol generator Compressor (AM 400D) 1 Polystyrene Latex StandardWater particle removal solution of aerosol air supply Dryer (TX15K) 1 3 Water removal of aerosol air supply (Standard Dryer particle (TX15K) 15 mL) 1 Air supply and flow control of Compressor (AM 400D) 1 aerosol generator Water removal of aerosol air supply Dryer (TX15K) 1

Figure 2. ExpandedExpanded picture picture of standard solution through a scanning electron microscope (SEM).

Figure 2. Expanded picture of standard solution through a scanning electron microscope (SEM).

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The height of the PM concentration measuring device for measuring changes in the concentration of fine particulate matter in the breathing zone of occupants was set to 1.5 m by considering the Int. J. Environ. Res. Public Health 2020, 17, x 4 of 14 breathing area of occupants while standing. 2.2. Arrangement of the Air Purifier Considering Draft 2.2. Arrangement of the Air Purifier Considering Draft There are methods to control the concentration of various indoor pollutants such as fine particulateThere are matter, methods including to control generation the concentration control, ofelimination various indoor control, pollutants and dilution such as finecontrol. particulate If fine matter,particulate including matter generation is the target control, pollutant, elimination the control,concentration and dilution can be control. controlled If fine by particulate eliminating matter the issource the target or collecting pollutant, fine the particulate concentration matter can disper be controlledsed in the by space. eliminating Therefore, the source the most or collecting effective finemethod particulate for reducing matter the dispersed concentration in the space.of indoor Therefore, fine particulate the most matter effective is methodto locate for and reducing operate the an concentrationair purifier close of indoor to the fine source particulate of pollutants. matter isHo towever, locate andoutdoor operate fine an particulate air purifier matter close to can the flow source in ofthrough pollutants. openings However, or cracks outdoor in the fine building, particulate andmatter fine particulate can flow inmatter through are openingsgenerated or from cracks a variety in the building,of activities and by fine occupants, particulate matterincluding are generatedcooking, exercising, from a variety and of ventilation. activities by Thus, occupants, the includingsource of cooking,pollutants exercising, cannot be andeasily ventilation. characterized. Thus, Furtherm the sourceore, of many pollutants measurement cannot besensors easily are characterized. required to Furthermore,detect the source many of measurement the indoor fine sensors particulate are required matter to early detect [22–25]. the source This of study the indoor selected fine a particulatemethod of matterdelivering early air [22 discharged–25]. This studyfrom the selected air purifier a method toward of delivering the breathing air discharged zone of occupants from the airto improve purifier towardthe reduction the breathing performance zone ofof occupantsthe concentration to improve fine theparticulate reduction matter performance by operating of the the concentration air purifier. fineIn this particulate case, occupants matter bycould operating inhale purified the air purifier. air regardless In this of case, the occupantssource. Contrarily, could inhale occupants purified could air regardlessfeel discomfort of the from source. drafts Contrarily, if the velocity occupants of air could currents feel is discomfort extremely from high, drafts and therefore, if the velocity this needs of air currentsto be considered. is extremely Gong high, et andal. performed therefore, thisthe needsstudy toon be the considered. allowable Gongwind etvelocity al. performed range needed the study for onfinding the allowable human recognition wind velocity for range the local needed airflow, for finding under human isothermal recognition and non-isothermal for the local airflow, conditions, under isothermaland designed and individual non-isothermal ventilation conditions, of the and tropic designedal regions individual through ventilation the experiment, of the tropical and suggested regions throughthat a wind the experiment,velocity of minimum and suggested 0.3 m/s that and a wind maximum velocity 0.9 of minimumm/s is acceptable 0.3 m/s andbased maximum on the comfort 0.9 m/s isof acceptable occupants based[26]. on the comfort of occupants [26]. This study set the installation height of the PM measuring device to 1.5 m by considering the This study set the installation height of the PM2.52.5 measuring device to 1.5 m by considering the breathingbreathing areaarea ofof occupantsoccupants whilewhile standing.standing. InIn thethe casecase wherewhere purifiedpurified airair isis dischargeddischarged towardtoward thethe breathingbreathing zonezone of of occupants, occupants, the the air air purifier purifier was was arranged arranged to to maintain maintain the the velocity velocity of theof the discharged discharged air thatair that reaches reaches the 1.5the m-high 1.5 m-high measuring measuring device device at 0.8 at m 0.8/s, asm/s, shown as shown in Figure in Figure3. The 3. speed The speed set here set takes here thetakes precedence the precedence over the over performance the performance of reducing of reducing fine dust fine concentrationsdust concentrations in the in breathing the breathing zone zone and notand over not theover human the human thermal thermal comfort. comfort. The air The purifier air pu usedrifier in used this studyin this inhales study inhales air from air the from bottom the andbottom discharges and discharges from the from top, the and top, if the and discharge if the disc angleharge is angle changed is changed toward toward the breathing the breathing zone of zone the occupants,of the occupants, the velocity the velocity may change may change due to due the pressureto the pressure loss. Accordingly, loss. Accordingly, a separate a separate discharge discharge outlet wasoutlet made was with made a 3Dwith printer a 3D toprinter form to the form same the face same velocity face velocity regardless regardless of the discharge of the discharge angle, and angle, it is shownand it is in shown Figure 4in. Figure 4.

