The Efficacy of Existing AirPurification Technologies in the

Dental Environment; A BriefLiterature Review.

Contents

1 Background 2

2 Types of purification technologies 52.1 Mechanical filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Ionisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.4 UVC Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 Methods 7

4 Results 94.1 A Review of Efficacy of Air Purification Technologies in Removing Particulate

Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.2 Review Of The Efficacy Of Antimicrobial Efficacy Of Air Purification Tech-

nologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

5 Discussion 13

6 Emerging Technologies and Adjunctive Technologies 146.1 Photocatalytic Oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.2 Cold plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.3 Zeolites with Metallic Silver Adsorbent Technology . . . . . . . . . . . . . . 14

7 Conclusions 15

References 16

1

1 Background

That several dental procedures generate aerosols is a fact established widely both amongstdental professionals and their patients. This cloud is often clearly visible when performinginterventions such as those involving the high speed handpiece or the ultrasonic scaler. Theaerosol visible consists of particulate matter and liquids aerosolised both from the dentalwater lines, and the site of treatment; making a more apt description of the resultant aerosola bioaerosol, which contains microorganisms and other organic substances such as blood andsaliva alongside dental material particulates.

Given the ubiquitous nature of bioaerosol production within dentistry, dental profession-als are understandably familiar with universal protection measures such as masks and visorsto prevent, at least to some extent, the inhalation of the bio aerosol generated during prac-tice, with the potential inhalation rate of bioaerosol when unimpeded by masks reported tobe as high as 0.12microlitres of aerosolised saliva in a 15-minute period (Bennett et al., 2000), and although the efficacy of the face masks routinely used in dentistry is contentious, withthe reduction in aerosol inhalation varying greatly between reports, it is clear that they havesome effect. In conjunction with visors and high-volume evacuation suction during the pro-cedure, the bioaerosol released into the environment or inhaled can be greatly minimalised(Micik et al., 1969) (Harrel and Molinari, 2004).

In their pioneering studies, Micik et al postulated that in the dental environment, weshould divide the general aerosol produced into true aerosols which are particles of a diameterof less than fifty micrometres in size, and ‘splatter’ which consists of larger particles. Thereis a wide consensus that when we consider the risk of airborne infection and inhalation ofparticulate contaminants in dentistry, it is mainly from the true aerosol particles, due to theirability to remain airborne for an extended period, reportedly up to thirty minutes (Hinds,1982), and the ability of the smaller aerosol particles within this subset - zero point five toten micrometres in size - to penetrate the respiratory tract.

With the widespread use of resin composites in dentistry, comes a new risk associatedwith dental aerosol. The sensitisation of dental professionals to methacrylates, the ma-jor constituent of resin composites, has been well documented (Schedle et al., 2007) (Syedet al., 2015), and though patients show few adverse effects, dental professionals due to theirincreased exposure to these materials have been shown to develop contact dermatitis andairway related disease (Hamann et al., 2005) (Piirila et al., 2002) (Jaakkola et al., 2007), thelatter being the focus here. One vector for inhalation of particle associated methacryclatesis in the respirable ‘dust’ (Cokic et al., 2020) aerosolised by dental procedures, and parts ofthis fall into the size category of <10 micrometres, which allows them to penetrate deeplyinto the respiratory system (Nilsen et al., 2019). This particulate matter, if inhaled, notonly has the potential to cause some airway diseases, such as adult onset asthma (Jaakkolaet al., 2007), but in an in an alarming in vitro study on the effects of the respirable fraction ofcomposite dust on human bronchial cells showed that even in sub-toxic levels, the particulatematter was capable of causing genotoxic changes (Cokic et al., 2020).

2

The spread of the ‘splatter’ also becomes a matter of concern given the advent of infectiousdiseases such as SARS, and the resurgence of tuberculosis in some areas, as these diseases areusually spread via droplet nuclei. These droplet nuclei are larger than the aerosol particlesand behave in a ballistic manner, meaning that they are ejected with a greater momentumthan aerosols from the site of propagation, and continue until they contact a surface orfall to the floor (Micik et al., 1969) (Miller and Micik, 1978) (Miller et al., 1971) . Thisbehaviour is preferable to aerosol suspension as surfaces are consistently cleaned betweenpatients in a dental environment, unfortunately, in the case of droplet nuclei, as the dropletstarts to evaporate its constituent microorganisms gain the ability to remain airborne asaerosols (Harrel and Molinari, 2004).

