2021/10/13 Aerosol Session 3[R9]03

The 8th Asian Particle Technology Symposium(APT2021)
in Knowledge Capital Congres Convention Center, Osaka

Evaluation of the droplet removal
performance by a small on-desk aircleaner with photocatalyst
Masafumi Akiyoshi *, Satoru Watano (Osaka Prefecture Univ.)
Tsuyoshi Ochiai (Kanagawa Institute of Industrial Science andTechnology, KISTEC )Web sute: http://anticovid19.starfree.jp/* akiyoshi@riast.osakafu-u.ac.jp（In Japanese）

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

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

Droplets Remover
The combination of filter andphotocatalyst catches droplets
and oxidizes and decomposesviruses.
The system is useless unless it isinstalled between people who arefacing each other and within thedistance of the flying droplets, soa large number of units arerequired.

Droplets
Large droplets withlarge amounts of virus

Stay Home

Droplets larger than 5 μm emitted from the oralcavity are called droplets and have a distribution
peak at about 120-150 μm. These droplets can bescattered over a distance of up to 2 meters in a few
seconds. They can be dispersed not only by
coughing or sneezing, but also by normal talking.
To prevent the release of droplets, masks
are effective. Non-woven or cloth masks
can stop about 80% of the droplets, but
the remaining 20% are dispersed throughgaps. Therefore, there is a risk ofinfection when meeting within the rangeof droplets. In addition, it is difficult towear a mask during meals.

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

Aerosol

Air Purifier△Active disinfection with Chlorine dioxideand Ozone 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.)

Fly around for long periods
Particles smaller than 5 microns areof time and travel far on thecalled aerosols, and the solids thatremain after the droplets have dried are wind.
also called droplet nuclei. They stay inthe air for several minutes and canspread over a wide area. It can be
Upper-Room Ultraviolet Germicidal Irradiation
dispersed even by talking.
Even if you wear a mask, nearly half of
the aerosol is dispersed between fibers
and through gaps. Since they stay for along time, their concentration gradually
increases if ventilation is poor.

UV-C is harmful to the human body (strong
inflammation of the cornea of the eye and skin).
However, it is possible to inactivate viruses in the air
by irradiating UV-C in a space above where it will
not hit people. This UVGI is recommended by CDC.

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

Wash Hands

Disinfection

You never know where abooby trap might be lurking

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.

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. The virus is not transmitted by merecontact with the hands, but through ACE2 receptors
in the mucous membranes of the oral/nasal cavity
and eyes.

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-containing
photocatalysts, such as copper,
are effective for a certain period oftime even after dark. The simplest
way is to apply copper foil tape.

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

120～150μm
Talking

The distribution of particles sizes
released from the oral cavity is twopeaked, with 150μm "droplets" falling in
about two seconds and reaching only
about one to two meters, while 2μm"aerosols" drift in the air for a long time(without airflow, they do not move much).
In some cases, the droplets evaporate toform aerosol-sized droplet nuclei.

CoughingG.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.

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

Droplets that wouldusually 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)

What is Photocatalyst?
Since the discovery of the Honda-Fujishima effect in 1967, which decomposes water
into oxygen and hydrogen, "photocatalyst" has attracted attention as a technology
originating in Japan, and development has continued. Photocatalyst is a type ofsemiconductor that generates electrical energy when exposed to light, just like a solarcell. Rather than extracting that energy as an electric current, it creates active oxygen
such as superoxide anions and OH radicals on the surface of the tiny particles, andthrough its extremely strong oxidizing power, it decomposes organic matter, finally
breaking it down completely into water and CO2. Viruses and bacteria are noexception, and there have been no reports of ineffectiveness.
Conventionally used TiO2 (anatase type) has a band gap of 3.2 eV, so its response tovisible light is small, requiring irradiation with ultraviolet rays. On the other hand, WO3
with a band gap of 2.5 eV, which have been put to practical use in recent years,
respond to light of wavelengths below 480 nm (visible light region of green to blue).
Both TiO2 and WO3 have already been reported to inactivate SARS-CoV-2 inacademic papers.
By combining this visible light responsive tungsten-based photocatalyst with visible
light LEDs, we are able to manufacture a safe droplets remover with a simple
structure.

Light (Photon)
Particle ofPhotocatalyst

Reaction in photocatalystReductionO2Band 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.

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 bepowered 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.

Inactivation of viruses by visible light responsive
photocatalysts
Masashi Uema et al., "Effect of Photocatalyst under
Visible Light Irradiation in SARS-CoV-2 Stability on an
Abiotic Surface", Biocontrol Science, 26 (2021) 119-125.

Viral titerfrom Toshiba RENECATTM Website

without RENECAT

The amount of photocatalyst applied isabout 0.7 g/m2 in the standard
specification of Hikari Cleaner, but it isabout 14.3 g/m2 in the high-performance
filter that can be mass-produced.

with RENECAT

The illuminance on the filter surface ofthe Hikari cleaner is more than 68,500
lux. Therefore, the Hikari Cleaner is
expected to inactivate at a faster rate
than the above conditions.

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.

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 μm
or larger could be almost
completely captured.
Catching andslowly decomposing

The non-woven fabric filter used in this study

Number of Droplets / m-3109108107
106105109108107106105
104

L=90cm, Prefilter 2 layer
Without luminary cleaner
0.3-1.0μm
1.0-5.0μm
With luminary cleaner
0.3-1.0μm
1.0-5.0μm

10310Height = 30cm
Prefilter 2 layer
Particle size
0.3-1.0μm
1.0-5.0μm
5.0-25μm

104
30

40
50
60
70
Distance from Nebulizer / cm

Dependence of droplet concentration
on distance from the nebulizer.

Number of Droplets / m-3

Number of Droplets / m-3

Collection rate of droplets flying in space

108
107

2030

40

2030
Height / cm

40

L=90cm, Prefilter 2 layer
Without luminary cleaner
5.0-25μm
With luminary cleaner
5.0-25μm

106105104103
10

The droplet collection
examinations wereconducted in a clean
bench with a wind speedof about 0.6 m/s.
The number of large
particles decreased withdistance. This may be
due to both falling bygravity and shrinking byevaporation.
It was confirmed that thedroplet removal systemsignificantly reduced thenumber of droplets at thelevel of the face when aperson seated at a table.

HCHO density / mg・m3

Formaldehyde decomposition experiment

5k=8.84×10-4/min

k1=1.81×10-2/mink2=1.41×10-3/min

k=6.70×10-3/min
-2

10.50

k=2.15×10 /min
Blank
Standard nonwoven fabric filter
Ceramic fiber paper
Ceramic fiber paper after 500℃ baking

100200Time / min

300400Using a 38L size acrylic desiccator, changes in the concentration of formaldehyde (HCHO), a
kind of organic gas, were measured using a Formaldiameter htV-m.
Reliable decomposition performance was confirmed even with a standard type droplet removal
device with a simple structure and low cost, which is being considered for widespread use byself-made devices in educational and medical fields. In addition, the prototype system thatusing a filter with a high concentration of photocatalyst and inorganic materials showed aperformance that far surpassed that of commercially available small air purifiers.