Improved Photocatalytic Air Cleaner with Decomposition of Aldehyde and Aerosol-Associated Influenza Virus Infectivity in Indoor Air

Air pollution caused by fine particulate matter (PM2.5), volatile organic compounds, and bioaerosols is a major environmental risk to health. We developed a photocatalytic air cleaner for reducing the pollution levels of indoor air; we improved the photocatalytic system by using UV-LED for the removal of acetaldehyde and PM2.5 and by reducing the weight and size of the system. The efficiency of photocatalysis depends on the surface area and materials. Therefore, we prepared a nanosized titanium dioxide (TiO2)-coated aluminum plate irradiated by UV-LED lamps (wavelength: 375 nm) as a photocatalytic air cleaner. Passing air continuously through a TiO2-coated aluminum plate (5 × 10 × 1 cm) under black light for 200 min decomposed 90% of 5 ppm acetaldehyde (12.4 μmol h) and generated two carbon dioxide molecules (25.43 μmol h) at a molar ratio of 1:2, indicating complete decomposition of acetaldehyde with high efficiency. This photocatalytic air cleaner was applied to the decomposition of acetaldehyde and inactivation and removal of aerosolassociated influenza virus. Acetaldehyde (20 ppm) in a 1-m cubic space was eliminated in 60 min at a half-life of 8 min. The aerosol-associated infectivity and the RNA genome of influenza virus A/PR/8/1934 (H1N1) produced by a nebulizer in a 779-L cubic space were eliminated within 7 min; however, they were detectable for up to 28 minutes when the functional photocatalytic air cleaner was not used. The presence of intermediate breakdown products of influenza virus indicated that the virus was broken down by photocatalysis. Thus, the photocatalytic air cleaner efficiently decomposed and eliminated organic chemicals, acetaldehyde, and aerosol-associated influenza virus infectivity and viral RNA, indicating that it can clean and detoxify the indoor air in a closed space for maintaining a safer environment.