That the bioaerosol generated in dental procedures, which potentially contains infectiousmicroorganisms or contaminants such as methacrylate particulates, is able to remain airbornefor up to 30 minutes is alarming considering that many dental practitioners remove theirmasks following a procedure to converse with their patient, exposing themselves to airbornepathogens and adverse respiratory effects. There is also the risk of this potentially infectiousaerosol entering ventilation systems and therefore spreading to ‘clean’ zones and areas wherepersonal protection is not used.

A published example of a virus spreading throughout a medical practice via the venti-lation system was seen when a 12-year-old boy with measles was reported coughing in oneroom, and several people who had no close contact with the patient, and had in fact neverbeen in the same room as him, developed measles (Bloch et al 1985). Coughing, as seen inthis case, and sneezing are common methods of aerolisation of pathogens and are reportedto be a common method of transmission of several illnesses, including SARS and influenza,as well as the common cold. Dental aerosol production in comparison, by virtue of the highspeed of the motor within the dental unit, and the duration of its use in certain procedures,has the capability to generate a far greater volume of aerosolised pathogen when treating aninfectious patient (Micik et al., 1969) and this risk to patients and practitioners should betaken very seriously.

Furthermore, although bodies such as the American dental association state that takingthe proper universal protections in personal protective equipment should eliminate muchof the danger of the immediate aerosol and splatter propagated by operative procedures, astudy in 1988 showed an elevated serology for legionella in dentists (Reinthaler et al., 1988).Legionella is a bacterium that is commonly found in dental unit water lines left stagnant, andgood practice in dentistry currently is frequent flushing of the dental unit water lines. Thesalient information we can take from this study is related to the fact that several studiesshow that legionella is a bio-aerosol dependent hazard to elderly patients and those withrespiratory difficulties (Zemouri et al., 2017). The increased level of exposure to dentalaerosols by the dental team suggests that presence of legionella in the aerosol would affectthem before the patients, and this was reflected in the elevated serological levels of antibodiesfor legionella in dentists, 11% compared to 5% in the general population (Reinthaler et al.,1988). The relevance here is the evidence in this study of bio- aerosol related spread ofinfectious disease in the dental environment despite the use of personal protective equipment.A more recent study found this serology to be inconsistent with dentists surveyed in London

3

in 2003, however, in this study in only one practice surveyed were they able to isolatelegionella in the water supply, and the serology was perhaps consistent with this (Pankhurstet al., 2003).

This suggests that with the advent of widespread resin composite use, and in light of thenovel threat of airborne respiratory diseases such as SARS and the coronavirus, in additionto the existing protective protocols in place in the dental environment, novel strategiesto minimise aerosol inhalation and spread should be adopted. A strategy that has beensuggested in previous literature on the subject of dental bioaerosol (Harrel and Molinari,2004), and subject of this literature review, is the use of air purification technologies in thedental environment to mitigate the risk of aerosol related spread.

4

2 Types of purification technologies

In this review, we will consider the comparative and synergistic efficacy of the following airpurification technologies, which are frequently used either in isolation or in combination inseveral commercially available air purifiers.

2.1 Mechanical filtration

This is the integral part of air purification which focusses on the removal of particulate matterfrom the air, and these filters can be subdivided by particle filtration efficiency and the sizeof particles removed. Commonly used filters are pre-filters which remove larger particles,and are usually used in conjunction with high efficiency particulate air (HEPA) filters. Thelatter are the most commonly used type of mechanical filtration method in commercial airpurifiers. True HEPA filters have a standardised efficiency of 99.7% (Schroth, 1996) rating forremoving particles equal to 0.3 micrometres and above, which should mean for every 10,000particles of 0.3 micrometres or larger only 3 should be able to pass unimpeded through thefilter. Rutala et al showed that when in room portable purifiers were used, 90% of particleswith a size of more than or equal to 0.3 micrometres were cleared within 5-8 minutes, a greatreduction from both the value quoted in Hinds’ study of 30 minutes (Hinds, 1982), and thecontrol value in this study in a non-ventilated chamber of 120 minutes (Rutala et al., 1995).

