key: cord-0795329-7jd6mjv0
authors: Vernon, J. J.; Black, E. V. I.; Dennis, T.; Devine, D. A.; Fletcher, L.; Wood, D. J.; Nattress, B. R.
title: Dental mitigation strategies to reduce aerosolization of SARS-CoV-2
date: 2021-03-26
journal: nan
DOI: 10.1101/2021.03.24.21254254
sha: 41d3786f575fe29ae8e78c6505f7acd9f6ac7678
doc_id: 795329
cord_uid: 7jd6mjv0

Limiting infection transmission is central to the safety of all in dentistry, particularly during the current SARS-CoV-2 pandemic. Aerosol-generating procedures (AGPs) are crucial to the practise of dentistry; it is imperative to understand the inherent risks of viral dispersion associated with AGPs and the efficacy of available mitigation strategies. In a dental surgery setting, crown preparation and root canal access procedures were performed with an air turbine or electric speed-controlled hand-piece, with mitigation via rubber dam or high-volume aspiration and a no mitigation control. A phantom head was used with a 1.5 mL flow of artificial saliva infected with {Phi}6 bacteriophage (a surrogate virus for SARS-CoV-2) at ~108 plaque forming units mL-1, reflecting the upper limits of reported salivary SARS-CoV-2 levels. Bioaerosol dispersal was measured using agar settle plates lawned with the bacteriophage's host, Pseudomonas syringae. Viral air concentrations were assessed using MicroBio MB2 air sampling, and particle quantities using Kanomax 3889 GEO particle counters. Compared to an air turbine, the electric hand-piece reduced settled bioaerosols by 99.72%, 100.00% and 100.00% for no mitigation, aspiration and rubber dam, respectively. Bacteriophage concentrations in the air were reduced by 99.98%, 100.00% and 100.00%, with the same mitigation strategies. Use of the electric hand-piece with high-volume aspiration, resulted in no detectable bacteriophage, both on settle plates and in air samples taken 6-10-minutes post-procedure. To our knowledge, this study is the first to report the aerosolization of active virus as a marker for risk determination in the dental setting. Whilst this model represents a worst-case scenario for possible SARS-CoV-2 dispersal, these data showed that the use of electric hand-pieces can vastly reduce the risk of viral aerosolization, and therefore remove the need for clinic fallow time. Furthermore, our findings indicate that the use of particle analysis alone cannot provide sufficient insight to understand bioaerosol infection risk.

associated with AGPs and the efficacy of available mitigation strategies. 23

In a dental surgery setting, crown preparation and root canal access procedures were 24 performed with an air turbine or electric speed-controlled hand-piece, with mitigation via 25 rubber dam or high-volume aspiration and a no mitigation control. A phantom head was used 26 with a 1.5 mL flow of artificial saliva infected with Φ6 bacteriophage (a surrogate virus for 27 SARS-CoV-2) at ~10 8 plaque forming units mL -1 , reflecting the upper limits of reported 28 salivary SARS-CoV-2 levels. Bioaerosol dispersal was measured using agar settle plates 29 lawned with the bacteriophage's host, Pseudomonas syringae. Viral air concentrations were 30 assessed using MicroBio MB2 air sampling, and particle quantities using Kanomax 3889 31

GEOα particle counters. 32

Compared to an air turbine, the electric hand-piece reduced settled bioaerosols by 99.72%, 33 100.00% and 100.00% for no mitigation, aspiration and rubber dam, respectively. 34

Bacteriophage concentrations in the air were reduced by 99.98%, 100.00% and 100.00%, 35

with the same mitigation strategies. Use of the electric hand-piece with high-volume 36 aspiration, resulted in no detectable bacteriophage, both on settle plates and in air samples 37 taken 6-10-minutes post-procedure. 38

To our knowledge, this study is the first to report the aerosolization of active virus as a 39 marker for risk determination in the dental setting. Whilst this model represents a worst-case 40 scenario for possible SARS-CoV-2 dispersal, these data showed that the use of electric 41 hand-pieces can vastly reduce the risk of viral aerosolization, and therefore remove the need 42 for clinic fallow time. Furthermore, our findings indicate that the use of particle analysis alone 43 cannot provide sufficient insight to understand bioaerosol infection risk. 44

The potential nosocomial spread of SARS-CoV-2 coronavirus and other pathogens through 46 oral fluid aerosolization provides a significant risk to the safety of patients, dentists and oral healthcare teams. During the current SARS-CoV-2 pandemic, extensive constraints have 48 been placed on dentistry across the world, with a particular focus on aerosol-generating 49 procedures (AGPs).

