Photocatalytic Oxidizers as a Novel Device for Bacterial Reduction in Dental Surgical Bays – Dr. Margaret Scarlett and colleagues

Abstract

Background
The COVID-19 pandemic has underscored the need to reduce air and surface pathogens in publicly occupied indoor spaces. This study evaluates the effects of an advanced photocatalytic (APC) air purification system on both the air and surface microbial burden in a dental school oral surgery department.

Methods
Samples were collected from two operatories, where aerosol generating procedures were regularly performed, for three consecutive days prior to use of APC units, and for three consecutive days two weeks after a CASPR Compact APC unit was installed and running continuously in each room. Total aerobic bacterial counts were measured in the air and on surfaces using agar settling plates (n = 36) and cellulose environmental testing sponges (n = 60).

Results
The APC units yielded a 76% reduction (p = 0.0015; 95% confidence interval, 17 to 529 CFUs) in the aerobic bacterial count measured on surfaces, and a 58% reduction (p = 0.0012; 95% confidence interval, 35 to 95 CFUs) in the aerobic bacterial count measured in the air.

Conclusions
The APC air purification system provides a promising solution to reduce air and surface microbial burden in dental environments, where aerosol generating procedures are regularly performed.

Practical Implications
Dental clinics produce high amounts of bioaerosols and likely present higher risks for disease transmission. Implementing APC air purification technology may help to mitigate risk.

Key Words
Photocatalytic Oxidizers, PCO, CASPR, Aerosols, COVID-19, SARS-CoV-2, Dental Clinic, Oral Surgery


Introduction

Since the 1960s and 1970s, environmental microbiologists have measured the distribution and content of fluid particles from the mouths of patients undergoing dental procedures. This early research reported contamination from air-turbine handpieces, air-water syringes and other equipment, and recommended dentists use protective shields and glasses to reduce contamination.^1-4

The spread of COVID-19, caused by the virus SARS-CoV-2, has reawakened interest in the respiratory microbial burden of ambient air in general, and contamination from aerosol generating dental procedures (AGDPs) in particular. SARS‐CoV‐2 has been shown to remain infectious in aerosols for hours, and on surfaces for days.^5,6 In one laboratory study, viruses in aerosols remained viable for up to 16 hours.⁷ Wide dispersal of viral load has been documented in hospital and isolation units, contaminating nearly 100% of floors under beds and 80% of airducts.^8,9 While the exact risk of AGDPs to dental personnel and patients is not known, dental aerosols represent an undeniable inhalation risk for SARS‐CoV‐2, as well as other respiratory pathogens. These include tuberculosis, influenza, measles and mumps.¹⁰

In September 2020, an evaluation of AGDPs noted that bacterial contamination occurs some distance from the field of dental operation, namely, 360 cm for air turbine handpiece operating at 400000 rpms, 300 cm for contra-angle handpiece, and 240 cm for ultrasonic scaler. It should be noted that the authors concluded that “No surface of the operative environment was free from the tracer after the use of the air turbine.”¹¹

Various ventilation, filtration and de-densifying methods have been recommended by different bodies to prevent transmission of respiratory pathogens in buildings and hospitals. Many are based on theoretical models or formulas, rather than empirical testing.^12-19 This study evaluates the efficacy of an advanced photocatalytic (APC) air purification system (Figure 1) on bacterial presence in air and on surfaces.

Figure 1: CASPR Compact unit

Photocatalytic oxidation (PCO), first patented for air cleaning by NASA in 1977,²⁰ uses UV light and a catalytic surface to oxidize gaseous molecules in the air. Many of the reaction products are highly reactive radicals which can combine to form hydrogen peroxide (H2O2) and Ozone (O3).^{21, 22} These oxidizing species can then react with viral envelopes and bacterial cell membranes, leading to death of the pathogen.^{22, 23} Since the technology was first developed, catalytic processes with greater efficiency have been developed and tested in hospital settings, reducing numbers of multidrug-resistant bacteria and decreasing hospital absenteeism caused by healthcare-associated infections.^24-26 Although these systems have been proven effective in hospital settings, dental offices have lower ventilation regulations and more frequent AGDPs. For this reason, is important to evaluate the effects of an APC system in a real-life dental environment. Here we evaluate the effects using a CASPR Compact stand-alone unit. This APC system was chosen because of its ability to alternate between low-ozone and non-ozone producing modes for continued and safe occupation of surgical rooms.

