H.Matsui

The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023

How I do it

Build a simple and inexpensive air purifier
Hideo Matsui, M.D.1

Abstract
To prevent airborne transmission of SARS-CoV-2 indoor spaces, it isimportant to use appropriate ventilation systems in buildings. Growing
evidences about other technologies that remove infectious particles fromthe air showed that effectiveness of air purifiers fitted with high-efficiency
filters. In the present study, we built a simple air purifier combining a box
fan and a MERV13 filter, and investigated the performance of the device.
The airflow rate achieved 0.41-0.71 m3/s depending on fan speed. The
device generated more than 50dB noise even at the lowest airflow. The
amount of PM0.3 was reduced by 40-50% depending on fan speed, thus theestimated clean air delivery rate ranged from 343 to 751 cfm. The cost ofthe device was U.S.$128. The performance of this simple and inexpensive
air purifier is comparable to that of other do-it-yourself air purifiers such as
the Corsi-Rosenthal Box.

Key W ords : SARS-CoV-2, airborne transmission, ventilation, quality ofindoor air, clean air delivery rate (CADR), air change per hour (ACH),
do-it- yourself air purifier, the Corsi-Rosenthal Box

Introduction
When COVID-19 became pandemic status in early 2020, the WHO dismissed therole of airborne transmission, and health officials didn’t pay much attention to therisks of indoor air, therefore rarely considered importance of ventilation. In the 316
classrooms in the Marche region of Italy, that had mechanical ventilation with ratesof 1.4 to 14 litres/sec/person, the students’ risk of infection was reduced by at least
74% compared with that for students in classrooms that relied on naturalventilation1). One of the growing evidences about other technologies that removes

1

Director and Chairman, Kawasaki Takatsu Shinryo-jo and VHC243

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023

infectious particles from the air showed that ventilation combined with air cleaning,
equivalent to six air change per hour (ACH) in total, reduced exhaled aerosol
concentrations in a gym to 5 to 10% of what they would have been whithout thesemeasures2). Recently, Raymenants J, et al.3) in Belgium found factors associatedwith respiratory pathogen detection by indoor air surveillance in community
settings. According to their results, high CO2 and low natural ventilation wereindependent risk factors for detection of targeted pathogens including SARS-CoV-2.
Furthermore, CO2 concentration and portable air filtration were independently
associated with pathogen concentration. The effects of such as mask wearing,vocalization and mechanical ventilation were not significant. Their results supportthe importance of ventilation and air filtration to reduce transmission.
The primary transmission route of SARS-CoV-2 is airborne4),5), therefore,
countermeasures for infection route should be focused on airborne transmission
occurring especially indoors. Because droplets and small particles that people
breathe out can contain viral particles, it is important to use appropriate ventilation
systems which bring fresh, outdoor air into rooms, filter or disinfect the air there,
thus improve quality of indoor air (QIA)6). Center for Disease Control andPrevention (CDC) recommends to aim to deliver 5 or more ACH of clean air to rooms,
and to use filters rated minimum efficacy reporting value (MERV)7)13 or higher
grade6). Using higher-rated filters in the HVAC system can remove more germs in
the air, although incur costs and burden on the systems. Similar airborne
mitigation standards are 6 ACH (The Lancet COVID-19 Commission)8) or 10
litres/sec/person (WHO) outside health-care settings9). There should be ongoingverification that the systems are operating properly. The best and easy way to do
this is by real time monitoring ventilation with CO2 that should be less than800ppm3).

