key: cord-0271326-u5dxglzb
authors: Allison, J. R.; Dowson, C.; Pickering, K.; Cervinskyte, G.; Durham, J.; Jakubovics, N.; Holliday, R.
title: Local Exhaust Ventilation to Control Dental Aerosols and Droplets
date: 2021-07-22
journal: nan
DOI: 10.1101/2021.07.21.21260917
sha: a8a01c1cf04511daf9a81b8a20fa0b8cb9d85061
doc_id: 271326
cord_uid: u5dxglzb

Dental procedures produce aerosols which may remain suspended and travel significant distances from the source. Dental aerosols and droplets contain oral microbes and there is therefore potential for major disruption to dental services during infectious disease outbreaks. One method to control hazardous aerosols often used in industry is Local Exhaust Ventilation (LEV). The aim of this study was to investigate the effect of LEV on aerosols and droplets produced during dental procedures. Experiments were conducted on dental mannequins in an 825.4 m3 open plan clinic, and a 49.3 m3 single surgery. 10-minute crown preparations were performed with an air-turbine handpiece in the open plan clinic, and 10-minute full mouth ultrasonic scaling in the single surgery. Fluorescein was added to instrument irrigation reservoirs as a tracer. In both settings, Optical Particle Counters (OPCs) were used to measure aerosol particles between 0.3 - 10.0 m and liquid cyclone air samplers were used to capture aerosolised fluorescein tracer. Additionally, in the open plan setting fluorescein tracer was captured by passive settling onto filter papers in the environment. Tracer was quantified fluorometrically. An LEV device with High Efficiency Particulate Air (HEPA) filtration and a flow rate of 5,000 L/min was used. LEV reduced aerosol production from the air-turbine handpiece by 90% within 0.5 m, and this was 99% for the ultrasonic scaler. OPC particle counts were substantially reduced for both procedures, and air-turbine settled droplet detection reduced by 95% within 0.5 m. The effect of LEV was substantially greater than suction alone for the air-turbine and was similar to the effect of suction for the ultrasonic scaler. LEV reduces aerosol and droplet contamination from dental procedures by at least 90% in the breathing zone of the operator and it is therefore a valuable tool to reduce the dispersion of dental aerosols.

Dental procedures produce aerosols and droplets containing microbes from the oral cavity (Meethil et al. 2021; Zemouri et al. 2020) . This is of particular relevance during infectious disease outbreaks, where concerns over dissemination of human pathogens (e.g., SARS-CoV-2) in dental aerosols have the potential to cause significant disruption to dental service provision. The potential for dispersion of pathogens during Aerosol-Generating Procedures (AGPs) is also an issue in healthcare more widely, for example during procedures such as endotracheal intubation and extubation, surgery using powered instruments, and endoscopy (Tran et al. 2012 ).

The literature relating to airborne transmission of infectious diseases has recently been subject to some scrutiny, and although the received wisdom is that droplets greater than 5 µm diameter do not remain airborne for significant periods of time or travel further than 2 m from the source ((WHO) 2014), this has been questioned by some (Tang et al. 2021) . In fact, there is evidence that that droplets 60 -100 µm can remain suspended for some time and therefore travel significant distances from the source, thus posing an inhalation risk to others in the area or those entering the area thereafter (Xie et al. 2007 ).

Several methods of mitigating the dispersion of aerosols and droplets during dental procedures have been proposed to reduce risk of the transmission of pathogens.

Effectiveness Programme 2021). LEV systems capture airborne contaminants, thus minimising the risk of them being inhaled by the operator or escaping into the wider . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101 https://doi.org/10. /2021 environment. LEV is widely used in the manufacturing sector to protect workers from exposure to dust, fumes, and gases produced from tasks such as welding and soldering (Health and Safety Executive 2017) . These devices have previously been referred to in the context of dentistry as "extra-oral suction/scavenging", however LEV is a more correct term and is used throughout this paper. Previous studies of LEV for dental procedures have reported promising findings, however to our knowledge, no studies have evaluated both settled droplets and suspended aerosols together (Ehtezazi et al. 2021; Shahdad et al. 2020) . The aim of this study is to investigate the effect of LEV on the distribution of aerosols and droplets produced during dental procedures.

Open plan setting Experiments using an air-turbine handpiece were conducted in the Clinical Simulation Unit at the School of Dental Sciences, Newcastle University (Newcastle upon Tyne, United Kingdom). This is an 825.4 m 3 dental clinical teaching laboratory situated within a large dental teaching hospital. The setting has a supply and extract Heating, Ventilation and Air Conditioning (HVAC) system which provides 6.5 Air Changes per Hour (ACH; as assessed by an external engineering contractor) through ceiling mounted vents. A rig was constructed around a dental mannequin as previously described comprising platforms spaced at 0.5 m intervals along eight, 4 m, rigid rods, laid out at 45° intervals supported by a central hub (figure 1). This created an 8 m diameter circle around the mannequin, with the . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101 https://doi.org/10. /2021 centre of the circle 28 cm superior to the mannequin's mouth and 73 cm above the floor. All windows and doors remained closed and only the operator and assistant were present inside the experimental area, leaving immediately after completing the procedure. 