The mechanism of action of HEPA filters is threefold: larger particles, i.e. those of a sizeover 10 micrometres are filtered in the traditional sense, in that in they do not fit the holesin the filter’s fibre meshwork and so are directly intercepted. Inertial impaction is employedto remove particles between 0.5 and 10 micrometres, these include typical bacteria, and dueto their density, which is greater than that of air, they deviate from the normal laminarflow of air and adhere to the filter by virtue of their inertia. Particles much smaller thanthis, those less than around 0.1 micrometre behave differently, by merit of their reduced size,the effect of Brownian motion is far stronger on these particles and so their typical coursethrough air can be likened to a zigzag. Therefore, although they can fit through the poresin the filter, their non-laminar flow causes them to be caught on the fibres which constitutethe filter, this is known as diffusional interception. A study conducted at the university ofMinnesota showed that HEPA filters had a 99.99% efficacy when removing particles below5 nanometres. The reason that the focus is placed on particles 0.3 micrometres in size isbecause at this size, they are less susceptible to Brownian motion and inertial impaction,and so have the greatest potential to pass through the filter. A true HEPA filter musttherefore be tested using di-octyl phthalate to show 99.97% efficiency for these most difficultto capture particles.

When placed into the dental context, in theory these filters should remove most particu-late content, as well as most microbial content as bacteria tend to fall into the size bracketof 0.5 to 1.5 micrometre and viruses between 0.01 and 0.4 micrometres in size, although theHEPA filter does not in itself have any mechanism for sterilisation once these microorganismshave been captured.

5

Ultra-low particulate air (ULPA) filters may have an efficiency even higher than that ofthe HEPA filter, with reported values at 99.999% (Jamriska et al 1997), but these are notyet in widespread use.

2.2 Adsorption

This technology involves the use of solid adsorbent materials, such as the commonly usedactivated carbon, and is particularly suited to removing volatile organic compounds andother contaminants in the gaseous phase, which attach themselves to the surface of theadsorbent. However, its efficiency is variable and VOCs attach to the filter so over time itsefficiency reduces and the filter requires regular replacement. Humidity has also been shownto reduce the efficacy of carbon filters (Cabrera et al., 2014).

2.3 Ionisation

It has been shown that the emission of ions has a role in air purification (Lee et al 2004),ions are though to bind to airborne particles, thereby giving them an electrical charge,which should repel other similarly charged airborne particles, and encourage them to attachthemselves to indoor surfaces (Blackhall et al., 2012), where they can then be cleaned away inregular disinfection more effectively. This can be done by the release of negative or positiveions, both of which have shown some efficacy in reducing particulates in air. Negative iongenerators have caused some concern over their potential to generate ozone as a toxic byproduct.

2.4 UVC Irradiation

UV light can be divided into different categories based on its wavelength, with the wave-lengths of UVC waves ranging between 100-280 nm (Green and Scarpino, 2001). It is usedin purification units in the form of one or many UVC emitting mercury lamps, or LEDs,within the unit, and its mechanism of action is through the damage of the DNA or RNAof microorganisms such as viruses and bacteria, and its efficacy is largely dependent on theintensity of the light, the duration of exposure of the microorganism and the humidity of theroom. This UVC is usually used in conjunction with another filtration technique as it hasno effect on particulate matter.

6

3 Methods

Given that the particulate matter of concern in dental aerosols tends to be less than fiftymicrometres in size, due to their ability below this size to remain in airborne for up to thirtyminutes (Hinds, 1982), with respirable particles being between less than 10 micrometres insize, when evaluating the efficacy of air purification technologies, studies will be includedwhich detail the ability of each technology to effectively remove particulate matter of this sizefrom the air. Another consideration when evaluating efficacy in the dental environment is theantimicrobial properties of the purification technology, considering the potentially infectiousbio aerosol created during dental procedures. We may also consider the ability of thesetechnologies to remove volatile organic compounds from the air, these include malodourousconstituents of bioaerosol and are often present in dentistry due to the nature of someprocedures.

A particular difficulty encountered when conducting this literature review was the paucityof literature on the efficacy of air purification in the dental environment. However, as thepurpose of this review is to assess the efficacy of the purification technologies in essence onbio-aerosols, studies were included which assessed the technologies for removal of particulatematter, antimicrobial efficacy, and ability to remove malodourous and toxic VOCs.

Given the particular nature of the different filtration methods above and their differentmethods of action, they also have different targets, as shown generally in table 1 (Liu et al.,2017). Pollutants can be subdivided into three main categories, larger particulate mattersuch as respirable dust, VOCs such as ammonia and microorganisms such as bacteria andviruses. Respirable dust and microorganisms are most pressing in the dental environment.

It would follow that certain types of filters are used for removal of particulates and othershave primarily antimicrobial effects, and reviewing the efficacy of each for both functionsis beyond the scope of this review on dental efficacy. More in depth reviews have beenconducted on these topics (Wang and Zhang, 2011) (Bahri and Haghighat, 2014) (Luengaset al., 2015) (Zhong and Haghighat, 2015) (Siegel, 2016).