[1] These constraints impact widely upon how dentistry can be delivered 50 in both dental practices or offices and on multi-occupancy teaching clinics. There is a paucity 51 of robust data supporting some of these restrictions and further research is essential to 52 investigate the efficacy of mitigation strategies and the requirement for fallow time between 53 patients. 54

In dental settings, aerosols are generated from the use of dynamic dental instruments such 55 as ultrasonic scaling tips, high-speed dental hand-pieces and 3:1 syringes. The aerosols are 56 known to contain saliva/blood, microorganisms (including viruses) from the oral cavity of the 57 patient and are created by air mixture from the hand-piece and water flowing from the dental 58 unit water line. [2-4] This generation of aerosols is considered to be unavoidable due to the 59 use of rapidly rotating high-speed dental hand-pieces. Exposure to aerosol particles of 50µm 60 or smaller pose the greatest risk because the smaller the particle size, the more likely they 61 are to remain airborne for long periods. Therefore, they are more capable of entering the 62 nasal passages and penetrating deep into the respiratory system. [5] 63

Aerosol composition also varies from patient to patient, the site in which the dental 64 procedures are carried out and the type of procedure performed. Ninety percent of the 65 aerosols generated are known to have a mass median aerodynamic diameter of <5µm, and 66 as the dental environment is known to be contaminated with various microorganisms, the 67 resulting aerosols pose a threat to dental workers. [4, 6] SARS-CoV-2 represents the most 68 significant threat to date. Droplets/aerosol particles may transfer from the patient's mouth to the breathing zone or 74 body surface of the dental team, thereby contributing to the spread of infections. It is the 75 'perfect storm' of the routine production of these aerosols, combined with patients that might 76 be asymptomatic carriers of SARS-CoV-2, plus the lack of good evidence regarding 77 measures to mitigate the effects of dental aerosols that is at the root of the issues facing 78 dental practitioners and dental schools. These impact dentists in terms of their clinical 79 practise, use of PPE and an arbitrarily determined length of fallow time, leading to a 80 reduction in the number of patients that can be treated. Following the suspension of routine 81 dental treatment, advice was provided from dental councils around the world that AGPs 82 should be stopped (avoid/defer) unless necessary e.g. for emergency procedures.

[8] 83

Various SOPs have since been published by a number of learned and professional 84 organisations to guide dentists on safe working practice, however many of these 85 acknowledge that there is only a limited evidence base. 86

Various methodologies for determining aerosolization in dental environments have been 87 implemented, including the use air particle measurement, [9, 10] biological air sampling, [11, 88 12] culture of settle plates [6, 13, 14] and fluorescent markers. [9, 15, 16] Each of these 89 methods offers insight into bioaerosol production, but each has its limitations. For instance, 90 settle plates cannot account for the smallest particles that will not settle out of the air, air 91 particle data cannot distinguish "clean" particles from the dental water unit line and those of 92 biological origin, and the use of fluorescent dyes cannot offer information on viability of any 93 potential biological component. Therefore, none of these methods alone can proffer robust 94 findings as to the dispersal of active SARS-CoV-2. 95

Here we report a novel viral model for bioaerosol enumeration, utilising the bacteriophage 96 Φ6 (Phi6) as a surrogate for SARS-CoV-2. Structurally the Φ6 virus particle is similar to is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint

The copyright holder for this this version posted March 26, 2021. ; https://doi.org/10.1101/2021.03.24.21254254 doi: medRxiv preprint 6 density at 600nm of 0.6 (OD600 0.6) using a spectrophotometer (Jenway 6305, UK). 500 μL of 128 each bacteriophage stock dilution and 200 μL of P. syringae culture were added to 6 mL of 129 molten top-layer agar, consisting of 33g L -1 TSB and 6.6 g L -1 Agar (Sigma, UK), swirled and is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint

