2022/06/14 Bioaerosol-II OS10-5

Asia Aerosol Conference 2022 (AAC2022)
Taipei / On line

Development of a small droplet
remover for infectious disease control
Masafumi Akiyoshi *, Satoru Watano (Osaka Metropolitan Univ.)
Tsuyoshi Ochiai (Kanagawa Institute of Industrial Science andTechnology, KISTEC )Web sute: http://anticovid19.starfree.jp/* akiyoshi-masafumi@omu.ac.jp（In Japanese）

Particle size distribution of droplets
released from the oral cavity
2μm

150μm
Talking

The actual distribution of particles sizes
released from the oral cavity is twopeaked, with 150μm "droplets" falling in
about two seconds and reaching only
about two meters, while 2μm "aerosols"
drift in the air for a long time.
In some cases, the droplets evaporate toform aerosol-sized droplet nuclei.

Coughing

G.R. Jhonson et al., Modality of human expired aerosol size distributions,
J. Aerosol Science, 42(2011)839-851.

Nobuyuki Takegawa, Aerosol, Droplet Transmission, andAirborne Transmission, Earozoru Kenkyu, 36 (2021) 65-74.

Investigation of Engineering Countermeasures against COVID-19 (1)CrowdedplacesSocial distance

Since the droplets only travel about 2meters, you can protect yourself fromflying droplets by keeping a distancefrom others.

Requirementfor Droplets
RemoverDroplets
Large droplets withlarge amounts of virus

Stay Home

Wet particles larger than 5 μm emitted from the oralcavity are called droplets and actually have a
distribution peak at about 150 μm. These large
droplets can be scattered over a distance of up to 2
meters in a few seconds.
To prevent the release of droplets, wear
mask is effective. Non-woven or clothmasks can stop about 80% of thedroplets, but the remaining 20% aredispersed through gaps. Therefore, there
is a risk of infection when meeting within
the range of droplets. Furthermore, it isdifficult to wear a mask during meals anddrinking.

Wear Mask

Investigation of Engineering Countermeasures against COVID-19 (2)ClosedSpace
EnoughVentilation
Carbon dioxide concentrationis one indicator of ventilation
status.

Aerosol

Air Purifier△Active disinfection with Chlorine dioxide orOzone is not recommended

Various types air purifier are available, including thoseusing photocatalyst, UV-C, and high-performance filters,
which collect aerosols and inactivate viruses contained inaerosols.
Inactivation is also possible with fan heaters and stoves,which are subject to high temperatures. (Air conditioners
cannot be used for this purpose.)

Particles smaller than 5 μm are calledaerosols, and the solids that remain
Fly around in the air for
after the droplets have dried are also
long periods of time.
called droplet nuclei. They stay in theair for several minutes and can spreadover a wide area. It can be dispersed
even by talking.
Even if you wear a mask, nearly half of theaerosol is dispersed between fibers and gaps.
Since they stay in air for a long time, theirconcentration gradually increases if ventilationis poor.

Investigation of Engineering Countermeasures against COVID-19 (3)Closecontact
settings

Wash Hands

Falled droplets cause
contact infection

Contact infection fromobject surfaces
UV-C irradiation toPPE
Since the risk of removing
personal protective equipment(PPE) is high in medical fields
dealing with infectious diseases,
UV-C irradiation to PPE at theboundary to the Cold area will
reduce the risk of infection.

Disinfection

Coronaviruses are the typethat have an envelope, alipid membrane, on theirsurface, so it is important todissolve the lipid. Just
physically washing it away
is effective.

Depending on the environment, a virus attached tothe surface of an object may retain its infectivity forseveral days. Dispersed droplets fall in several
second, but the virus contained in the dropletsremains on the rurface.

UV-C irradiation to object surfaces
Various papers have confirmed that it ispossible to inactivate SARS-CoV-2 in ashort time.
It cannot be used in the presence ofpeople because it is harmful to thehuman body. There are some products
using 222 nm UV that have very little
effect on the human body, but they have
not yet been certified as completely safe.
It is necessary to understand various
problems before using these products.

Application of photocatalysts and metal
particles such as copper and silver to thesurface of shared items

The application of photocatalyst to
the surface of an object always
produces a gradual deactivation
effect. Some metal-containingphotocatalysts, such as copper,
are effective for a certain period oftime even after dark. The simplest
way is to apply copper foil tape.