Methods

Set-up, Site Selection, and Schedule
Testing took place in the enclosed oral surgery operatories no. 4 and no. 5 at the Texas A&M University College of Dentistry between November 17 and December 4, 2020. Both surface and air sampling were completed using the methods described below. The two operatories (surgical bays) are 3.8 x 4.2 m2 (49 m3) and 3.8 x 4.1 m2 (48 m3), respectively. The rooms have forced air ventilation at a rate of 14 air exchanges per hour.

A week prior to the start of testing, surfaces were identified using ATP surface testing (UltraSnap Surface ATP Test with SystemSURE Plus luminometer, Hygiena) in areas that were unlikely to have regular direct contact or cleaning. This was to ensure consistent and representative samples were collected. The upper side of the bottom base portion of medical carts positioned next to the oral surgery patient chairs were selected as the testing surfaces (Figure 2).

Figure 2: Medical Cart (orange arrow indicated area of surface testing)

The APC units were placed in each operatory on top of built-in cabinetry at a height of 2.3 m from the floor with the airflow directed horizontally in the direction of the patient’s chair and the medical cart identified for surface sampling. Space was allocated next to the units for the settling plates used for passive microbial air sampling. The medical carts were located at a distance of 2.6 m in room 4, and 2.5 m in room 5, from the APC units. Settling plates were placed on the built-in cabinetry between 20 cm and 60 cm adjacent to the APC units, at a location not in the path of the air flow induced by the APC system.

Baseline testing (No CASPR) was completed for three consecutive days of regular surgical occupancy to allow for adequate data collection in case of random daily variation or contamination of the testing sites. The rooms were used by oral surgery staff and residents throughout the day for procedures such as wisdom tooth removal, dental implant placement, and other dentoalveolar surgeries. Surface testing was performed after the post-surgical cleaning of the last surgery of each day, but prior to the regular over-night cleaning of the building, which includes the surgical rooms. Cleaning involved routine wiping down of surfaces and chairs with disinfectant after every patient and procedure. At the end of the day, floors are vacuumed, and surfaces are once again disinfected. Settling plates were placed at the time of surface testing and collected 24 hours later. After baseline testing, the APC units were placed in the operatories and set to HIGH mode with a room size indication on the display of the unit of 250 ft2 and a fan speed of 50%. HIGH mode produces trace amounts of ozone. The units were operated on this setting for two days, over the weekend, to saturate the testing environment. After 48 hours, the units were switched to normal PCO (non-ozone producing) mode, with a fan speed of 67%. The units ran in this mode for the remainder of the testing. Follow-up testing (CASPR) was performed in the same manner as the baseline testing (No CASPR) two weeks after the start of baseline testing, also for three consecutive days of regular surgical occupancy. As during baseline testing, on the days of follow-up testing oral surgery staff and residents used the rooms throughout the day for typical dentoalveolar surgeries.

All 60 sponge test samples and 36 agar settling plates were sent to an accredited third-party microbiological laboratory for incubation and evaluation (Eurofins, Desoto, TX). The results were reported as a measurement of the total number aerobic bacterial colony forming units (CFUs). Individual bacterial strains were not differentiated or determined.

Surface Testing
Using the guidelines described above for set-up, site selection and schedule, five samples were collected on each of the collection days in each surgical bay (Table 1). Sampling was completed in a grid to ensure that the different sampling areas did not overlap (Table 1, Figure 3). Sponge environmental surface samples were collected using 3M cellulose sponges in accordance with NIOSH guidelines.²⁷ Due to the limited space, surface swabbing area was reduced to 21 cm2 to avoid overlap in samples. Samples were taken at a similar time (about 5 PM local time) each day for consistency. Processing of samples was performed using sponge AOAC 966.23 methods.²⁸ Two negative controls were also collected to evaluate the quality/validity of the results. One such control was obtained by wiping the surface with a disinfecting wipe, then sampling. The other, by measuring CFU units of a sponge not exposed to any surfaces.

Summary of surface testing.
Sampling was performed concurrently in each surgical bay with surface sponge testing.

Figure 3: Grid visual of surface sponge testing on medical cart. Surface area: 28 cm x 46 cm. Measurement dates are indicated under Table 1.