FigureFigure 3.3. Air puriﬁerpurifier ofof dischargedischarge directiondirection towardtoward breathing.breathing.

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(a) (b) (a) (b) Figure 4. Discharge outlet made with a 3D print.: (a) Discharged upward (b) Discharged toward Figureoccupants. 4. Discharge outlet made with a 3D print.: (a) Discharged upward (b) Discharged toward occupants. 2.3. Real-time Monitoring and Transfer Unit Production Using an Arduino Board 2.3. Real-time Monitoring and Transfer Unit Production Using an Arduino Board To reduce the concentration of fine particulate matter in the breathing zone of occupants through the operationTo reduce of the an concentration air purifier, it of is fine necessary particulate to respond matter into the the breathing movement zone of occupantsof occupants or throughmonitor thereal-time operation changes of an of air the purifier, concentration it (isa) necessary of fine particulateto respond matter,to the( movementb as) well as changesof occupants in the or discharge monitor real-time changes of the concentration of fine particulate matter, as well as changes in the discharge angleFigure of the 4. air Discharge purifier. outlet A data made communication with a 3D print.: module ( a)) Discharged for real-time upward monitoring ( b) Discharged of the measurement toward angle of the air purifier. A data communication module for real-time monitoring of the measurement sensoroccupants. for indoor fine particulate matter and a remote transfer unit for the air purifier were produced. sensorFigure for5 shows indoor the fine overview particulate of matterthe data and communication a remote transfer module unit for system, the air andpurifier Figure were 6 produced.shows the Figure 5 shows the overview of the data communication module system, and Figure 6 shows the 2.3.diagram Real-time of the Monitoring remote transfer and Transfer unit. UnitUnit Production UsingUsing anan ArduinoArduino BoardBoard diagram of the remote transfer unit. ToThe reduce PM2.5 theconcentration concentration value of finemonitored particulate through matter the in ca themera breathing sensor zoneis transferred of occupants to the through server ToThe reduce PM2.5 concentrationthe concentration value of finemonitored particulate through matter the in ca themera breathing sensor zoneis transferred of occupants to the through server thevia operationESP 32 WiFi of module. an air purifier, Whether it isthe necessary air purifier to respondneeds to be to themoved movement is determined of occupants based on or the monitor PM2.5 thevia ESPoperation 32 WiFi of module. an air purifier, Whether it theis necessary air purifier to needsrespond to beto movedthe movement is determined of occupants based on or the monitor PM2.5 real-timeconcentration changes data of received, the concentration and if its movement of fine particulate is necessary, matter, the as remote well as transfer changes unit in sends the discharge a signal real-timeconcentration changes data of received, the concentration and if its movement of fine particulate is necessary, matter, the as remote well as transfer changes unit in sendsthe discharge a signal angleto the ofArduino the air purifier.board to Amove data it. communication module for real-time monitoring of the measurement angleto the ofArduino the air purifier.board to Amove data it. communication module for real-time monitoring of the measurement sensorFigure for indoor 7 shows fine particulatethe data mattercommunication and a remote module transfer and unit remote for the transfer air purifier unit were for produced.real-time sensorFigure for indoor 7 shows fine particulatethe data mattercommunication and a remote module transfer and unit remote for the transfer air purifier unit were for produced. real-time Figuremonitoring5 shows of the the movable overview air of purifier the data made communication for the experiment module of system,this study. and Figure6 shows the Figuremonitoring 5 shows of the the movable overview air ofpurifier the data made communication for the experiment module of thissystem, study. and Figure 6 shows the diagram of the remote transfer unit. The PM2.5 concentration value monitored through the camera sensor is transferred to the server via ESP 32 WiFi module. Whether the air purifier needs to be moved is determined based on the PM2.5 concentration data received, and if its movement is necessary, the remote transfer unit sends a signal to the Arduino board to move it. Figure 7 shows the data communication module and remote transfer unit for real-time monitoring of the movable air purifier made for the experiment of this study.