Recently,AmericanSocietyofHeating,

RefrigeratingandAir-Conditioning Engineers (ASHRAE) introduced a more detailed new standard(ASHRAE 241-2023) focusing on the control of infectious aerosol in buildings10).
In-room portable air purifiers fitted with high-efficiency particulate absorbing(HEPA) filters have potential to augment primary decontamination strategies suchas ventilation systems and eliminate airborne SARS-CoV-211). The Association ofHome Appliance Manufacturers (AHAM) provides assurance to the public thatmanufacturer’s claimed ratings of Clean Air Delivery Rate (CADR) have been244

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023

independently verified in accordance with Standard AC-1, which applies to portableelectric room cleaners that remove particulate matter (PM) from the air that have
CADR ratings with the range from 10 to 600 cubic feet per minutes (cfm) for tobaccosmoke with particle size between 0.10µm and 1.0µm 12).
Due to the cost to introduce high grade air cleaners fitted with HEPA filters,different types of do-it-yourself (DIY) air purifiers using box fans and filters havebeen devised, however it is not enough to consider whether these devices are
effective in the real world13), 14). In this paper, we build a simple air purifier at low
cost and evaluate efficacy of the device in terms of cost, sufficient air flow, noise,and removal of particles.
M aterials and M ethods
1) Preparation of the electric fan and high quality air filter
A portable electric fan (LASKO BOX FAN 3733, LASKO Products, LLC, PA,USA) was purchased from the company through an agent. It has five blades and theair volume increases from dial 1 to 3 (Fig.1). Sizes of the fan were 20.75"D×22.2"W×4.5"H. The rotation diameter of the blade was 20”. High quality air filters ofMERV13 were purchased from BNX Converting LLC (TX, USA) (Fig. 2). Thesefilters are electrostatic pleated air conditioner heating, ventilation, and airconditioning (HVAC) AC furnace filters, which remove mainly pollen, mold,bacteria and smoke. Sizes of the filter were 20”D×20”W×1”H.

Fig.1 A portable electric box fanFig.2 An HVAC filter of MERV13245

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023

2) How to build a simple air purifier
The HVAC filter was sealed to the inlet side of the box fan with the plastic ducttape (3M Scotch® 3920-WH duct tape, 3M Japan). The tape was applied
perpendicular to the fan to cover the four corners of the filter.
3) Evaluation of the air velocity and the airflow rate before and after filtration andcalculation of the air change per hour (ACH) and the estimated clean air delivery
rate (est.CADR)
To assess the airflow rate after installing of the filter, we measured the airvelocity before and after installing the filter by Anemometer (OTraki GM816,
ShenZhen QiYu Technology Co., Ltd., China). Data were measured at least twice
and the average value was recorded. The air flow rate was calculated with thefollowing formula15): Qair=uair*π.D2/4
where uair: air velocity in conveying fan of diameter D (m/s), Qair: air volumetric flowrate m3/s on the surface of the fan, D: fan diameter (m)The ACH was calculated as follows6): ACH=F*60/A*H
where F: the airflow through the system (cfm), A: the area of the room=length (ft) *width (ft), H: the height of the room (ft)The est.CADR (cfm) was calculated as follows6), 14): est.CADR =Qair*35.3*60*particle
removal rate (PRR)
where PRR: 1-(Number of PM0.3 particles when filter is on)/(Number of PM0.3

particles when filter is off)
4) Measurement of the noise generated by the fan
To evaluate the noise generated by the fan, National Institute for Occupational
Safety & Health (NIOSH) sound level meter (v.1.2.6.42) was used. This is asmartphone-based sound level measurement application developed by NIOSH andEA Lab16). The measurement was done for 10 seconds at 1 ft from the device withthe microphone pointed at 45°angle toward the device. The A-weighted EquivalentSound Level (LAeq), Maximum Level measured during the current run time(Max.Level), and the C-weighted Peak Sound Pressure Level (LCpeak) wereautomatically displayed in decibels on the smartphone.

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 20235) Measurement of particulate matter (PM) in the air after filtration
To examine the efficiency of the filtration, we measured three kinds of particulatematter (PM) with sizes 0.3, 2.5 and 10 µm by using the principle of laser scattering(Tc-8100, Hangzhou Shuiyue Technologies Co., Ltd., China). Counting or weighing
mode was displayed as Piece/L or µg/m3. Measurement was done in the room of 124
sqft (8-ft ceiling) under the central HVAC with temperature of 28.5℃ and humidityof 69%. At least two measurements were taken for 10 seconds at one ft from the fan
and the average value was recorded.