Air Sampling
Using the guidelines described above for set-up, site selection and schedule, air sampling was performed in each of the surgical rooms using passive air sampling methods established by the EPA.²⁹ Three standard methods agar settling plates were collected per operatory during each collection day (Table 2). A control negative was also sent in for each sampling period to evaluate the quality/validity of the results by measuring CFU units for sedimentation plates not exposed to the air. Processing of samples was performed using BAM Chapter 3 methods.³⁰

Summary of air testing (per surgical bay).

Hydrogen peroxide (H2O2) and ozone (O3) levels were monitored during testing using Dräger gas monitoring tubes (0.1-3 ppm Hydrogen Peroxide Dräger tube and 0.05-1.4 ppm Ozone Dräger tube, Dräger) to ensure proper use of APC units and safety of healthcare personnel and patients.

Results

Surfaces
As described above, surfaces were tested using sponge environmental testing methods. The data from sponge surface testing between each day and each surgical bay was compared within each group (No CASPR or CASPR) using a pairwise Wilcoxon rank sum test. No significant difference was found between test days or locations of the same group. For this reason, data was pooled between each of the three test days, and two operatories, before operating the APC unit (No CASPR) and the three days after the APC unit was turned on (CASPR). Results yielded an average (mean) of 273 CFUs (n = 30; 95% confidence interval, 17 to 529) before operating the APC unit and an average (mean) of 65 CFUs (n = 30; 95% confidence interval, 35 to 95) while operating the APC unit (Figure 4), giving an average reduction of 76%. The data did not follow assumptions of normality of equality of variance, so a non-parametric Mann-Whitney U-test was performed yielding a p-value of 0.0015. The negative controls were <10 CFUs, validating the quality of the results. Subsequently, measurements less than 10 CFUs were counted as zero. Note that none of the outliers were removed since they are indicative of a real-life surface, and the large range in bioburden that can exist in slightly different locations or times.

Figure 4: Surface pathogen reduction measured in CFUs before and after operating PCO. Here (A) shows all data points, where (B) is a scaled representation for easier viewing.

Air Sampling
Air sampling was completed as described above. One of the samples was damaged and not included in the analysis. The data from the settling plates between each day and each surgical bay was compared within each group (No CASPR or CASPR) using a pairwise Wilcoxon rank sum test. No significant difference was found between test days or locations of the same group. For this reason, data was pooled between the three test days, and two operatories for each treatment group. Results yielded an average of 24 CFUs (n = 18; 95% confidence interval, 15 to 33) before operating the APC unit and 10 CFUs (n = 17; 95% confidence interval, 8 to 12) while operating the APC unit (Figure 5). This gave an average reduction of 58%. The data did not follow assumptions of normality of equality of variance, so a non-parametric Mann-Whitney U-test was performed yielding a p-value of 0.0012. The negative controls showed <1 CFUs, validating the quality of testing.

Figure 5: Air pathogen reduction measured in CFUs before and after operating PCO.

Results from Dräger tubes showed safe operating levels throughout the experiment. Concentrations of 0.05 ppm O3 and <0.10 ppm H2O2 were measured for the first 48 hours of operation. During this period, the room was unoccupied. The APC units ran on HIGH mode, with a room size indication of 250 ft2, and the fan speed of 67%. Measurements were taken in the center of the rooms (~2.0 m from units). After 48 hours, APC units were set to normal PCO mode with the fan running at level 67%. Concentrations of 0.00 ppm were measured for both O3 and H2O2 throughout this period of time (4 measurement points over two weeks).

Discussion

The APC system showed a promising reduction in the bacterial burden in the air and on surfaces, which may have implications for reducing risks for dental personnel and patients. Sponge surface testing showed a 76% in measured CFUs, with a significance value of p<0.01. This means that we can say with 99% confidence that there was a statistically significant difference in the measured values. Conversely, data collected from the settling plates yielded a 58% reduction in measured CFU values with p<0.01. Undetectable concentrations of H2O2 and O3 in the normal non-ozone producing PCO mode, and measured values of 0.05 ppm O3 and <0.10 ppm H2O2 for optional HIGH mode (when unoccupied), provide evidence for the safe operations of the APC units in occupied spaces.^31-32 Although the levels of H2O2 and O3 are at almost undetectable concentrations, results from this study provide evidence that they are still able to significantly reduce bioburden in air and on surfaces, most likely through an oxidative membrane process previously described in other publications.^22,23