Figure 5. Overview of the data communication module system. Figure 5. Overview of the data communication module system.

Figure 5. Overview of the data communication module system.

FigureFigure 6.6. Diagram of the Arduino-based remoteremote transfertransfer unit.unit. Figure 6. Diagram of the Arduino-based remote transfer unit. The PM2.5 concentration value monitored through the camera sensor is transferred to the server via ESP 32 WiFi module. Whether the air puriﬁer needs to be moved is determined based on the PM2.5 concentration data received, and if its movement is necessary, the remote transfer unit sends a signal to the Arduino board to move it.

Figure 6. Diagram of the Arduino-based remote transfer unit.

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Figure7 shows the data communication module and remote transfer unit for real-time monitoring ofInt.the J. Environ. movable Res. Public air puriﬁer Health 2020 made, 17, x for the experiment of this study. 6 of 14 Int. J. Environ. Res. Public Health 2020, 17, x 6 of 14

(a) (b) (a) (b) Figure 7. Data communication module and remote transfer unit used in the experiment: (a) Data Figurecommunication 7.7. Data communicationmodule, (b) Remote module transfer and unit. remote transfertransfer unit used inin thethe experiment:experiment: (a) DataData communication module, ((b)) RemoteRemote transfertransfer unit.unit. 3. Measurement Experiment with Fixed Air Purifier 3. Measurement Experiment with Fixed Air PurifierPurifier 3.1. Case Setting 3.1. Case Setting This experiment was conducted to evaluate thethe air purifiers’purifiers’ performance in reducing the This experiment was conducted to evaluate the air purifiers’ performance in reducing the concentration ofof finefine particulateparticulate matter matter in in the the breathing breathing zone zone of of occupants occupants according according to to the the location location of concentration of fine particulate matter in the breathing zone of occupants according to the location theof the fixed fixed air air purifier purifier and and the the direction direction of of the the discharged discharged air. air. The The air air purifierpurifier waswas operatedoperated toto make theof the background fixed air purifier PM2.5 concentration and the direction of the of experimental the discharged space air. Theto be air less purifier than 30 was μg/m operated33. As shown to make in the background PM2.5 concentration of the experimental space to be less than 30 µg/m . As shown in the background PM2.5 concentration of the experimental space to be less than 30 μg/m3. As shown in Figure8 8,, thethe experimentexperiment generatedgenerated pollutantspollutants throughthrough thethe aerosolaerosol generatorgenerator afterafter 1010 minmin fromfrom thethe Figure 8, the experiment generated pollutants through the aerosol generator after 10 min from the start of measurement, measurement, and and the the air air purifier purifier was was oper operatedated for for 150 150 min min through through a remote a remote control control after after 30 start of measurement, and the air purifier was operated for 150 min through a remote control after 30 30min min from from the theoperation operation of the of theaerosol aerosol generator. generator. The Thegeneration generation of pollutants of pollutants was discontinued was discontinued after min from the operation of the aerosol generator. The generation of pollutants was discontinued after after90 min. 90 min. 90 min.

Figure 8. Experiment progress. Figure 8. Experiment progress. The case conditions according to the location of the air purifierpurifier and changes in the direction of dischargeThe case are shownconditions in Table according2 2.. InIn CaseCase to the 1,1, thethe location airair purifierpu ofrifier the ofof air upward purifier dischargedischarge and changes isis locatedlocated in the inin direction the center, of anddischarge it is the are control shown group in Table to compare2. In Case with 1, the other air pu casesrifier in of which upward the discharge location and is located discharge in the direction center, ofand the it airis the purifierpurifier control were group changed.changed. to compare In Case with 2, other the air cases purifierpurifier in which of the the upward location discharge and discharge was located direction on theof the nearby air purifier wall that were was changed. 1.4 m away In Case from 2, the the P air 44 (Point(Point purifier 4)4) measuringmeasuringof the upward devicedevice discharge andand waswas was setset located toto analyzeanalyze on the nearby reduction wall of that the was concentration 1.4 m away of from finefine particulatethe P 4 (Point matter 4) measuring in the breathing device and zone was of set occupants to analyze by installingthe reduction the purifierpurifierof the concentration nearnear occupantsoccupants of regardlessregardlessfine particulate ofof thethe matter locationlocation in ofof the thethe breathing pollutantpollutant zone source.source. of occupants by installing the purifier near occupants regardless of the location of the pollutant source.

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Table 2. Experiment case conditions.