In the open plan setting, experiments were conducted on a dental simulator unit (Model 4820, A-dec; OR, USA) with a mannequin containing model teeth (Frasaco GmbH; Tettnang, Germany). The mouth of the mannequin was positioned 83 cm above the floor. One operator (RH, height: 170 cm) completed an anterior crown preparation of the upper right central incisor tooth for a full coverage crown for ten minutes using a high-speed air-turbine (Synea TA-98, W&H (UK) Ltd.; St Albans, UK). The coolant flow rate was 29.3 mL/min. Fluorescein sodium tracer was introduced into the irrigation reservoir of the dental unit as a 2.65 mM solution. In all experiments in this setting, an assistant operated dental suction with an 8.3 mm . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint internal diameter suction tip at a flow rate of 133 L/min of air measured using a flow meter (Ramvac Flowcheck, DentalEZ; PA, USA); this equates to "medium volume suction" according to UK national guidelines (NHS Estates 2003) . Three replicates were conducted for each experiment as well as for a negative control condition where no procedure was occurring.

In the single surgery setting, a dental mannequin (P-6/3 TSE, Frasaco GmbH; Tettnang, Germany) was attached to a dental chair (Pelton and Crane Spirit Series, Charlotte, USA) and the mouth of the mannequin positioned 90 cm above the floor. 

A DentalAIR UVC AGP Filtration system (DA-UVC1001; VODEX Ltd., UK) was used as the LEV device. This device uses a High-Efficiency Particulate Air (HEPA) filter and is compliant with EN1822 standards. The device has an air flow rate of 5,000 . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint L/min of according to the manufacturer and includes a 254 nm integrated UVC source providing a 27 mW/cm 2 UV dose to the airflow within the device before filtration. The device has a transparent shield attached to the inlet, which is positioned over the patient's mouth during dental procedures. In experiments using LEV, the device was positioned with the centre of the inlet 10 cm inferior to the chin of the mannequin, and 4 cm above the plane of the mannequin's mouth as in Figure   1 .

Three complementary methods were used to measure aerosols and droplets: active sampling with optical particle counters (non-specific measurement of suspended aerosols), active sampling with air samplers and subsequent fluorometric analysis (measurement of suspended fluorescein-containing aerosols), and passive sampling using filter papers to collect fluorescein-containing droplets and settled aerosols. were used to measure suspended aerosols. OPCs had six particle-size channels (0.3, 0.5, 1.0, 2.5, 5.0, and 10.0 µm) with a sampling flow rate of 2.83 L/min and were calibrated by the manufacturer to ISO 21501-4 standards. The instruments were set to sample continuously at 5-second intervals beginning 2 minutes before the procedure, continuing during the 10-minute procedure, and for 20 minutes after . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint (32 minutes total). OPCs were placed in two positions during each experiment ( Figure 1) . In the open plan setting, this was 0.5 m inferior to the mouth of the mannequin, and to the left of the mannequin at 2 m; in the single surgery setting, this was 0.5 m to the right of the mannequin and 2 m at the foot of the dental chair. Both

OPCs were positioned with sampling nozzles at 87 cm above the floor. Data were presented as normalised particle counts (particles/ m 3 ) over the time-course of the experiment and total particle counts were summed across all particle size channels.

As experiments were conducted in real clinical settings, background particle counts were variable. All OPC data were therefore normalised to an internal baseline by subtracting the average counts during the 2 minutes before the procedure from all particle counts. These instruments were also used to measure temperature and relative humidity at the same intervals. (chest), 0.5 m to the right of the mannequin, 1 m on the dental chair, and 2 m at the end of the dental chair. 20 mL of distilled water was added to the sampling vessels before operation. BioSamplers were operated at an air flow rate of 12.5 L/min using . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint a sampling pump (BioLite+, SKC Inc.; PA, USA) and were calibrated using a rotameter (SKC Inc.; PA, USA). Sampling began at 2 minutes before the 10-minute dental procedure and continued until 20 minutes after the end of the procedure (32 minutes in total). 100 showed that these methods eliminate risk of carry-over of fluorescein ). Fluorescein was recovered from filter papers by adding 350 µL deionised water. Immersed samples were shaken for 5 minutes at 300 rpm using an orbital . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. 