The recognition of the role of indoor air and aerosols on the transmission of SARS-CoV-2 was established by mid-2020, when it was shown that virus particles travel by air many meters from a source patient.^33-35 Respiratory droplets and aerosols from patients range from 0.1 to 1000 µm36, and airflow, air currents and the movements of people in and out of the operatory may carry aerosols long distances from the site of a dental procedure. All of these may impact the trajectory of particles within the air.³⁷ Various parameters affect aerosol movement, including initial velocity and Stokes law. The fact that handpieces operate at 200000 rpms for electric models, or 400000 rpm for air turbine models, means that the initial velocity and acceleration of any virus particles is quite high.

Complex airflow dynamics, turbulence, evaporation and settling impact indoor air quality when performing AGDPs on patients.³⁸ Generally, ventilation is believed to be a key factor in mitigating safety, in that improved ventilation helps in dilution of airborne particles. Ventilation with room air exchange of 14 for the study site in this oral surgery clinic is believed to be much higher than most dental offices. Physical characteristics of dental offices which influence ventilation patterns highly variable, as offices are often in converted homes, or in commercial buildings not originally designed as health care facilities. The room air exchange rates in this study are likely to be far higher than in most dental facilities. Additional research is needed to assess additional benefits of this system on air cleansing in less-than-optimal ventilation systems and within a variety of dental offices.

The test results evidence a significant reduction in the bacterial burden in the air and on surfaces following the utilization of the APC system in oral surgery operatories, indicating a further reduced risk for the dental professionals, patients and bystanders. Optimizing engineering controls after aerosol-generating procedures is recommended in current CDC guidance for dental settings. In addition to HEPA filtration, this system may be considered as an adjunct air cleansing method after achieving proper ventilation and filtration.¹²

Conclusion

The results of this study indicate that the implementation of the CASPR Compact APC air purification technology can improve indoor air quality and a higher surface disinfection rate through continuously de-densifying the pathogenic burden in the air and on surfaces.

__________

Justin Bernstein
Environmental Testing Manager, 2C MedTech
Dr. Likith Reddy
Clinical Professor and Department Head, Oral and Maxillofacial Surgery, Texas A&M College of Dentistry
Dr. Margaret Scarlett
Infectious and Chronic Disease Prevention Specialist and Corresponding Author
T: +1 404 808 9980
mscarlett@scarlettconsulting.com