Cases 1 2 3 4 Variables Location of air puriﬁer A B A B Direction of discharge Upward Toward point 4 Initial PM concentration 27 µg/m3 26 µg/m3 29 µg/m3 30 µg/m3 Temperature/Relative Humidity 25 2.2 C/28 1.8% ± ◦ ±

Case 3 and 4 were set to analyze the reduction of the fine particulate matter in local areas according to changes in the discharge direction of the air purifier. Case 3 set the direction of the discharge from the central location toward the P 4 measuring device as in Case 1, and the wind velocity of the air current reaching the P 4 measurement point was 0.23 m/s. Case 4 installed the air purifier near the occupants while changing the direction of the discharge, and the wind velocity of the air current reaching the P 4 measurement point was 0.81 m/s. The wind velocity value was the average value measured every 10 s for a total of 60 times with TSI 9565 (TSI Inc., Shoreview, MN, USA; Velocity).

3.2. Experiment Results

Figure9 shows changes in the PM 2.5 concentration according to the time by measurement point of each case. The changes in the concentration of fine particulate matter in the local areas according to the location of the air purifier were compared and analyzed with the results of Case 1 and Case 2. In all the measurement points of Case 1 and 2, the PM2.5 concentration has increased according to the pollutants, and the rising curve of the PM2.5 concentration was maintained consistently. When the generation of pollutants was discontinued, the PM2.5 concentration was decreased by the air purifier. In Case 1, the average PM2.5 concentration difference in P 1 (Point 1), P 2 (Point 2), P 3 (Point 3), and P 4 (Point 4) was about 25 µg/m3 from 40 min to 100 min. This is due to the distance from the source of pollutants to the measurement points. In Case 2, the PM2.5 concentration in P 4 that is close to the air purifier of the upward discharge was higher than the PM2.5 of the other measurement points. This was due to the movement of pollutants to the discharge outlet of the air purifier. In Case 3, P 4 also had the highest PM2.5 concentration and this meant that the P 4 point assumed as the breathing zone of the occupants did not fall within the influence of purified air discharged from the air purifier. The concentration of PM at points other than point P 4 was lower than that of P 4, but compared to Case 1, the PM concentration in P 3 decreased by 21 µg/m3, whereas points P 1 and P 2 maintained similar levels. 3 Meanwhile, the average PM2.5 concentration in P 4 from 40 min to 100 min was about 77 µg/m lower than other measurement points. In particular, the PM2.5 concentration in P 4 was maintained at less than 50 µg/m3 since the operation of the air purifier, and it reached 30 µg/m3 within 20 min after discontinuing the operation of the aerosol generator. It could be believed that the concentration of PM at point P 3 increased due to the diffusion of pollutants from the pollution source, and points P 1 and P 2 were similar to that of Case 1. Thus, the air purifier must be located near occupants, and the breathing zone of occupants must be located within the influence of purified air discharged from the air purifier to reduce the concentration of fine particulate matter in the breathing zone of occupants. Int. J. Environ. Res. Public Health 2020, 17, 5561 8 of 14 Int. J. Environ. Res. Public Health 2020, 17, x 8 of 14

FigureFigure 9. Comparing 9. Comparing PM PM2.52.5 concentrationsconcentrations by by measurement measurement points points (Case (Case 1–4). 1–4 ).