A specialized device that removes droplets
flying between person-and-person

Droplets that would
usually reach thefacing person

Droplets released fromthe oral cavity

1) Suction
Silent type fan
Contaminated
air contains
droplets

Droplets captured on the filter

The filter with visiblelight-responsible
photocatalyst

2) Capture
Purified air

Visible LED modules

3) Decompose

Within 2m(Typical meeting table width isabout 80cm)

Ultra-low-cost droplet removal system "Hikari Cleaner"using visible light responsive photocatalyst
NOT commercial productShading leaked light with Japanese paper

Luminary light between
person-and-person

The size is 12 cm square and5 cm high. Fan noise is only
19 dB. Power consumption isless than 5 W, and can be
powered by a mobile battery

Using visible light-responsive photocatalyst, it does notneed to be completely shielded from leaking light and canbe made with a simple structure. It is made by combining
commercial PC parts, therefore it cost only 1200 yen perunit. The photocatalyst filter can be manufactured with asimple non-woven fabric filter and Toshiba's "Renecat"
spray, which is commercially available. The suction
performance can be improved by using a more powerful fan.

Light (Photon)
Particle of
Photocatalyst

Reaction in photocatalyst

Reduction

O2Band Gap

Conduction band

Excitedby light

Valence band・O2-H2Oh+need UV-lightsuperoxideanionsOxidizing power
higher than ozone

e- Electronhole
TiO2(anatase) 3.2eV

Visible light（380nm～, 3.1eV～）・OH

Oxidation

WO3 2.5eV

Chemicallydecompose a varietyof organic materials
by oxidation
Finally organic materials are breaking down
completely into water and CO2

OH radicals
Because reactive oxygen
species disappear in about
one millionth of a second,
they can only move to a few
microns from the surface.

Visualization of droplet suction by
special imaging system
Special video recording wasconducted to visualize droplets inthe air. Within a range of about
50cm, we can see that droplets
emitted by speech from the oralcavity with a "booming" sound areinhaled and stopped by the filterin the same way as a mask.

Collection rate of droplets flying in space
Air flow
0.3 m/s

300mm

500mm
PCXNebulizer:
nozzle height
400mm

Small Droplet
remover: puton the desk

Particle counter:nozzle height
400mm

Large clean booth: 1.5×1.5×2.4m

Density of Particles / m-3

109

Performance of the small droplet remover inopen space was estimated using large cleanbooth. Particle counters were placed offcenter axis of nebulizer and the remover. The
nozzle of the nebulizer was set horizontallyand the mist flied almost straight with afollowing wind of 0.3 m/s from air purifier units.

108107106105

Particle Counter Height 400 mm
Particle size 1-5μm, Cleaner OFF
Particle size 1-5μm, Cleaner ON
Particle size 5-25μm, Cleaner OFF
Particle size 5-25μm, Cleaner ON104
103

0100

200

PCX: position of Particle Counter / mm

Large droplets with diameters of 5.0 to 25μm were reduced to about 1/10 in all
position. Aerosols of 1.0 to 5.0 μm, which
are close to the peak diameter of the aerosol
emitted from the actual oral cavity, were also
reduced to about 1/10 from center to 15cm,but at 20 and 25cm almost no reduction wasobserved.

Transmittance rate of droplets to a filter
A duct was set up in a cleanbooth using HEPA filter unit, andthe rate at which ultrasonicsprayer mist, simulating dropletsfrom the oral cavity, was captured
by a non-woven fabric filter wasevaluated. As a result, it wasconfirmed that droplets of 5 μmor larger could be almost
completely captured.
Catching andslowly decomposing

The non-woven fabric filter used in this study

A solution containing bacteriophage Qβ
was sprayed with a nebulizer in a 370 L
glove box to make an aerosols. At themeasurement time, 10 L air was sampledthrough a gelatin filter and the infectivity
was evaluated by the plaque method.
As a result, the infectivity decreased in
the blank from 1.6 × 106 PFU to 1.1 ×
106 PFU in 20 minutes, a decrease ofabout 30%.
On the other hand, the use of the small
droplets remover with an inorganic highperformance filter resulted in a decrease
from 7.2×105 PFU to 3.3×105 PFU after
10 minutes and 1.8×105 PFU after 20minutes, approximately half in every 10minutes.

Infectivity / PFU

Removal performance for virus in aerosol

106
Blank
With small droplets remover

105

010Time / min20

Although aerosols suspended in the
air for long time cannot be caught by
the filter, it was suggested thatviruses contained in aerosols can beremoved by the small droplet removerusing a photocatalyst.