INTRODUCTION
Air pollution caused by fine particulate matter (PM 2.5 ), volatile organic compounds (VOCs), and bioaerosols is a major environmental risk to health (Hess et al., 2016;Nazaroff, 2016;Rohr and McDonald, 2016;WHO, 2006;Mukherjee and Agrawal, 2017).By reducing air pollution levels, countries can reduce the disease burden resulting from stroke, heart disease, lung cancer, and both chronic and acute respiratory diseases, including asthma (WHO, 2016a), as well as the burden caused by VOCs, particulate matter, and pathogens present in the environment that affect the health of humans.Photocatalysis appears to be one of the means of eliminating these risk factors and has been reported to decompose organic chemicals by reactive oxygen species generated by photocatalysis with titanium dioxide (TiO 2 ) and light (Fujishima and Honda, 1972;Ochiai et al., 2014).We previously developed a highly efficient photocatalytic air cleaner and characterized its efficacy in the decomposition of aldehyde and the inactivation of influenza virus infectivity.
Photocatalysis by TiO 2 produces negative electrons (e -: electron hole) and positive holes (h + ) and generates superoxide anions (O 2 + e -→ O 2 -) and hydroxyl radicals (OH -+ h + → •OH).TiO 2 on the surface is exposed to ultraviolet light with a wavelength shorter than 400 nm, and e -and h + are generated on the surface of aluminum.
The generated e -reacts with oxygen (O 2 ) in air and generates a superoxide anion (O 2 -).Then, h + reacts with H 2 O and generates a hydroxyl radical (•OH).These O 2 -and •OH radicals react and decompose organic chemicals and inactivate microbes.The efficient photocatalytic activity for the complete decolorization of methylene blue is shown in Fig. 1 and Supplementary Material.Titanium is noncorrosive and environmentally safe; it is used as a food additive and in cosmetics, particularly sun blockers.
Experimental and clinical evidence suggests that influenza infection caused by a small-particle aerosol (particle diameter < 10 µm) is more potent than that caused by larger droplets or contact infection (McLean, 1961;Schulman and Kilbourne, 1962;Moser et al., 1979;Knight, 1980;Goldmann, 2000).Transmission of influenza occurs mostly through aerosols generated by coughs and sneezes.A study reported that 72% of 54 passengers aboard an airplane were infected with influenza when the ventilation system was inoperative during a 3-h delay, and the closed space with inadequate ventilation accounted for the high infection rate (Moser et al., 1979).Infection can be most efficiently delivered by an aerosol-associated virus.The 50% human infectious dose for wild-type influenza A virus administered intranasally as drops is 127-320 of the median tissue culture infective dose (TCID 50 ), and that of the virus delivered by an aerosol is 0.6-3 TCID 50 (Alford et al., 1966;Couch et al., 1971;Murphy et al., 1973;Couch and Kasel, 1983).The sizes of the largest droplets that evaporate entirely before landing 2 m away are between 60 and 100 µm for respiratory exhalation flows (Xie et al., 2007).Thus, aerosols produced by coughs and sneezes are crucial for air-borne infections.
In our previous study, we developed a photocatalysis system by using a porous ceramic substrate coated with nanosized TiO 2 .This photocatalysis system resulted in the decomposition of acetaldehyde, refractory dioxins, and aerosol-associated influenza virus infectivity; in particular, 7.5 and 2.8 pg m -3 of particulate dioxins (40 pg m -3 ) and gaseous dioxins (16 pg m -3 ) were removed, respectively, indicating that photocatalysis decomposed more than 80% of dioxins (Daikoku, 2015).Although this first photocatalytic air cleaner provided satisfactory results, we have improved it as follows.
This improved photocatalysis cleaner is equipped with Hepafilter to remove fine particulate matter (PM 2.5 ).Compared with a ceramic filter, aluminum is more suitable as a substrate for TiO 2 coating in a photocatalytic air cleaner, and an aluminum filter is uncrackable, compact, lighter, and cheaper than porous ceramic.The light source was changed from black light (peak wavelength: 352 nm) to a UV-LED array (wavelength: 375 nm) to increase the power of photocatalysis because of the increased light quantity and longer lifespan of UV-LED.Thus, we developed a compact photocatalytic air cleaner for indoor air and improved the photocatalysis system by coating an aluminum plate surface with nanosized TiO 2 with a UV-LED array and an HEPA filter.In this study, the highly efficient photocatalytic performance of the air cleaner was characterized by the decomposition of acetaldehyde as an organic chemical and the inactivation of aerosol-associated influenza virus infectivity.

Photocatalytic Air Cleaner
The improved cleaner meets the standard of Japan Electrical Manufacturers (JEM1467) defined by the Japan Electrical Manufacturers' Association (JEMA)."The removal performance test method and calculation method of household air cleaner for PM 2.5 using 32 m 3 ," In addition, a photocatalysis system was built into this air cleaner for indoor air.The aluminum plate was purchased from UACJ Corporation, Japan.The photocatalytic TiO 2 was purchased from Ishihara Sangyo Kaisha Ltd., Japan.The coating of TiO 2 on the surface of an aluminum plate was performed by APS Japan Co., Ltd., by fixing nanosized TiO 2 particles on the satin-finished surface of the aluminum filter.The photocatalytic activity of the TiO 2 -coated aluminum plate was shown by the complete decolorization of sprayed methylene blue at UV-LED (wavelength: 375 nm; Fig. 1).The total surface area of the filter (6 × 48 × 0.03 cm × 2 pieces) was estimated to be approximately 300 m 2 (Grätzel, 2003); the cleaner comprises two sets of TiO 2 -coated aluminum filters and 12 UV-LED lamps (operated at 375 nm and 2 W; Nichia Corporation, Tokushima, Japan) and an HEPA filter (HEPA H-11 that can remove a maximum of 5% of 0.1 micron particles per liter of air; Airclean Ltd., Kent, UK) with active carbon and TiO 2 coating (Fig. 2).
The improved aspects of the cleaner are its compactness and enhanced performance.The size (435 × 412 × 180 mm) and weight (5.2 kg) of the cleaner were reduced by 32.7% (460 × 306 × 75 mm) and 59.3% (3.2 kg), respectively, by replacing the photocatalytic porous ceramic filter (800 g) with a photocatalytic aluminum filter (64 g).Furthermore, substituting black light (peak wavelength: 352 nm) with the UV-LED system as the light source increased the effective UV dose by 10 times, and the lifespan of 3,000 h provided by black light was quadrupled to 12,000 h by the UV-LED array.The air flow ranges from 126 to 66 m 3 h -1 with three channels; the air quality and flow are monitored and controlled by a gas sensor SP3S-AQ2 (FIS Inc., Itami, Hyogo, Japan) as an air quality control (VOCs) and PPD42NJ (Shinyei Technology Co. LTD.Kobe, Japan) as a particle sensor (over 1.0 µm).