4. Measurement4. Measurement Experiment Experiment with with Movable Movable AirAir Purifier Puriﬁer

4.1. Operating4.1. Operating Method Method of Movableof Movable Air Air Purifier Purifier In thisIn this section, section, the the real-time real-time monitoring monitoring device and and the the remote remote transfer transfer unit unit were were applied applied to to allowallow the airthe purifier,air purifier, which which delivers delivers purified purified air to the the breathing breathing zone zone of ofoccupants, occupants, to respond to respond to to the movementthe movement of occupants of occupants and and the the reduction reduction performanceperformance of of fine fine particulate particulate matter matter in the in breathing the breathing zonezone of occupants of occupants was was evaluated. evaluated. In In addition, addition, thethe experiment experiment was was conducted conducted through through the real-time the real-time occupantoccupant tracking tracking method method and and the the zone zone controlling controlling method method byby considering the the threshold threshold according according to to the moving path of the air purifier. the moving path of the air purifier. 4.1.1. Real-Time Occupant Tracking Method 4.1.1. Real-Time Occupant Tracking Method Firstly, a controlling method of tracking occupants to reduce the concentration of fine particulate matterFirstly, in a the controlling breathing method zone of ofoccupants, tracking regardle occupantsss of tothe reduce surrounding the concentration concentration of was fine set. particulate The matterperformance in the breathing evaluation zone of this of method occupants, was conducted regardless through of the the surrounding measurement concentration experiment. was set. The performanceThe target evaluation experimental of thisspace method and location was conducted for measuring through the PM the2.5 measurement concentrations experiment. were the sameThe targetas those experimental in the experiment space conditions and location of Sect forion measuring2, but an additional the PM device2.5 concentrations for measuring were the the sameconcentration as those in theof fine experiment particulate conditions matter was of installed Section 2at, butthe anP 5 additional(Point 5), which device was for an measuring assumed the concentrationbreathing zone of fine of the particulate occupant. The matter measuring was installed device for at the the concentration P 5 (Point of 5), fine which particulate was an matter assumed breathingand the zone movable of the air occupant. purifier were The fixed measuring in the loca devicetion, forwhich the was concentration assumed as ofthe fine breathing particulate zone of matter the occupants. Figure 10 displays an implementation of a controlling method of tracking occupants and the movable air purifier were fixed in the location, which was assumed as the breathing zone of in real-time. For the experiment, the aerosol generator and the air purifier were operated at the same the occupants. Figure 10 displays an implementation of a controlling method of tracking occupants in time after 10 min from starting the recording of each measurement device, and the operation of the real-time.aerosol For generator the experiment, was discontinued the aerosol after generator 190 min. and The the movement air purifier of the were measuring operated device at the in same the time afterbreathing 10 min from zone starting of the occupants the recording was set of to eachmove measurement to random points device, every and 15, 20, the and operation 30 min to of analyze the aerosol generatorthe reduction was discontinued effect of the after concentration 190 min. Theof fine movement particulate of matter the measuring in the breathing device inzone the of breathing the zoneoccupants of the occupants in terms of was controlling set to movethe real-time to random tracking points of occupants. every 15, 20, and 30 min to analyze the reduction effect of the concentration of fine particulate matter in the breathing zone of the occupants in terms of controlling the real-time tracking of occupants.

4.1.2. Zone Controlling Method Meanwhile, a method of controlling occupant tracking of a movable air purifier has limitations in its actual use. such as the moving path of the air purifier or changes in the number of occupants. Accordingly, the study conducted a measurement experiment for performance verification by setting Int. J. Environ. Res. Public Health 2020, 17, x 9 of 14

Int. J. Environ. Res. Public Health 2020, 17, 5561 9 of 14 the moving locations of the air purifier by zone, which allows the air purifier to move to the set location with the high concentration of fine particulate matter or the area where occupants are located and reduce the concentration of fine particulate matter in the breathing zone of occupants within that area. The experimental space and the locations of the device for measuring the concentration of fine particulate matter are the same as those in the measurement experiment for the occupant tracking method. Int. J. Environ. FigureRes. Public 10. Health A method 2020, 17of, ximplementing the air purifier that tracks occupants in real-time. 9 of 14

4.1.2. Zone Controlling Method Meanwhile, a method of controlling occupant tracking of a movable air purifier has limitations in its actual use. such as the moving path of the air purifier or changes in the number of occupants. Accordingly, the study conducted a measurement experiment for performance verification by setting the moving locations of the air purifier by zone, which allows the air purifier to move to the set location with the high concentration of fine particulate matter or the area where occupants are located and reduce the concentration of fine particulate matter in the breathing zone of occupants within that area. The experimental space and the locations of the device for measuring the concentration of fine particulate matter are the same as those in the measurement experiment for the occupant tracking method. As shown in Figure 11, the experiment generated pollutants after 10 min from the start of FigureFigure 10. 10. AA method method of of implementing implementing the the air air purifier purifier that that tracks occupants in real-time. measurement, and the air purifier was operated at the same time. The generation of pollutants was 4.1.2.discontinuedAs Zone shown Controlling after in Figure190 min.Method 11 Figure, the experiment 12 shows the generated moving path pollutants and location after 10of the min air from purifier, the startand the of measurement,air purifier was and set theto be air located purifier near was the operated walls of at each the samezone using time. Thethe moving generation path of of pollutants the wall. was Meanwhile, a method of controlling occupant tracking of a movable air purifier has limitations discontinuedThe air purifier after 190 was min. relocated Figure 12 every shows 30 the min moving to analyze path changes and location in the of concentration the air purifier, of and indoor the in its actual use. such as the moving path of the air purifier or changes in the number of occupants. airfine purifier particulate was setmatter, to be according located near to the the moving walls of path each and zone location using the (Figure moving 13). path of the wall. Accordingly, the study conducted a measurement experiment for performance verification by setting the moving locations of the air purifier by zone, which allows the air purifier to move to the set location with the high concentration of fine particulate matter or the area where occupants are located and reduce the concentration of fine particulate matter in the breathing zone of occupants within that area. The experimental space and the locations of the device for measuring the concentration of fine particulate matter are the same as those in the measurement experiment for the occupant tracking method. As shown in Figure 11, the experiment generated pollutants after 10 min from the start of measurement, and the air purifier was operated at the same time. The generation of pollutants was discontinued after 190 min. Figure 12 shows the moving path and location of the air purifier, and the air purifier was set to be located near the walls of each zone using the moving path of the wall. Figure 11. Experiment Progress (Real-Time Occupant Tracking Method and Zone Controlling Method). Int. J. TheEnviron.Figure air 11.Res. purifierExperiment Public Healthwas Progressrelocated2020, 17, x (Real-Time every 30 Occupant min to Trackinganalyze Methodchanges and in Zonethe concentration Controlling Method). of indoor10 of 14 fine particulate matter, according to the moving path and location (Figure 13).