7

In this review, we will focus on HEPA filters and ionising purification technologies andtheir efficacy in removal of particulate matter. For their reported antimicrobial effects, wereviewed ionising purification systems and UVC irradiation

Studies were reviewed which assessed both clean air delivery rate (CADR) and first passefficacy. 36 studies were returned in various literature searches and of these 15 were retained.

8

4 Results

n.b. Although adsorption methods such as activated carbon filters do not remove particulatesor have anti-microbial effects in any notable way, and are not the focus of this review theyhave been shown to have great efficacy in removing volatile organic compounds (VOCs)which can cause unpleasant odours in a dental setting. This system has been shown to havean efficacy of 70-80% by (Sidheswaran et al., 2012) and 90% by (Jo and Yang, 2009)). Thiscould be a useful adjunct to any major purification technology.

4.1 A Review of Efficacy of Air Purification Technologies in Re-moving Particulate Matter

9

10

4.2 Review Of The Efficacy Of Antimicrobial Efficacy Of Air Pu-rification Technologies

11

12

5 Discussion

HEPA filtration systems showed consistently increased efficacy independent of ambient con-ditions when removing particulate matter of all sizes in comparison to ion generating purifi-cation systems, however, values were generally lower than those quoted by manufacturerswhich suggests not all HEPA filters are ‘true’.

UVC irradiation shows consistently higher antimicrobial efficacy when compared to iongenerating systems. Both systems are affected by room humidity and rate of airflow, andboth have a diminishing effect on efficacy. There is a suggestion that increased humidityis protective in some way for microorganisms. In light of this, it could be postulated thatthe adjunctive use of a dehumidifier or desiccant in a purifier which contains either of thesebiocidal technologies could improve efficacy.

In the current climate, the efficacy of these technologies on the novel coronavirus is likelyto be a question of some urgency, however, there is little in the literature on this subject.However, UVC irradiation is in use in sterilisation of N95 masks and has shown efficacy inSARS, a virus very similar in structure to COVID-19.

13

6 Emerging Technologies and Adjunctive Technologies

6.1 Photocatalytic Oxidation

Photocatalytic oxidation is the process of promoting, at ambient temperatures, the stepwisefree radical decomposition of several organic compounds in a stepwise reaction and producingwater and carbon dioxide as by products. UV light is used on a semi-conductor, oftentitanium dioxide (Zhong and Haghighat, 2015) (Huang et al., 2016). The appeal of thistechnology is clear, as then use of photocatalysis as s viable means of purifying air and waterhas long been considered. In theory, this technology requires little maintenance and are costeffective (Shiraishi and Ishimatsu, 2009), and it has also been shown to have an antiviraland bacterial effect. However, this process has been shown to produce some undesirableproducts which can deactivate the catalyst, making it ineffective over longer periods of time.Performance is also usually less than other methods of filtration. However, tests outside oflab conditions have yielded greatly different results and suggest that perhaps the technologyis not ready to be used in isolation, but could be useful as an adjunctive method of filtrationonce the by products are better understood (Hay et al., 2015).

6.2 Cold plasma

This method of purification relies on the use of high voltages to eliminate particulate matterand microorganisms using redox reactions and precipitation. There have been reports of anefficacy of up to 95% (Liang et al., 2012) in the elimination of bacterial and fungal species.However, this is not yet commonly found in commercial units due to poor energy efficiencyand the formation of nitrous oxides and ozone (Mista and Kacprzyk, 2008).

6.3 Zeolites with Metallic Silver Adsorbent Technology

(Cheng et al., 2012) impregnated zeolite with metallic silver and found that it had an an-timicrobial efficacy of up to 95% in two hours and around 90% at one hour.

14

7 Conclusions

Given the specific concerns with airborne contaminants, it follows that the purification tech-nology most effective in the dental environment is the one which is most efficient in removingparticulate matter from at least zero point five micrometres and larger from the air, and whichhas an antimicrobial effect to reduce the bioaerosol effect generated by dental procedures.

In the context of this, the combination of HEPA filters and UVC technologies seemsto be the best synergistic technology to improve safety in the practice of dentistry. TheHEPA filter was shown in all studies to be effective in removing the largest proportionof particulate matter from the air and greatly reducing the time interval in which thesecontaminants remain airborne (Rutala et al., 1995). It has also shown efficacy in removingmicro-organisms from the air, however it is not able to inactivate them and this could leadto a risk of accumulation of bacterial colony forming units and viruses on the filter andredistribution. In combination with UVC irradiation, HEPA filtration is able to eradicatemicroorganisms which pass through the unit. However, neither is effective in the removal ofVOCs such as odours from the air which can be unpleasant for both practitioner and patient,the addition of a carbon filter to this base technology could resolve this issue.