Performance Evaluation of the Photocatalysis of Acetaldehyde
The performance of decomposition of acetaldehyde by Methylene blue solution was sprayed on the surface of TiO 2 -coated aluminum, and after drying, the blue color on the surface was decolorized on exposure to the UV-LED array, indicating the efficient decomposition of methylene blue by photocatalytic activity.This process can be seen in the Supplementary Material "Video Photocatalysis".photocatalysis was evaluated in both flow system and closed system.The flow system was used to characterize the efficiency of photocatalytic activity of a photocatalytic aluminum filter and the closed system was used to examine the efficacy of the air cleaner system to remove aldehyde in the closed space.The decomposition of acetaldehyde to carbon dioxide was analyzed by passing it through a photocatalytic aluminum filter (49.5 mm × 99.5 mm × 9.5 mm) under 1 mW black light (wavelength: 351 ± 2 nm) per 1 cm 2 , according to the Japanese Industrial Standard (JIS) R1701-2 procedure defined by the JIS Committee.
Gas containing 5 ppm acetaldehyde and 1.56% water was passed through a photocatalytic aluminum filter (49.5 mm × 99.5 mm × 9.5 mm) at a rate of 1 L min -1 at 25°C ± 2°C and humidity of 50%, and acetaldehyde and carbon dioxide were directed to a methanizer, reduced to CH4 by means of nickel catalyst, and detected using a flame ionization detector (GC-2014AFF, Shimadzu Corporation, Japan) with the use of nitrogen (< 0.1 ppm carbon dioxide) as a carrier gas at the Kanagawa Academy of Science and Technology (Daikoku, 2015).The decomposition of acetaldehyde was analyzed by measuring the acetaldehyde concentration in a 1-m 3 container, and acetaldehyde was detected using a gas detector tube (92 L; Gastec Corporation, Japan) at the Japan Food Research Laboratories (JFRL, Tokyo, Japan).Acetaldehyde (20 ppm) was introduced into the container, and the acetaldehyde concentration was measured at 0, 10, 30, and 60 min.This study compared the efficacy of photocatalysis with and without a fan and UV-LED array in the air cleaner.

Inactivation of Aerosol-Associated Infectivity of Influenza Virus
Fig. 3 shows the system used for assaying the aerosolassociated infectivity of influenza virus in a closed space.The photocatalytic air cleaner used was experimentally manufactured by APS Japan Co., Ltd.(Japan).The air was filtered through an aluminum plate coated with TiO 2 under an electric fan in the photocatalytic air cleaner.
Aerosol-associated influenza virus was inactivated by the air cleaner, as described previously (Daikoku, 2015).A spraying nebulizer was used to produce and distribute particles (7-15 µm in diameter).Influenza virus solution (14 ML) containing 9.8 × 10 9 plaque-forming units (PFU) was nebulized into a 92 × 92 × 92-cm cubic space (779 L).Subsequently, 45 L of air this space was collected in an air bag, and the viral infectivity and amount of viral RNA in the bag were measured at 0, 3, 7, 15, and 28 min.The trapped air was filtered through a 0.45-µm cellulose-acetate membrane filter (47 mm in diameter, Merck Millipore, Germany) by using an aspirator vacuum pump.One set of experiments (with and without an air cleaner) was performed once a day at 22°C and 50%-60% humidity, with both experiments performed under similar conditions.The filter was immersed in and then vigorously rinsed with 3-mL VDM.The medium was serially diluted 10-fold and inoculated into MDCK cells in six-well plates for the plaque assay to determine the infectivity of the virus trapped on the filter.