Preliminary testing was carried out to ensure fluorescein was captured by the LEV device and not redistributed into the environment which could lead to spurious results. Three filter papers were placed 11 cm away from the two exhaust vents on both sides of the LEV device, and one BioSampler was placed 22 cm away from each vent (total 6 filter papers and 2 BioSamplers). The LEV was switched on, and after four minutes, an air turbine handpiece was operated with fluorescein tracer as described above, with the spray directed into the LEV nozzle. The handpiece was used for six minutes before stopping, and after 10 further minutes the LEV was turned off and samples were collected. A negative control condition with plain water instead of fluorescein was also conducted and both conditions were conducted in triplicate. Samples were processed as described above.

Data were collected using Excel (2016, Microsoft; WA, USA) and analysed with SPSS (version24, IBM Corp.; NY, USA) using descriptive statistics.

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The copyright holder for this preprint this version posted July 22, 2021. 

No difference was seen in fluorescein detection between the negative control and when fluorescein tracer was used for pilot experiments, confirming that fluorescein does not pass through the LEV's HEPA filter. These data are shown in Appendix Table 1 and Appendix Figure 1 .

The mean (SD; minimum -maximum) temperature in this setting was 23.7 °C (0.5; 22.6 -25.1 °C) and the relative humidity was 28.8 % (6.3; 20.0 -38.6 %).

Active sampling with an optical particle counter (suspended aerosols) OPC data were collected at two positions in the open plan setting: 0.5 m to sample aerosols present in the breathing zone of the operator and assistant, and at 2 m to sample aerosols at the minimum distance between dental chairs recommended by current UK infection prevention and control guidance for multi-chair dental clinics (Public Health England 2020). Particle counts were substantially lower during all conditions at the 2 m sampling location compared to at 0.5 m. At both 0.5 m and at 2 m, the use of LEV was associated with a substantial reduction in particle counts from the dental procedure using an air-turbine handpiece. Figure 2 shows illustrative data from one repetition at the 0.5 m location. Data from all repetitions, including from the 2 m sampling location and negative control, are available in the supplementary appendix.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint Active sampling with BioSamplers and fluorometric analysis BioSampler data from the open plan setting are presented in Table 1 and Figure 3 .

Detection of fluorescein decreased with increasing distance from the procedure. The use of LEV was associated with a 75 -91% reduction in aerosolised fluorescein from the air-turbine handpiece dependant on location. The percentage reduction decreased with increasing distance from the dental procedure, and this was 90% within 0.5 m; this distance represents the breathing zone of members of the dental team. (Table 2 and Appendix Figure 8 ). Within first 0.5 m there was a 95% reduction in settled fluorescein from the air-turbine handpiece when LEV was used. Between 1 -2m there was a 69% reduction in settled fluorescein detection when LEV was used. Between 2.5 -4m there was a 78% reduction in settled fluorescein detection when the LEV used.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint The mean (SD; minimum -maximum) temperature during experiments in this setting was 24.1 °C (0.8; 22.7 -26.4 °C) and relative humidity was 38.5 % (6.2; 26.0 -45.0 %).

Active sampling with an optical particle counter (suspended aerosols) Particle counts were substantially lower during all conditions at the 2 m sampling location compared to at 0.5 m. At 0.5 m and at 2 m, the use of LEV was associated with a substantial reduction in particle counts from the ultrasonic scaler. Figure 2 shows illustrative data from one repetition at the 0.5 m location. Data from all repetitions, including from the 2 m sampling location and negative control, are available in the supplementary appendix.

Using the BioSampler, detection of fluorescein decreased with increasing distance from the procedure (Table 1 and Figure 3 ). At all locations, the use of LEV was associated with a 98.7 -100.0% reduction in aerosolised fluorescein from the ultrasonic scaler.

Overall, three complementary sets of data at multiple sampling locations, with different dental procedures across two different clinical settings robustly demonstrate that LEV is effective in capturing aerosols and droplets from dental procedures and reducing the dispersion of these in the clinical environment. This reduction was most . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101 https://doi.org/10. /2021 significant closest to the procedure, in the breathing zone of the operator and assistant.

Previous studies have evaluated the effectiveness of LEV in dentistry by studying droplet dispersion using a non-fluorescent tracer (Shahdad et al. 2020 ) and aerosols using particle counting instruments alone (Ehtezazi et al. 2021 ). These studies demonstrate substantial reductions in respective measures when LEV is used, however the present study is the first to examine the effect of LEV on both settled droplets and suspended aerosols simultaneously, and the first to do so using a method of capturing suspended aerosols with a tracer specific to the dental procedure. The positioning of LEV in the above cited studies was also more distant from the procedure, (15 -20 cm) whereas in the present study the LEV nozzle was positioned in an optimal position for aerosol capture (10 cm). Relative reduction in aerosol was most pronounced for the ultrasonic scaler and we hypothesise that this is because the high frequency oscillation of the ultrasonic device produces particles with less momentum than those forced out under compressed air from the airturbine; for this reason, we propose that particles from the scaler are more easily captured by LEV, explaining the more marked reduction.