Figure 11. Experiment Progress (Real-Time Occupant Tracking Method and Zone Controlling Method). Figure 12. Moving path of the air puri puriﬁerfier that controls zones.

Figure 13. Location of the air purifier at each time and moving path from the previous location (Zone Controlling Method).

4.2. Experiment Results

Figure 14 shows changes in the PM2.5 concentration by measurement point according to the operation of the air purifier that tracks occupants in real-time. The PM2.5 concentrations were compared and analyzed at P 5 (Point 5), which is considered as the breathing zone of occupants and continuously received purified air currents discharged from the air purifier and other measurement points. The PM2.5 concentrations at all measurement points had increased after 10 min from the start of the measurement and reached about 100 μg/m3 at P 1, P 2, and P 4 at 40 min. The upward slopes of P 5 and P 3, which were closest to the discharge outlet of the air purifier, were relatively low compared to those of other points and showed concentrations of 40 μg/m3 and 55 μg/m3, respectively. During the overall experiment time, the average concentration at

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The air puriﬁer was relocated every 30 min to analyze changes in the concentration of indoor ﬁne particulate matter, accordingFigure 12. to theMoving moving path pathof the and air puri locationfier that (Figure controls 13 ).zones.

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P 5 was reduced by 51 μg/m3 compared to the average concentration at other measurement points, which demonstrated that the operation of an air purifier that tracks occupants in real-time could reduce the concentration of fine particulate matter in the breathing zone of occupants even if the concentration of fine particulate matter in the surrounding area is high. In addition, the concentration of fine particulate matter in measurement points near the discharge outlet was lower than other measurement points. This means the operation of an air purifier that tracks occupants in real-time Figure 13. LocationLocation of of the the air air purifier puriﬁer at each at each time time and andmoving moving path pathfrom fromthe pr theevious previous location location (Zone could reduce the concentration of fine particulate matter in the breathing zone of occupants (ZoneControlling Controlling Method). Method). regardless of the surrounding concentration of fine particulate matter. The result of measuring the 4.2. Experiment Results PM4.2.2.5 Experiment concentration Results at measurement points near the discharge outlet proves that the concentration of fine particulateFigure 14 showsmatter changescould be in reduced the PM if2.5 theconcentration location is close by measurement to the air puri pointfier and according is within to the the influenceoperationFigure of of the14 the showspurified air puriﬁer changes air di thatscharged in tracks the PM occupantsfrom2.5 concentrationthe air in purifier. real-time. by measurement point according to the operation of the air purifier that tracks occupants in real-time. The PM2.5 concentrations were compared and analyzed at P 5 (Point 5), which is considered as the breathing zone of occupants and continuously received purified air currents discharged from the air purifier and other measurement points. The PM2.5 concentrations at all measurement points had increased after 10 min from the start of the measurement and reached about 100 μg/m3 at P 1, P 2, and P 4 at 40 min. The upward slopes of P 5 and P 3, which were closest to the discharge outlet of the air purifier, were relatively low compared to those of other points and showed concentrations of 40 μg/m3 and 55 μg/m3, respectively. During the overall experiment time, the average concentration at

Figure 14. Comparing PM concentrations by measurement points according to the operation of the Figure 14. Comparing PM2.52.5 concentrations by measurement points according to the operation of the airair purifier puriﬁer that that tracks tracks occupants.