Real-Time Quantitative Reverse Transcription Polymerase Chain Reaction of Influenza Virus in the Air
Influenza virus RNA was quantified using a method reported in a previous study (Daikoku, 2015).Viral RNA in the 45-L bag was extracted from 1-mL VDM containing influenza virus trapped on the membrane filter; the extraction was performed using the High Pure Viral RNA kit (Roche Diagnostics, Germany) according to the manufacturer's protocol.To create a standard curve, viral RNA was extracted from 1 mL of the supernatant of infected MDCK cells containing 7.0 × 10 8 PFU.Reverse transcription polymerase chain reaction (PCR) was performed through the ReverTra Ace qPCR RT Kit (Toyobo Bio, Japan) by using the influenza viral RNA-specific primer Uni12 (5′-agcgaaagcagg-3′) in a 20-µL reaction mixture at 37°C for 15 min.Subsequently, 2 µL of the reverse transcription PCR mixture was employed for real-time PCR (RT-PCR) performed by using an influenza M segment-specific primer set-which comprised a forward primer (5′-ctaatcagacatgagaacagaatggt-3′) and a reverse Fig. 3. System for assaying aerosol-associated infectivity of influenza virus.Aerosol-associated influenza virus was injected by a nebulizer into a 92 × 92 × 92-cm cubic space (779 L), and 45 L of the air in this space was slowly collected in an air bag at the indicated times.The air in the 45-L air bag was filtered through a 47-mm diameter, 0.45-µm pore cellulose-acetate filter, and the filter was immersed in virus dilution medium (VDM).Next, the trapped infectious virus was suspended, diluted in VDM and inoculated into Madin-Darby canine kidney (MDCK) cells for determining the plaque number or used for quantifying viral RNA through real time-polymerase chain reaction.The assay was performed with or without the developed photocatalytic air cleaner.primer (5′-ctagcctgactagcaacctccatg-3′)-and SYBR Premix Ex Taq II (Tli RNaseH Plus; Takara Bio, Japan).The quantitative PCR program was as follows: an initial denaturation step at 95°C for 30 s, followed by 40 cycles of PCR amplification at 95°C for 5 s and 60°C for 30 s; the levels of PCR products were monitored using a Takara Dice thermal cycler for the RT-PCR system and analyzed using Takara RT-PCR software (Takara Bio, Japan).PCR products were used for determining viral copy numbers.

Decomposition of Acetaldehyde by Photocatalysis
Aldehyde concentrations of 5 and 20 ppm are considerably high for indoor air; however, these concentrations are defined as analysis conditions for aldehyde decomposition by the JIS R1701 and JFRL.This also indicates that lower aldehyde concentrations in indoor air are decomposed similarly.
Fig. 4 illustrates a profile of the decomposition of acetaldehyde to carbon dioxide.When gas containing 5 ppm acetaldehyde was passed through the photocatalytic aluminum plate, 90% of acetaldehyde was stably removed in a single passage for 200 min according to the procedure defined by the JIS Committee.The decomposition of acetaldehyde to carbon dioxide and water by photocatalysis is outlined as follows: CH 3 CHO + 6OH + O 2 → 2CO 2 + 5H 2 O. Thus, the molar ratio of input acetaldehyde to output carbon dioxide is one acetaldehyde molecule and two carbon dioxide molecules; 90% of 5 ppm acetaldehyde (12.4 µmol h -1 ) was decomposed into two carbon dioxide molecules (25.43 µmol h -1 ; carbon dioxide conversion rate = 92.5%)efficiently and continuously for 200 min when acetaldehyde was passed through the TiO 2 -coated aluminum (5 × 10 × 1 cm) under black light (Fig. 3).The decomposition efficiency was 90%, and the ratio of acetaldehyde to carbon dioxide was consistent with the decomposition of the product, with the molar ratio being one acetaldehyde (12.40 µmol h -1 ) to two carbon dioxide (25.43 µmol h -1 ) molecules and water, indicating complete decomposition of acetaldehyde.
Fig. 5 shows the results of two experiments conducted for examining the decomposition of acetaldehyde in a closed space (1 × 1 × 1 m) by a photocatalytic air cleaner.The acetaldehyde concentration (20 ppm) did not change for 60 min when the photocatalysis system was not operational or the fan alone was operational.When the photocatalysis system was operated using a UV-LED array, acetaldehyde was stably removed in 60 min with a half-life of 8 min.Aldehyde stably persisted without photocatalytic activity, indicating no adsorption, leakage, or spontaneous decomposition of aldehyde in the testing space including the air cleaner.As expected, the photocatalytic air cleaner efficiently decomposed and removed aldehyde in the space through the decomposition of 5 ppm aldehyde in a one-pass experiment (Fig. 4).