In experiments with the air-turbine handpiece, dental suction was used during the control condition and with LEV; the effect of LEV was marked for the air-turbine even above the effect of suction. With the ultrasonic scaler, the effect of suction was also measured separately to LEV. The effect of LEV with the scaler was similar to the effect of LEV, however it was difficult to measure the effect of LEV in addition to suction due to how substantial the effect of suction alone was. This supports the . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint hypothesis that particles from ultrasonic scalers are more easily controlled with suction and LEV than those from air-turbines. Importantly, the effect of dental suction may vary depending on the performance of the system and the actions of the operator, which is not the case for LEV.

In this study, we assessed the effectiveness of LEV for the containment of aerosols during dental procedures and we used dental suction during experiments to most accurately simulate standard clinical practice. Previous studies using a similar methodology demonstrate the significant benefit of dental suction Holliday et al. 2021 ) and the present study clearly demonstrates the additional benefit of LEV. This study did not assess the practicality of using LEV for routine dentistry or the acceptability of the device for patients; however, in the authors' opinion, the device is very unobtrusive and there are unlikely to be significant barriers to clinical use.

The present study was conducted using a dental mannequin rather than in patients, and respiratory activities, which are significant aerosol sources (Wilson et al. 2021 ), were therefore not modelled using this methodology. However, the study aimed to understand the effect of the LEV on the additional aerosols produced by the dental procedure, over and above normal clinical contact-an experimental design using a mannequin is ideal to allow this.

In this study, the tracer showed where any aerosols from dental instruments were distributed to and the effect of LEV on these. Clearly, it is not the instrument aerosols themselves which pose a risk of infection, but the pathogens from saliva carried . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint within these aerosols. Our previous work has shown that 'saliva', modelled with fluorescein tracer, is dispersed by aerosols from dental instruments Llandro et al. 2021) . We chose to measure the aerosols from instruments themselves as dispersed 'saliva' is likely to be highly diluted; the model used in the present investigation therefore allowed us to demonstrate the effect of LEV with greater sensitivity than if a 'saliva'-based model were used.

The use of a fluorescent tracer is a reasonably straightforward approach to examine the distribution of dental aerosols, however the biological characteristics of bioaerosols cannot be examined, such as the infectivity of any dispersed viruses or other microbial pathogens within these bioaerosols. Future studies should utilise biological tracers to validate the findings from non-biological models such as those used in the present study.

Particle counts from 0.3 -10 μ m OPC channels were combined, as this provides an easily comparable measure across experiments, and is consistent with measures used in air-quality monitoring combining particles < 10 μ m, for example, PM 10 (although this uses particle mass instead of number as in the present study). It is likely that particles of differing size behave different ways; however, it was not the aim of the present study to examine this.

This study demonstrates that LEV reduces aerosols produced from dental procedures by at least 90% within 0.5 m of the procedure. LEV seems to be more effective at capturing aerosols from ultrasonic scalers where particles are likely to be . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101 https://doi.org/10. /2021 less energetic, compared to with an air-turbine handpiece. LEV therefore shows promise in reducing aerosols from dental procedures and should play a role in reducing risks from dental bioaerosols.

This work was commissioned and funded by VODEX Ltd. We would like to thank Philippa McClen for her support with this work. Contribution: JRA, RH-conception, design, data acquisition and interpretation, drafted and critically revised the manuscript; CD, KP, GC-data acquisition and interpretation, critically revised the manuscript; JD, NSJ-conception, design, drafted and critically revised the manuscript . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. Data adjusted for background fluorescence by subtraction of the background reading (Open plan setting: 25.2 Relative Fluorescence Units, RFU; n = 12; single surgery setting: 25.8 RFU; n = 12) from all data. All air-turbine experiments also used dental suction. *Actual reading was below zero (-1 RFU) after subtraction of background reading but limited to zero for this table.

. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.

The copyright holder for this preprint this version posted July 22, 2021. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

The copyright holder for this preprint this version posted July 22, 2021. ; https://doi.org/10.1101/2021.07.21.21260917 doi: medRxiv preprint A rig to support filter papers is shown as black lines radiating from a centre above the mannequin. Filter papers were spaced at 0.5 m intervals on each of the eight rods. B: Plan view of single surgery setting as above. The star indicates the location of the aerosol generating procedure. C: Positioning of the LEV device in relation to the dental mannequin. . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)

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