_____________________________________________________________________________

References

1. Miller RL, Micik RE, Abel C, Ryge, G. Studies on Aerobiology: II microbial Splatter Discharged rom the Oral Cavity of Dental Patients. JDR. 1971 50: 3:621-625.
2. Micik RS, Miller Rl, Mazzarella MA and Ryge G. Studies on Dental Aerosobiology: I. Bacterial Aerosols Generated During Dental Procedures. JDR 1969. 48: 29-56.
3. Traveglini, EA and Larato DC.: A Disposable Face Mask with a Plastic Eye Shield for Operating with the Air Turbine Drill. J Proth Dent 1965. 15:525-525.
4. Caldarone CV: A Protective Shield for High Speed Equipment. J Proth Dent 1966. 47: 583-584.
5. van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, Tamin A, Harcourt JL, Thornburg NJ, Gerber SI, Lloyd-Smith JO, de Wit E, Munster VJ. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N Engl J Med. 2020 Apr 16;382(16):1564-1567. doi: 10.1056/NEJMc2004973. Epub 2020 Mar 17. PMID: 32182409; PMCID: PMC7121658.
6. Ong SWX, Tan YK, Chia PY, et al. Air, Surface Environmental, and Personal Protective Equipment Contamination by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) From a Symptomatic Patient JAMA. Apr 28; 323(16), 1610-1612. doi:10.1001/jama.2020.3227
7. Fears SC, Klimstra WB, Duprex P, Hartman A, Weaver SC, Plante KS, et al. Persistence of severe acute respiratory syndrome coronavirus 2 in aerosol suspensions. Emerg Infect Dis. 2020 Sep.26(9); 2168-2171. Epub 2020 Jun 22. https://doi.org/10.3201/eid2609.201806
8. Guo ZD, Wang ZY, Zhang SF, et al. Aerosol and Surface Distribution of Severe Acute Respiratory Syndrome Coronavirus 2 in Hospital Wards, Wuhan, China, 2020 [published online ahead of print, 2020 Apr 10]. Emerg Infect Dis. 2020;26(7):10.3201/eid2607.200885. PMID: 32275497
9. Santarpia JL, Rivera DN, et al. Transmission Potential of SARS-CoV-2 in Viral Shedding Observed at the University of Nebraska Medical Center. 2020. Preprint here.
10. Tellier R, Li Y, Cowling BJ, Tang JW. Recognition of aerosol transmission of infectious agents: a commentary. BMC Infect Dis. 2019 Jan 31;19(1):101. doi: 10.1186/s12879-019-3707-y. PMID: 30704406
11. Ionescu AC, Cagetti MG, Ferracane JL, Garcia-Godoy F, Brambilla E. Topographic aspects of airborne contamination caused by the use of dental handpieces in the operative environment, JADA 2020. 151 (9): 660-667. ISSN 0002-8177, https://doi.org/10.1016/j.adaj.2020.06.002.
12. Centers for Disease Control and Prevention (CDC), Sehulster LM, Chinn RYW, Arduino MJ, Carpenter J, Donlan R, Ashford D, Besser R, Fields B, McNeil MM, Whitney C, Wong S, Juranek D, Cleveland J. Guidelines for environmental infection control in health-care facilities. Recommendations from CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC). Chicago IL; American Society for Healthcare Engineering/American Hospital Association; 2004.
13. Endo A. Centre for the Mathematical Modelling of Infectious Diseases COVID-19 Working Group, Abbott S, Kucharski AJ, Funk S. Estimating the overdispersion in COVID-19 transmission using outbreak sizes outside China. Wellcome Open Res. 2020 Jul 10;5:67. doi: 10.12688/wellcomeopenres.15842.3. PMID: 32685698; PMCID: PMC7338915.
14. Jayaweera M, Perera H, Gunawardana B, Manatunge J. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environ Res. 2020 Sep;188:109819. doi: 10.1016/j.envres.2020.109819. Epub 2020 Jun 13. PMID: 32569870
15. Li Y, Huang X, Yu IT, Wong TW, Qian H. Role of air distribution in SARS transmission during the largest nosocomial outbreak in Hong Kong. Indoor Air. 2005 Apr;15(2):83-95. doi: 10.1111/j.1600-0668.2004.00317.x. PMID: 15737151
16. Li Y, Leung GM, Tang JW, Yang X, Chao CY, Lin JZ, Lu JW, Nielsen PV, Niu J, Qian H, Sleigh AC, Su HJ, Sundell J, Wong TW, Yuen PL. Role of ventilation in airborne transmission of infectious agents in the built environment – a multidisciplinary systematic review. Indoor Air. 2007 Feb;17(1):2-18. doi: 10.1111/j.1600-0668.2006.00445.x. PMID: 17257148
17. Li Y, Qian H, Hang J, et al. Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant. medRxiv 2020;2020.04.16.20067728.
18. Chen W, Zhang N, Wei J, Yen H, Li Y. Short-range airborne route dominates exposure of respiratory infection during close contact Building and Environment. 2020; 176:106859-.
19. REHVA. REHVA COVID-19 guidance document. How to operate HVAC and other building service systems to prevent the spread of the coronavirus (SARS-CoV-2) disease (COVID-19) in workplaces. November 17, 2020. Available at: https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID-19_guidance_document_V4_23112020_V2.pdf