Inactivation of Aerosol-Associated Influenza Virus Infectivity by Photocatalysis
The effect of photocatalytic activity on the aerosolassociated infectivity of influenza virus was evaluated in a 779-L cubic space by performing a plaque assay with or without an operational photocatalytic air cleaner.In the two experiments, the aerosol-associated infectivity levels in the space were 1.2 × 10 6 and 2.0 × 10 6 PFU at time 0, respectively; when the photocatalytic air cleaner was not operational, these levels decreased to 3.4 × 10 4 and 2.4 × 10 4 PFU, respectively, at a time of up to 28 min, and this .By contrast, when the photocatalytic air cleaner was used, the aerosol-associated infectivity was undetectable at 7 min after Hepafilter trapping of influenza virus and photocatalysis with TiO 2 irradiated by UV-LED lamps, whereas infectivity levels of 3.8 × 10 5 and 2.6 × 10 5 PFU were detected at 7 min without the air cleaner until 28 min (Figs. 6(a) and 6(b)).Thus, the aerosol-associated infectivity of influenza virus in the space was rapidly and efficiently inactivated by the photocatalytic air cleaner.
Because the aerosol-associated infectivity of influenza virus was rapidly inactivated, we examined the fate of influenza virus RNA in virus particles through quantitation with RT-PCR.Similar to the result of the aerosol-associated infectivity in the influenza virus plaque assay, the viral genomes were quickly reduced within 7 min by the photocatalytic air cleaner; by contrast, they gradually decreased for up to 28 min in the absence of the photocatalytic cleaner (Figs.6(c) and 6(d)).Thus, viral RNA was rapidly eliminated from the space by the photocatalytic air cleaner.The influenza M segment used for PT-PCR was 1 kb, and the RT-PCR product size was 116 bp, comprising 12% of the M segment.An inability to detect the 116-bp length of the M segment indicated the degradation of infectivity and the decomposition of the RNA genome.
The infectious influenza virus is an enveloped virus particle with intact full-length viral RNA including the M gene.Influenza virus infectivity was inactivated, and viral RNA was decomposed by photocatalysis in this study.RNA copies larger than 116 bp released from virus particles were detected in the air at 3 min and further degraded to an undetectable level at 7 min (Figs.6(e) and 6(f)).The number of RNA copies per infectivity and their ratio were preserved at time 0 and in the course of spontaneous decay, whereas the photocatalytic air cleaner increased the ratio of RNA copies per infectivity by 2.2 and 5.8 times through photocatalysis.Photocatalysis broke the enveloped particles, and the inner viral RNA was consequently scattered in the air space, resulting in an increased ratio of RNA copies per infectivity.Thus, viral infectivity was first lost by the degradation of the viral structure with less degradation of RNA, and the infectious virus treated with photocatalysis required approximately 2-5 times more viral RNA copies for infectivity compared with that without photocatalysis.The detection of the intermediately degraded influenza virus indicated that viral infectivity was eliminated by photocatalysis in addition to trapping the virus in the HEPA filter in the improved air cleaner.