Results
It took less than ten minutes to complete the build of an air purifier (Fig.3). The
structure of the air purifier is very simple, which consists of a classical electric box
fan and a MERV13 filter, and costs approximately U.S.$128 (as of August 28th,2023, including cost of import).

Fig.3 Completed photo of the air purifier we prototyped

The air velocity (m/s) of each dial increased with increasing airflow (Dial 1-3). Theslight decrease in air velocity was observed in filtered conditions at dial 2 (mediumairflow) and 3 (high airflow) due to pressure drop (Fig.4).

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023

Fig.4 The air velocity at different air flow with or without the filter.

The airflow rate (Qair) of filtered condition on the device was 0.41-0.71 m3/s
depending on fan speed (Table1). The number of ACH of the room was estimated as
51.9-90.8 /h, which corresponded to 6.4-11.3 /h of those typical floor size of a U.S.
classroom of 1,000 ft2 (Table 1).
Table 1 The airflow rate (Qair), the air change per hour (ACH),
and the est.CADR by fan speed

The noise generated by the device was evaluated using LAeq, Max.Level, andLCPeak. The data taken from the quiet room was considered as a control value. TheLAeq increased with air volume, however the LCPeak was reached a plateau in themedium airflow (dial 2). Even the lowest airflow (dial 1) generated the LAeq valueof 53.5 dB (Table2).

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Table 2 The noise generated by the device at different fan speeds

PM in the air was measured at one ft from the device at low airflow (Fig. 5a). The
amount of PM0.3 was dramatically reduced after filtration from 11,710 to 7,090
Piece/L, which corresponded to a 40% reduction. The amount of PM2.5 was very low
before (7 Piece/L) and after (4 Piece/L) filtration and the amount of PM10 wasnegligible. The value of PM0.3 reduced with increasing level of airflow, however nodifference was observed between the medium (6,000 Piece/L) and the high (5,995
Piece/L) airflow levels (Fig. 5b). Therefore, a 50% reduction of PM0.3 was observedin the medium and the high airflow levels. Assuming that the PRR was 0.4 at Dial1,
and 0.5 at Dials 2 and 3, estimated CADR ranged from 343 to 751 cfm depending on
fan speed (Table1).

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The Bulletin of Kawasaki Takatsu Shiryo-jo 4(2): 243-252, 2023bFig. 5 The amount of PM in the air before and after filtration (a) and after filtration atdifferent fan speeds (b)Discussion
DIY air purifiers are generally assembled from box fans and square HVAC filtersand different configurations can be found on line that use from 1 to 5 filters6). In
general, all assembly methods are easy, however it is important to avoid air leaks
resulting in filter bypass. In our case, a square-shaped MERV13 filter protrudes
from the curved corners of a box fan, therefore it is important to seal corners as wellas each side by the duct tape. The cost of this device is $128, which is cheaper thanthose of high end portable ones with HEPA filters usually cost $500-$1,80017).
To achieve ACH of 5/h, minimum CADR required in the present room setting (124sqft, 8-ft ceiling) would be 83 cfm. Therefore, our data (751 cfm) at high airflow,which is less than the value of manufacturer's test report (819 cfm), is sufficient
numerical value for air exchange in U.S. standard classrooms with ACH of 11.3/h.
The second important factor of the device is noise. Even the lowest wind force
produces more than 50dB noise, which is comparable to the sound of a loud office orrefrigerator. Therefore, some people may find it noisy and turn off or operate at alower fan setting, resulting in an inappropriate function. Decreasing the noise levelof the device without compromising filtration performance is necessary to improve250