The copyright holder for this this version posted March 26, 2021. ;

Further P. syringae-lawned settle plates were used with two MicroBio MB2 air sampling devices 181 (Cantium Scientific, UK), set 30 cm either side of the oral cavity ( Figure 1 ). These devices draw 182 air onto the P. syringae-lawned plates at 100 L min -1 and were set to sample 400 L of air during 183 each of the four four-minute periods when the dental hand-piece was active. Two further, 184 delayed air samples of 400 L were taken during the fallow period, between the six and ten-185 minute timepoints. Air sample counts were adjusted by a positive hole correction factor. [25] 186

Procedures were carried out in series, to enable different mitigation strategies to be compared 187 with each other and a no mitigation baseline. Fresh PPE was donned for each procedure to 188 prevent cross contamination between experiments and only the dentist and investigator (also 189 acting as dental nurse) were present during AGPs. Post-procedure, a third investigator (wearing 190 PPE) sealed settle plates as a final anti-cross contamination measure. 191

Two Kanomax 3889 GEO α particle counters (Kanomax, Japan) were used to monitor size and 193 quantity of particles in six size ranges simultaneously; diameters 0.3 μm, 0.5 µm, 1.0 μm, 3.0 194 µm, 5.0 μm and 10 μm. One particle counter was positioned directly behind the dentist, and 195 another was stationed between the dental chair and the door ( is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint 

Bioaerosol dispersal during AGPs with a series of mitigation strategies 208

Bioaerosol was detected at all sampling points with an air turbine and no mitigation control 209 ( Figure 2) . Each applied mitigation reduced levels of bioaerosol recovered from settle plates 210 and air samples (Table 1) is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint Procedures employing the air turbine with high-volume aspiration demonstrated levels of 226 bacteriophage of 21 pfu and 11.25 pfu/m 3 , for post-AGP settled bioaerosols and microbiological 227 air samples respectively (Table 1) . The use of an electric speed-controlled hand-piece reduced 228 both to zero; (p=0.037 and p=0.037, respectively). The use of rubber dam for either hand-piece 229 also resulted in undetectable bioaerosol for the post-procedure, fallow period. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint Table 1 ). Significant differences were observed 238 between the hand-pieces with no mitigation for several particle size ranges; p=0.017, p=0.005, is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint

The copyright holder for this this version posted March 26, 2021. ;

The length of time, post-AGP for particle levels to return to pre-procedure baseline levels was 244 highly variable, with 22.2% of procedures not reaching baseline figures within 25 minutes. 245

There were no discernible differences between air turbine and electric hand-pieces for the time (Table 2) . Particle data was also vastly reduced in posterior 255 endodontic experiments versus anterior procedures for all particle sizes and sampling positions; 256 (Table 3, Appendix Table 2 ). 257 is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint introduced above the apex of the tooth, representing a worse-case scenario. Different mitigation 277 strategies using both air turbine and electric hand-piece were used for each dental procedure. 278

There was a clear distinction between the amount of aerosolized saliva dispersed around the 279 dental surgery using the traditional air turbine and an electric speed-controlled hand-piece. 280

Bioaerosol levels were clearly diminished when using the electric hand-piece over the air 281 turbine. Whilst the electric hand-piece was used at 60,000 rpm, compared to higher speeds with 282 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.

is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint

The copyright holder for this this version posted March 26, 2021. ; https://doi.org/10.1101/2021.03.24.21254254 doi: medRxiv preprint the air turbine, the absence of compressed air appears to be key to a reduced bioaerosol 283 production. 284

Air sampling was also implemented with the bacteriophage methodology to ensure the 285 quantification of viable viral particles in the air, potentially too small to settle onto the clinic 286

surfaces.