DISCUSSION
In this study, we evaluated the ability of a photocatalytic air cleaner device to decompose acetaldehyde and inactivate aerosol-associated influenza virus infectivity.The results reveal that 90% of 5 ppm acetaldehyde was efficiently decomposed by photocatalysis at a molar ratio of one acetaldehyde to two carbon dioxide molecules and water, indicating complete decomposition of acetaldehyde with   high efficiency.Dioxins are difficult to decompose and require incineration at high temperatures (over 850°C; (WHO, 2016b).We previously reported that more than 80% of particulate and gaseous dioxins were decomposed by photocatalysis (Daikoku, 2015).This cleaner meets the standards of JEM1467 as a household air cleaner for PM 2.5 , as defined by the JEMA.In addition to meeting this criterion, this cleaner consists of two sets of photocatalytic aluminum plates coated with nanosized TiO 2 irradiated by a UV-LED array as a supporting material and a cost-saving and strong UV-LED with HEPA filter with active carbon and TiO 2 coating (Fig. 2).The photocatalytic metal is physically and chemically stable, and the power of photocatalysis on the TiO 2 -coated surface depends on the strength of the light and the cleanness of the surface.Sulfur and nitrogen compounds are decomposed to sulfuric acid and nitric acid, respectively, after the use of the photocatalytic cleaner for a longer time, and these compounds bind to and accumulate on the surface after decomposition, resulting in the inhibition of photocatalysis.These substances can be removed by washing with deionized water, which then restores the photocatalytic activity.
Experimental and clinical studies have reported the importance of aerosol particles as the major and most effective route of influenza transmission, particularly in closed public spaces with low ventilation.Aerosol particles measuring 1-3 µm are produced directly or through the evaporation of droplets exhaled during coughing or sneezing by a patient with influenza, and they are preferentially deposited in the lower airways of the lungs (Knight, 1980;Heyder, 1986;Isaacs and Martonen, 2005;Milton et al., 2013), resulting in efficient transmission.Thus, aerosols produced by sneezes and coughs as well as the continuous tidal expiration of patients may constitute a critical source for the transmission of influenza in a closed space with inadequate ventilation.
We reported the photocatalysis alone eliminates influenza virus infectivity in the original photocatalytic air cleaner (Daikoku, 2015).In this study using the improved photocatalytic air cleaner, we observed that the aerosolassociated infectivity of influenza virus was detectable for up to 27 min in a closed space that was filled once with infectious aerosol.The enveloped virus and RNA were eliminated by photocatalysis and influenza virus trap by Hepafilter at 3 min, and the ratio of RNA copies to PFU increased to 220% and 575% compared with that at time 0. The increased ratio of RNA copies to PFU with low infectivity indicates the destruction of the enveloped virus particle, and RNA larger than 116 bp was produced by the degradation of influenza virus particles.The elimination rates of the infectious virus and viral RNA were different, indicating that elimination was due to photocatalysis in addition to the capture of virus particles by the filter (Fig. 7).At 7 min, infectious virus and RNA were not detected in the space, indicating complete elimination of virus particles and viral RNA by photocatalysis.Photocatalysis promoted peroxidation of the polyunsaturated phospholipid component of the lipid membrane initially and induced a major disorder in the cell membrane of Escherichia coli (Maness et al., 1999).Influenza virus infectivity and its RNA appeared to be degraded by photocatalysis.
We improved the photocatalytic air cleaner for indoor air by changing the porous ceramic to an aluminum plate and black light to the UV-LED array, installing the HEPA filter, and reducing the weight and enhancing compactness.An aluminum plate for photocatalysis is cheaper, lighter, easier to process, and more stable than porous ceramic.Our results show that both viral infectivity and viral RNA were efficiently eliminated from the container because of Hepafilter trapping of virus and the complete destruction of virus particles and viral RNA.With regard to air pollution and health (WHO, 2006;Hess et al., 2016;Nazaroff, 2016;Rohr and McDonald, 2016;Mukherjee and Agrawal, 2017), the improved photocatalytic air cleaner can efficiently and rapidly inactivate bioaerosols, such as aerosol-associated influenza virus particles produced by sneezes and coughs, and air organic pollutants, such as aldehyde and dioxins, and preserve the indoor air quality for better health.
The photocatalytic system showed the complete decomposition of 5 ppm acetaldehyde by a single pass through the photocatalytic filter and has an advantage as a high efficiency system without forming intermediate compounds (Zhao and Yang, 2003;Hodgson et al., 2007;Mo et al., 2009;Tseng et al., 2010).Regarding the performance of the improved photocatalytic air cleaner, the performance of photocatalytic air cleaners have been reported but, for example, the decomposing ability test of acetaldehyde were different.(Yu et al., 2006;Sekine et al., 2011;Ochiai et al., 2014;Del Curto et al., 2016;Costarramone et al., 2017) Although it is difficult to compare the performance among photocatalytic air cleaners in the market, our photocatalytic air cleaner completely decomposed the acetaldehyde in the flow and closed air systems, indicating the excellent performance among photocatalytic air cleaners.