Figure 15 shows changes in the PM2.5 concentration according to the time of each measurement point with the operation of the air purifier that controls zones through the set moving path. The PM2.5 concentration in all the measurement points between 10–70 min located in the back of the room without moving an air purifier had increased to a similar level, reaching an average of 128 μg/m3 concentration at 70 min. At 70–100 min, when the moved air purifier was located at the P 4 wall point, the average PM2.5 concentration at P 4 was about 56 μg/m3 for 30 min, and the PM2.5 concentration at P 1, P 2, and P 3 were 129, 133, and 172 μg/m3, respectively. The reason for the high PM2.5 concentration in P 3 was because pollutants generated from the aerosol generator were spread to the P 3 point due to the direction of the air current discharged from the air purifier. At 100–130 min, when the air purifier was located by the nearby wall of P 3 using the same moving path, a similar result for 70–100 min was observed. The average PM2.5 concentration in the P 3 point was about 60 μg/m3 for 30 min due to controlling of purified air discharged from the air purifier. P 4 showed the highest level of 161 μg/m3. At 130–160 min when the air purifier was located at the nearby wall of P 1, the PM2.5 concentration at P 1, which was affected by the purified air discharged from the air purifier, was about 56 μg/m3 for 30 min, and the PM2.5 concentrations at the P 2, P 3, and P 4 points were 153, 124, and 145 μg/m3 for 30 min, respectively. At 160–190 min, the PM2.5 concentration in P 3, which was controlled by purified air discharged from the air purifier, was 55 μg/m3 for 30 min, and the PM2.5 concentrations at P 1, P 2, and P 4 were 125, 129, and 151 μg/m3 for 30 min, respectively. The average concentration of the fine particulate matter from 70 min to 190 min in measurement points within the influence of purified air discharged from the air purifier was reduced by about 68 μg/m3 compared to other measurement points. This is due to the decreased removal rate of fine particulate matter in the breathing zone of the occupants for the corresponding time because the air purifier for controlling zones moves around.

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The PM2.5 concentrations were compared and analyzed at P 5 (Point 5), which is considered as the breathing zone of occupants and continuously received purified air currents discharged from the air purifier and other measurement points. The PM2.5 concentrations at all measurement points had increased after 10 min from the start of the measurement and reached about 100 µg/m3 at P 1, P 2, and P 4 at 40 min. The upward slopes of P 5 and P 3, which were closest to the discharge outlet of the air purifier, were relatively low compared to those of other points and showed concentrations of 40 µg/m3 and 55 µg/m3, respectively. During the overall experiment time, the average concentration at P 5 was reduced by 51 µg/m3 compared to the average concentration at other measurement points, which demonstrated that the operation of an air purifier that tracks occupants in real-time could reduce the concentration of fine particulate matter in the breathing zone of occupants even if the concentration of fine particulate matter in the surrounding area is high. In addition, the concentration of fine particulate matter in measurement points near the discharge outlet was lower than other measurement points. This means the operation of an air purifier that tracks occupants in real-time could reduce the concentration of fine particulate matter in the breathing zone of occupants regardless of the surrounding concentration of fine particulate matter. The result of measuring the PM2.5 concentration at measurement points near the discharge outlet proves that the concentration of fine particulate matter could be reduced if the location is close to the air purifier and is within the influence of the purified air discharged from the air purifier. Figure 15 shows changes in the PM2.5 concentration according to the time of each measurement point with the operation of the air purifier that controls zones through the set moving path. Int. J. Environ. Res. Public Health 2020, 17, x 12 of 14

Figure 15.15. Comparing PM2.52.5 concentrationsconcentrations by by measurement measurement points according to the operation of thethe airair puriﬁerpurifier thatthat controlscontrols zones.zones.