[28] With high-volume aspiration, the differences displayed between air and electric 287 hand-piece AGPs were large, 637.4 pfu/m 3 versus 0.1, respectively. The latter represented a 288 solitary bacteriophage unit detected in a single experimental replicate. For all mitigations, the 289 settle plates closest to the mouth indicated the highest quantities of bacteriophage (Figure 2) . 290

This was due to splatter rather than bioaerosol and can be consider a lower transmission risk 291 than aerosolized virus. The data in this study were collected in an environment with mechanical 292 ventilation. Whilst many "high street" practices around the world do not have these facilities, 293 evidence indicates that aerosol accumulation is greater in practices with poor ventilation.

[29] 294

These differences are clearly supported by the particle measurement data, which indicated 295 lower levels of all particles sizes for the majority of mitigations; (Figure 3 ). Furthermore, by 296 recording particle measurements from two locations in the surgery, we saw that aerosols were 297 not localised and displayed similar, although slightly delayed trends, towards the extremities of 298 the clinic. This further highlights the necessity for good mitigation protocols. 299

The evidence presented here also corroborates data reported by others, suggesting that the 300 use of a rubber dam significantly reduces microbial aerosolization. [30, 31] Whilst these studies 301 report bacterial air contamination rates, our study documents the reduction of viral 302 aerosolization, which may be expected to behave differently due to their smaller size. 303

Conversely, the Aspi Jet 25, specialist aerosol extraction device was no better than high-volume 304 aspiration alone. Although it reduced the biological particles in the air, it seemed to increase air 305 turbulence without substantially removing the majority of bioaerosol. 306

To improve the accuracy of the phantom head unit over previous published models, [9, 15, 32] is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint The bacteriophage data in this study demonstrated that with air turbine and high-volume 332 aspiration, substantial amounts of both settle and aerosolized bacteriophage were detectable 333 between the six and ten minute fallow period; (Table 1) . However, use of the electric speed-334 controlled hand-piece eliminated any bioaerosol within six minutes of the procedure completion. 335 Therefore, this evidence strongly suggests there is no need for a prolonged fallow period with 336 this hand-piece. Where an electric hand-piece is not available, the use of a rubber dam was 337 . CC-BY-NC-ND 4.0 International license It is made available under a perpetuity.

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The copyright holder for this this version posted March 26, 2021. ; https://doi.org/10.1101/2021.03.24.21254254 doi: medRxiv preprint equally effective in reducing air contamination shortly after conclusion of an AGP. Assessing 338 both the particle and bacteriophage fallow data together, it becomes clear that particle data 339 alone cannot provide sufficient information to determine risk of airborne viral particles. Here we 340 see instances of baseline levels not being achieved post-AGP, but no detectable active 341 bacteriophage in the air. 342

We assessed the differences between anterior and posterior AGP positions. Both the 343 bacteriophage and particle data highlighted the importance of procedural position on risk. When 344 using an air turbine and high-volume aspiration, no bacteriophage particles were detected in the 345 air during the posterior procedures with a >84% reduction in all particle sizes observed from 346 behind the dentist. Together, these data support the interpretation that endodontic procedures 347 in the posterior of the mouth impart a lower risk of viral contamination and dispersal into the 348 environment, with bioaerosols most likely trapped inside the oral cavity. Conversely, dental 349 procedures in the anterior region pose the greatest risk. 350

Through the combined use of novel and established methodologies, the data described here 351 presents a clear picture of how risk of SARS-CoV-2 and other similar biological hazards can be 352 greatly attenuated by the use of electric speed-controlled hand-pieces. Whilst detection of a 353 single viral unit may not translate to an infective viral load, the reduction in levels with these 354 mitigating approaches is clear to see. This study further suggests that with these hand-pieces 355 and high-volume aspiration or the use of rubber dam, a prolonged fallow period is not 356 necessary in the clinical setting used. Equipping our dental surgeries with these tools will be 357 crucial to protecting the health, safety and future of dental teams and services. Finally, the data 358 presented here suggests that particle count data alone cannot provide accurate information as 359 to the dispersal and settlement of bioaerosols, with bacteriophage markers offering a greater 360 insight into infection risk. is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint is the author/funder, who has granted medRxiv a license to display the preprint in (which was not certified by peer review) preprint

The copyright holder for this this version posted March 26, 2021. ; https://doi.org/10.1101/2021.03.24.21254254 doi: medRxiv preprint

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