Fig. 1 .
Fig. 1.UV-LED-induced photocatalytic decolorization of methylene blue sprayed on the surface of TiO 2 -coated aluminum.Methylene blue solution was sprayed on the surface of TiO 2 -coated aluminum, and after drying, the blue color on the surface was decolorized on exposure to the UV-LED array, indicating the efficient decomposition of methylene blue by photocatalytic activity.This process can be seen in the Supplementary Material "Video Photocatalysis".

Fig. 2 .
Fig. 2. Schematic view of the structure of the air cleaner with two sets of filter systems.The air first passes through either of the two HEPA filters (HEPA H-11), which can pass a maximum of 5% of 0.1 micron particles per liter of air.Next, the air passes through the photocatalytic aluminum surface coated with TiO 2 with 6 UV-LED lamps (375 nm 2w, Nichia Corp., Tokushima, Japan) and proceeds to one common outlet as clean air.The photograph shows the detailed structures of the HEPA filter, photocatalytic aluminum plate, and UV-LED array and depicts the bellows structure of the HEPA filter and aluminum plate to increase the surface area.

Fig. 4 .Fig. 5 .
Fig.4.Decomposition of acetaldehyde to carbon dioxide and water by the aluminum plate coated with nanosized TiO 2 (size: 5 × 10 cm; depth: 1 cm) and irradiated with 1 mW black light (wavelength: 351 ± 2 nm) per 1 cm 2 .When the black light was turned on at 40 min (1), the concentration of acetaldehyde decreased and that of carbon dioxide increased.When the black light was turned off at 240 min (2), the concentration of acetaldehyde increased and that of carbon dioxide decreased to the level without the black light.R A is the acetaldehyde removal rate.R C is the carbon dioxide conversion rate.Q A is the amount of acetaldehyde removed per hour.Q C is the amount of carbon dioxide converted per hour.

Fig. 6 .
Fig. 6.Stability and decay of aerosol-associated infectivity of influenza virus in a closed space and its inactivation by the photocatalytic air cleaner.Influenza virus was sprayed into the cubic space (779 L), and 45 L of air was serially transferred from the space to a bag at 0, 3, 7, 15, and 28 min.The infectivity ((a), (b)), numbers of viral RNA copies ((b), (c)), and the ratio of viral RNA copies to infectivity ((e), (f)) of the air in the bag were determined using a plaque assay and reverse transcriptase real-time polymerase chain reaction (RT-PCR), respectively. and  indicate values derived with and without the photocatalytic air cleaner, respectively.Two independent experimental results are shown in panels (a), (c), and (c) and panels (b), (d), and (f).The ratio of viral RNA copies per infectivity ranged from 85 to 100 RNA copies/plaqueforming unit and is expressed as 100% at time 0 in Figs.6(e) and 6(f).Dotted lines in Figs.(a), (b), (c), and (d) indicate the detection limit of RT-PCR.

Fig. 7 .
Fig.7.Degradation of infectious virus particles and RNA copies during photocatalytic treatment.Enveloped infectious influenza virus containing eight segmented RNA genomes was placed in a cubic space with 77-107 RNA copies/PFU (expressed as 100% at time 0) before photocatalysis, as shown in the left panel.Enveloped virus and RNA were eliminated by photocatalysis and less than 1% of infectious virus and RNA were detected at 3 min, as shown in the middle panel.The ratio of RNA copies/plaque-forming unit (PFU) at 3 min increased to 220% and 575% compared with that at time 0 as shown in Fig.6(e) and 6(f).The infectious virus was still detectable, and RNA larger than 116 bp of the M gene for RT-PCR was present in the space.The increased ratio of RNA copies/PFU with low infectivity indicates the destruction of enveloped virus particles and the presence of RNA larger 116 bp.The infectious virus and RNA were not detected in the space at 7 min, indicating complete elimination of virus particles and viral RNA, as shown in the right panel.