5. ConclusionsThe PM2.5 concentration in all the measurement points between 10–70 min located in the back of the room without moving an air purifier had increased to a similar level, reaching an average of 128 µThisg/m3 studyconcentration performed at 70measurement min. experiments to evaluate the reduction performance of fine particulate matter concentration in the breathing zone for a method of delivering purified air At 70–100 min, when the moved air purifier was located at the P 4 wall point, the average PM2.5 discharged from an air purifier to the human3 breathing zone. The results are as follows: concentration at P 4 was about 56 µg/m for 30 min, and the PM2.5 concentration at P 1, P 2, and The method of installing a fixed3 air purifier at a location adjacent to occupants without changing P 3 were 129, 133, and 172 µg/m , respectively. The reason for the high PM2.5 concentration in P 3 wasthe discharged because pollutants direction generated cannot improve from the aerosolthe perfor generatormance werein reducing spread tothe the concentration P 3 point due of to fine the directionparticulate of thematter air currentwithin dischargedthe breathing from zone the airof purifier.occupants; At the 100–130 method min, of when delivering the air purifierpurified was air locateddischarged by thefrom nearby an air wall purifier of P 3can using better the reduce same moving the concertation path, a similar of fine result particulate for 70–100 matter min in was the breathing zone of occupants compared to the concentration of fine particulate3 matter in the observed. The average PM2.5 concentration in the P 3 point was about 60 µg/m for 30 min due to controllingsurrounding of area, purified but if air the discharged distance from from the the air air purifier purifier. to P the 4 showed controlling the highest point is level distant of 161andµ sog/ mthe3. velocity of airflow is not sufficient, there is no effect in reducing the fine particulate matter At 130–160 min when the air purifier was located at the nearby wall of P 1, the PM2.5 concentration at Pconcentration. 1, which was affected by the purified air discharged from the air purifier, was about 56 µg/m3 for In the case of a mobile air purifier, the real-time occupant tracking method was effective in terms of reducing the concentration of fine particulate matter in the breathing zone of occupants by 51 μg/m3 compared to the surrounding PM concentration, but there are limits in actual use regarding the moving path of the air purifier or the change in the number of occupants. On the contrary, the operation of the movable air purifier showed that the fine PM concentration of the occupant’s respiratory zone to be 68 μg/m3 lower than other measurement points. Thus, it is more effective to divide the target space by zone and move an air purifier around considering the number of occupants and the mobility of an air purifier. This study compared and evaluated an air purifier’s performance in reducing the concentration of fine particulate matter in the breathing zone of occupants against measurement points of other zones by setting measurement points of each zone in one place when delivering purified air discharged from an air purifier. However, the level of reduction performance of fine particulate matter could differ according to the range of influencing purified air currents within the same zone. In addition, in the case of the zone controlling method, there are also limitations regarding the moving path and the distance between the occupants. Accordingly, a follow-up study on the range of purified air currents discharged from an air purifier, method of expanding the range, method of installing air purifier on the ceiling, and changing control mode according to the number of occupants will be conducted in future.

Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2007165).

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3 30 min, and the PM2.5 concentrations at the P 2, P 3, and P 4 points were 153, 124, and 145 µg/m for 30 min, respectively. At 160–190 min, the PM2.5 concentration in P 3, which was controlled by purified 3 air discharged from the air purifier, was 55 µg/m for 30 min, and the PM2.5 concentrations at P 1, P 2, and P 4 were 125, 129, and 151 µg/m3 for 30 min, respectively. The average concentration of the fine particulate matter from 70 min to 190 min in measurement points within the influence of purified air discharged from the air purifier was reduced by about 68 µg/m3 compared to other measurement points. This is due to the decreased removal rate of fine particulate matter in the breathing zone of the occupants for the corresponding time because the air purifier for controlling zones moves around.

5. Conclusions This study performed measurement experiments to evaluate the reduction performance of fine particulate matter concentration in the breathing zone for a method of delivering purified air discharged from an air purifier to the human breathing zone. The results are as follows: The method of installing a fixed air purifier at a location adjacent to occupants without changing the discharged direction cannot improve the performance in reducing the concentration of fine particulate matter within the breathing zone of occupants; the method of delivering purified air discharged from an air purifier can better reduce the concertation of fine particulate matter in the breathing zone of occupants compared to the concentration of fine particulate matter in the surrounding area, but if the distance from the air purifier to the controlling point is distant and so the velocity of airflow is not sufficient, there is no effect in reducing the fine particulate matter concentration. In the case of a mobile air purifier, the real-time occupant tracking method was effective in terms of reducing the concentration of fine particulate matter in the breathing zone of occupants by 51 µg/m3 compared to the surrounding PM concentration, but there are limits in actual use regarding the moving path of the air purifier or the change in the number of occupants. On the contrary, the operation of the movable air purifier showed that the fine PM concentration of the occupant’s respiratory zone to be 68 µg/m3 lower than other measurement points. Thus, it is more effective to divide the target space by zone and move an air purifier around considering the number of occupants and the mobility of an air purifier. This study compared and evaluated an air purifier’s performance in reducing the concentration of fine particulate matter in the breathing zone of occupants against measurement points of other zones by setting measurement points of each zone in one place when delivering purified air discharged from an air purifier. However, the level of reduction performance of fine particulate matter could differ according to the range of influencing purified air currents within the same zone. In addition, in the case of the zone controlling method, there are also limitations regarding the moving path and the distance between the occupants. Accordingly, a follow-up study on the range of purified air currents discharged from an air purifier, method of expanding the range, method of installing air purifier on the ceiling, and changing control mode according to the number of occupants will be conducted in future.

Author Contributions: Conceptualization, H.P.; methodology, H.P. and S.P.; writing-original draft preparation, H.P.; writing—review and editing, H.P. and S.P.; Supervision, J.S.; All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B2007165). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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