Notably, one study found that the use of a ULPA filter in conjunction with UVC had a99.9995% efficacy in removing contaminants from the air, which could be an area for furtherexploration (Marier and Nelson, 1993). Another area of emerging interest is the increased useof nanotechnologies in filtration which appears to show promising results in current trials,and photo catalysis using titanium dioxide, which has shown efficacy in removing VCOs butis as yet no commonly used in indoor air purification.

A clear finding of this review was the lack of literature focusing on the effects of airpurification in reducing the risk of respiratory disease and spread of airborne pathogens,although it has been identified as a possible solution (Harrel and Molinari, 2004); given theaerosolising nature of dental procedures and the occupational risk of respiratory ailmentsfrom inhalation of these aerosols, this is an area into which there is a clinical need for furtherexploration.

15

References

AM Bennett, MR Fulford, JT Walker, DJ Bradshaw, MV Martin, and PD Marsh. Microbialaerosols in general dental practice. British dental journal, 189(12):664–667, 2000.

Rudolph E Micik, Robert L Miller, Maurice A Mazzarella, and Gunnar Ryge. Studies ondental aerobiology: I. bacterial aerosols generated during dental procedures. Journal ofdental research, 48(1):49–56, 1969.

Stephen K Harrel and John Molinari. Aerosols and splatter in dentistry: a brief reviewof the literature and infection control implications. The Journal of the American DentalAssociation, 135(4):429–437, 2004.

William C Hinds. Filtration, aerosol technology. 1982.

Andreas Schedle, Ulf Ortengren, Nicole Eidler, Michael Gabauer, and Arne Hensten. Doadverse effects of dental materials exist? what are the consequences, and how can they bediagnosed and treated? Clinical oral implants research, 18:232–256, 2007.

Meena Syed, Radhika Chopra, and Vinod Sachdev. Allergic reactions to dental materials-asystematic review. Journal of clinical and diagnostic research: JCDR, 9(10):ZE04, 2015.

Curtis P Hamann, Louis G DePaola, and Pamela A Rodgers. Occupation-related allergiesin dentistry. The Journal of the American Dental Association, 136(4):500–510, 2005.

Paivi Piirila, Ulla Hodgson, Tuula Estlander, Helena Keskinen, Anja Saalo, Risto Vouti-lainen, and Lasse Kanerva. Occupational respiratory hypersensitivity in dental personnel.International archives of occupational and environmental health, 75(4):209–216, 2002.

MS Jaakkola, T Leino, L Tammilehto, Pekka Ylostalo, E Kuosma, and Kd Alanko. Respira-tory effects of exposure to methacrylates among dental assistants. Allergy, 62(6):648–654,2007.

Stevan M Cokic, Manosij Ghosh, Peter Hoet, Lode Godderis, Bart Van Meerbeek, andKirsten L Van Landuyt. Cytotoxic and genotoxic potential of respirable fraction of com-posite dust on human bronchial cells. Dental Materials, 36(2):270–283, 2020.

Bo W Nilsen, Einar Jensen, Ulf Ortengren, Berit Bang, and Vibeke B Michelsen. Airborneexposure to gaseous and particle-associated organic substances in resin-based dental ma-terials during restorative procedures. European journal of oral sciences, 127(5):425–434,2019.

RL Miller and RE Micik. Air pollution and its control in the dental office. Dental Clinics ofNorth America, 22(3):453–476, 1978.

Robert L Miller, Rudolph E Micik, Cabot Abel, and Gunnar Ryge. Studies on dental aero-biology: Ii. microbial splatter discharged from the oral cavity of dental patients. Journalof dental research, 50(3):621–625, 1971.

16

FF Reinthaler, F Mascher, and D Stunzner. Serological examinations for antibodies againstlegionella species in dental personnel. Journal of Dental Research, 67(6):942–943, 1988.

Charifa Zemouri, Hans de Soet, Wim Crielaard, and Alexa Laheij. A scoping review onbio-aerosols in healthcare and the dental environment. PloS one, 12(5), 2017.