20. NASA filed by Ryason Porter R. Solar Photoolysis of Water. 1977 US Patent: US4105517A
21. Venkatadri, R., & Peters, R. Chemical Oxidation Technologies: Ultraviolet Light/Hydrogen Peroxide, Fenton's Reagent, and Titanium Dioxide-Assisted Photocatalysis. Hazardous Waste and Hazardous Materials. 1993. 10(2), 107.
22. Foster HA, Ditta IB, Varghese S, Steele A. Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity. Appl Microbiol Biotechnol. 2011;90(6):1847-1868. doi:10.1007/s00253-011-3213-7
23. Wang, T., Libardo, M., Angeles-Boza, A., & Pellois, J. Membrane Oxidation in Cell Delivery and Cell Killing Applications. ACS Chemical Biology. 2017. 12(5), 1170-1182.
24. Nerandzic, Michelle M., Cadnum, Jennifer L., Pultz, Michael J., & Donskey, Curtis J. Evaluation of an automated ultraviolet radiation device for decontamination of Clostridium difficile and other healthcare-associated pathogens in hospital rooms. 2010. BMC Infectious Diseases, 10, 197.
25. Totaro, M., Casini, B., Profeti, S., Tuvo, B., Privitera, G., & Baggiani, A. Role of Hydrogen Peroxide Vapor (HPV) for the Disinfection of Hospital Surfaces Contaminated by Multiresistant Bacteria. 2020. Pathogens, 9(5), 408.
26. Inman, T., Chansolme, D. Evaluation of a Continuous Decontamination Technology in an Intensive Care Unit. 2020. Infection Control & Hospital Epidemiology, 41(Suppl 1), 519
27. Surface sampling procedures for Bacillus anthracis spores from smooth, non-porous surfaces. The National Institute of Health and Safety; 2012. “https://www.cdc.gov/niosh/topics/emres/surface-sampling-bacillus-anthracis.html#a” Accessed Jan. 8, 2021
28. Association of Official Analytical Chemists. 1990. Official Methods of Analysis, 15th ed. AOAC, Arlington, VA.
29. Passive Samplers for Investigations of Air Quality: Method Description, Implementation, and Comparison to Alternative Sampling Methods. Environmental Protection Agency; 2015. “https://nepis.epa.gov/Exe/ZyNET.exe/P100MK4Z.txt?ZyActionD=ZyDocument&Client=EPA&Index=2011%20Thru%202015&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&UseQField=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5CZYFILES%5CINDEX%20DATA%5C11THRU15%5CTXT%5C00000015%5CP100MK4Z.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=hpfr&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1” Accessed Jan. 8, 2021
30. Maturin L, Peeler J, Aerobic Plate Count. BAM 2001; ch. 3.
31. CARBA California Air Resources Board, California’s Air Cleaner Regulation (AB 2276); 2008
32. OSHA Occupational Safety and Health Department. Permissible Exposure Limits. United States Department of Labor. “https://www.osha.gov/annotated-pels” Accessed Jan. 8, 2021
33. Morawska L, Cao J, Airborne transmission of SARS-CoV-2: The world should face the reality. Environm Int. June 2020. 139,
34. Zhang R, Li Y, Zhang AL, Wang Y, Molina MJ. Identifying airborne transmission as the dominant route for the spread of COVID-19. Proc Natl Acad Sci U S A. 2020 Jun 30;117(26):14857-14863. doi: 10.1073/pnas.2009637117. Epub 2020 Jun 11. Erratum in: Proc Natl Acad Sci U S A. 2020 Oct 13;117(41):25942-25943. PMID: 32527856
35. Lednicky JA, Lauzardo M, Fan ZH, Jutla A, Tilly TB, Gangwar M, Usmani M, Shankar SN, Mohamed K, Eiguren-Fernandez A, Stephenson CJ, Alam M, Elbadry MA, Loeb JC, Subramaniam K, Waltzek TB, Cherabuddi K, Morris JG Jr, Wu CY. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. medRxiv [Preprint]. 2020 Aug 4:2020.08.03.20167395. doi: 10.1101/2020.08.03.20167395. Update in: Int J Infect Dis. 2020 Sep 16;: PMID: 32793914
36. Prather KA, Wang CC, Schooley RT. Reducing transmission of SARS-CoV-2 Science. 2020; 368(6498):1422-1424. Available here: https://science.sciencemag.org/content/368/6498/1422
    37. 37. Lipinski T, Ahmad D, Serey N, Jouhara H. Review of ventilation strategies to reduce the risk of disease transmission in high occupancy buildings International Journal of Thermofluids. 2020; 7-8:100045.
37. Nielsen, P. V., & Liu, L. (2020). The influence of air distribution on droplet infection and airborne cross infection. Department of Civil Engineering, Aalborg University. DCE Technical Memorandum No. 77 Available at: https://vbn.aau.dk/ws/portalfiles/portal/332256833/The_influence_of_air_distribution_on_droplet_infection_and_airborne_cross_infection.pdf

This submission is included in the JADA+ COVID-19 monograph as a Clinical Observation entry and has not been peer reviewed.

Stock photo credits: Jose A. Bernat Bacete/Moment/Getty Images