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box fan type air purifier.
Most of the respiratory droplets and particles exhaled during breathing, talking,singing, and coughing is less than 5 µm in size6). Therefore CDC recommends using
MERV13 filters which is at least 50% efficient at capturing particles in the 0.3 µm
to 1.0 µm size range6). In the present study, we showed that 40- 50% efficient atcapturing PM0.3 with the MERV13 filter.
Original DIY air purifier, known as the Corsi-Rosenthal Box, consists of four orfive HVAC particulate air filters so as to form a cube and a box fan to draw air
upward through filters6), 13), 14), 18), 19). A recent report indicated that the CADR of thefive two-inch MERV13 filters was between 600 and 850 cfm depending on fan speed,demonstrating exceptional performance compared to most commercially available
air purifiers fitted with HEPA filters13). Srikrishna D. reported using simplified,
low-cost measurement tools that the CADR (PM0.3) of single-filter (one-inchMERV14 filter) configurations with a box fan raged from 244 to 460 cfm14). These
data are comparable to our present CADR ranging from 343 to 751 cfm. Derk RC, etal. reported that increasing number of filters, filter thickness, and fan airflow
signify enhanced the ACH, which resulted in exposure reductions of simulated
respiratory aerosols up to 73%19). Although the DIY air filter units can be aneffective means of reducing aerosol exposure, the performance can varyconsiderably depending on their constructions19). If future clinical trials prove they
reduce viral infections in school classrooms, for example, installing these DIY air
purifiers will gain acceptance.

References
1) Buonanno, G, et al. Front. Public Health. 10:1087087, 2022
2) Blocken, B, et al. Build. Environ. 193:107659, 2021
3) Raymenants J, et al. Nat. Commun. 14:1332, 2023
DOI: 10.1038/s41467-023-36986-z
4) Morawska L and Milton DK. Clin. Infect. Dis. 71(9):2311-2313, 2020
5) Zhang R, et al. Proc. Natl Acad. Sci. 117(26):14857-14863, 2020
6) CDC. Ventilation in buildings.
https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html

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(cited 28 Aug, 2023)
7) EPA. What is a MERV rating?
https://www.epa.gov/indoor-air-quality-iaq/what-merv-rating(cited 28 Aug, 2023)
8) The Lancet COVID-19 Commission Task Force on Safe Work, Safe School, andSafe Travel. November. 2022 (cited 16 March, 2023)
https://static1.squarespace.com/static/5ef3652ab722df11fcb2ba5d/t/637740d40f35a9
699a7fb05f/1668759764821/Lancet+Covid+Commission+TF+Report+Nov+2022.pdf
9) WHO. https://www.who.int/publications/i/item/9789240021280(cited 28 Aug, 2023)
10)ASHRAE.https://www.ashrae.org/about/news/2023/ashrae-publishes-standard-2
41-control-of-infectious-aerosols (cited 28 Aug, 2023)
11) Liu DT, et al. Otolaryngol Head Neck Surg. 166(4):615-622, 2022
DOI: 10.1177/01945998211022636
12) AHAM. https://www.ahamdir.com/room-air-cleaners/
13) Porto RD, et al. Aerosol Sci Technol. 56(6):564-572, 2022
DOI: 10.1080/02786826.2022.2054674
14) Srikrishna D. Sci Total Environ. 838:155884, 2022
DOI: 10.1016/j.scitotenv.2022.155884
15) https://blog.progressiveproductsinc.com/calculate-air-velocity(cited 28 Aug, 2023)
16) NIOSH. https://blogs.cdc.gov/niosh-science-blog/2017/01/17/slm-app/(cited 3 Sep, 2023)
17) https://homeguide.com/costs/whole-house-air-purifier-cost (cited 4 Sep, 2023)
18) WIKIPEDIA. Corsi-Rosenthal Box.
https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box (cited 28 Aug, 2023)
19) Derk RC, et al. Build Environ. 229:109920, 2023
DOI: 10.1016/j.buildenv.2022.109920

Build a simple and inexpensive air purifier (v4.3) by H. Matsui.
Published on line in September 16, 2023 in Kawasaki Takatsu Shinryo-jo kiyo
©Creative Commons Attribution 4.0 International (CC BY 4.0)
(revised in October 30 to v4.3)

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