CL Pankhurst, W Coulter, JJ Philpott-Howard, T Harrison, F Warburton, S Platt, S Sur-man, and S Challacombe. Prevalence of legionella waterline contamination and legionellapneumophila antibodies in general dental practitioners in london and rural northern ire-land. British dental journal, 195(10):591–594, 2003.

T Schroth. New hepa/ulpa filters for clean-room technology. Filtration & separation, 33(3):245–244, 1996.

William A Rutala, Suzanne M Jones, John M Worthmgton, Parker C Reist, and David JWeber. Efficacy of portable filtration units in reducing aerosolized particles in the sizerange of mycobacterium tuberculosis. Infection Control & Hospital Epidemiology, 16(7):391–398, 1995.

Karen Blackhall, Sarah Appleton, and Christopher J Cates. Ionisers for chronic asthma.Cochrane Database of Systematic Reviews, (9), 2012.

Christopher F Green and Pasquale V Scarpino. The use of ultraviolet germicidal irradiation(uvgi) in disinfection of airborne bacteria. Environmental engineering and policy, 3(1):101–107, 2001.

Guoliang Liu, Manxuan Xiao, Xingxing Zhang, Csilla Gal, Xiangjie Chen, Lin Liu, SongPan, Jinshun Wu, Llewellyn Tang, and Derek Clements-Croome. A review of air filtrationtechnologies for sustainable and healthy building ventilation. Sustainable cities and society,32:375–396, 2017.

Zhiqiang Wang and Jensen S Zhang. Characterization and performance evaluation of a full-scale activated carbon-based dynamic botanical air filtration system for improving indoorair quality. Building and Environment, 46(3):758–768, 2011.

Mitra Bahri and Fariborz Haghighat. Plasma-b ased indoor air cleaning technologies: Thestate of the art-r eview. CLEAN–Soil, Air, Water, 42(12):1667–1680, 2014.

Angela Luengas, Astrid Barona, Cecile Hort, Gorka Gallastegui, Vincent Platel, and AnaElias. A review of indoor air treatment technologies. Reviews in Environmental Scienceand Bio/Technology, 14(3):499–522, 2015.

Lexuan Zhong and Fariborz Haghighat. Photocatalytic air cleaners and materialstechnologies–abilities and limitations. Building and Environment, 91:191–203, 2015.

JA Siegel. Primary and secondary consequences of indoor air cleaners. Indoor air, 26(1):88–96, 2016.

17

Meera A Sidheswaran, Hugo Destaillats, Douglas P Sullivan, Sebastian Cohn, and William JFisk. Energy efficient indoor voc air cleaning with activated carbon fiber (acf) filters.Building and Environment, 47:357–367, 2012.

Wan-Kuen Jo and Chang-Hee Yang. Granular-activated carbon adsorption followed byannular-type photocatalytic system for control of indoor aromatic compounds. Separa-tion and purification technology, 66(3):438–442, 2009.

Yu Huang, Steven Sai Hang Ho, Yanfeng Lu, Ruiyuan Niu, Lifeng Xu, Junji Cao, andShuncheng Lee. Removal of indoor volatile organic compounds via photocatalytic oxida-tion: a short review and prospect. Molecules, 21(1):56, 2016.

Fumihide Shiraishi and Takaaki Ishimatsu. Toluene removal from indoor air using a miniatur-ized photocatalytic air purifier including a preceding adsorption/desorption unit. Chemicalengineering science, 64(10):2466–2472, 2009.

Stephen O Hay, Timothy Obee, Zhu Luo, Ting Jiang, Yongtao Meng, Junkai He, Steven CMurphy, and Steven Suib. The viability of photocatalysis for air purification. Molecules,20(1):1319–1356, 2015.

Yongdong Liang, Yan Wu, Ke Sun, Qi Chen, Fangxia Shen, Jue Zhang, Maosheng Yao, TongZhu, and Jing Fang. Rapid inactivation of biological species in the air using atmosphericpressure nonthermal plasma. Environmental science & technology, 46(6):3360–3368, 2012.

W Mista and R Kacprzyk. Decomposition of toluene using non-thermal plasma reactor atroom temperature. Catalysis Today, 137(2-4):345–349, 2008.

Hsin-Han Cheng, Chu-Chin Hsieh, and Chia-Hsi Tsai. Antibacterial and regenerated char-acteristics of ag-zeolite for removing bioaerosols in indoor environment. Aerosol and airquality research, 12(3):409–419, 2012.

Robert L Marier and Tim Nelson. A ventilation-filtration unit for respiratory isolation.Infection Control & Hospital Epidemiology, 14(12):700–705, 1993.

18