SARS-CoV-2: Characterisation and Mitigation of Risks Associated with Aerosol Generating Procedures (AGPs) in Dental Practices

Liverpool John Moores University, James Parsons Building, Byrom Street, Liverpool, L3 3AF Techceram Limited, 9b Sapper Jordan Rossi Park, Baildon, Shipley, BD17 7AX Woodbury Dental & Laser Clinic, Woodbury House,149 High Street, Tenterden, Kent, TN30 6JS University of Liverpool School of Dentistry, Research Wing, Pembroke Place, L3 5PS University of Manchester School of Dentistry, Coupland 3 Building, Oxford Road, M13 9PL University of Hertfordshire, College Lane, Hatfield, Hertfordshire, AL10 9AB


Introduction
Potentially infectious agents (e.g. bacteria, fungi and viruses) can be transmitted when droplets containing microorganisms generated from an infected person (example by breathing, talking or coughing) are propelled through the air and are directly inhaled, deposited on the skin or mucosal surfaces, or contaminate infrastructure. 1 High-speed dental instruments require effective cooling of the work area in order to avoid damage of the pulp dentine system. These instruments generate a dental aerosol, as cooling water and air are sprayed around the instruments and the oral cavity.
Dental aerosols are distributions of particle sizes from 0.001 to >10 µm in diameter. 2,3 Traditionally, dental airborne aerosols were defined as being small particles <50 µm, with larger ballistic/projectile particles (>50 -100 µm) being described as "splatter". 4 The WHO definition 5 of aerosols has been adopted in the dental field, which defines large projectile particles as being > 5µm, with smaller (< 5 µm) "droplet nuclei" particles forming through the evaporation of larger particles generating an airborne solid residue. Infectious droplets from saliva or blood may enter the aerosol and expose the dental team to an increased risk of infection though direct inhalation, contact with eyes, and contact with contaminated work surfaces. 6,7 Dental aerosols therefore have the potential to provide a path for the transmission of COVID-19 8,9 which may remain infectious for between 2 hours to 9 days in a humid environment. 7 Research on the influenza virus has also demonstrated that the total viral copies were 8.8 times more numerous in particles <5 µm than in particles ≥5 µm. 10 Previous studies have demonstrated the dispersion of bioaerosols to all areas of the treatment room 11 which remain airborne for 30 minutes following the procedure. 12 Therefore, there is a clear need for the effective removal of aerosols in dental practices. 13 Protocols exist to minimise the risk of infection to clinical staff during dental procedures. 12,14-17 These include: low volume suction (LVS) to remove saliva and excess coolant, coolant disinfectant, high-volume intra-oral suction (HVS (IO)), personal protective equipment (PPE) and improved ergonomics and techniques (e.g. dental dams). A range of additional aerosol removal treatments have been proposed for use in dental procedures including extra-oral high volume suction (HVS (EO)), air cleaning systems (ACS), designed to filter, purify and recirculate room air) and ventilation systems. 7,14,18,19 However, their effectiveness within a diverse range of dental practice environments is difficult to predict. 13 A wide range of ACS with different air flow rates and cleaning technology are commercially available or being marketed for dental use. However, dental practices have no clear standards or specifications to refer to before making an investment. HVS(EO) and ACS 20,21 that contain high efficiency particulate air (HEPA) filters are effective in removing airborne particles with sizes greater than 0.3 µm: viruses, such as coronaviruses, are in the size range of 0.05 -0.15µm 22 and thus may evade filtration. Hence, ACS have evolved to include the addition of technology such as UV-C lamps (99.97% killing of H3N2 influenza virus), negative ion generators, and high pressure/voltage electrostatic plasma, which eliminate particles greater than 0.0146 µm. The efficiency of these air purifiers has not been evaluated for the removal aerosol particles in the presence of high volume intra-oral or extra-oral suction.
Whilst researchers have studied aerosol removal treatments, few studies have examined their effectiveness across the full dental aerosol particle size distribution. For example, the use of HVS(IO) at air flow rates of 250 -300 L min -1 is an established means of controlling dental aerosols but its effectiveness is based on a qualitative assessment of visible particles or particles greater than 0.65 µm. 19,23 Viruses are smaller than 0.65 µm and therefore the efficacy of HVS(IO) studies are not relevant to COVID 19.
The objectives of this current study were to characterise the aerosols generated by standard dental procedures and to investigate the effectiveness of different combinations aerosol management interventions across the particle distribution range from 0.0062 to 10 µm diameter to provide evidence for establishing a revised fallow time. A sequence of six standard dental procedures were performed in series to assess the effectiveness of four combinations of interventions based on HVS (IO), HVS (EO) and an ACS. The effectiveness of each intervention group was measured using a high-resolution particle size analyser, with air samples taken over a 36-minute period from six locations within a standard dental surgery.

Method
The study was performed within a dental surgery (dimensions 4.4 x 3.1 x 2.6 m: Figure 1). Real-time aerosol analysis was performed with a high-resolution Electric Low-Pressure Impactor particle sizer (HR-ELPI: "ELPI+", Dekati, Kangasala, Finland). The instrument recorded the concentration of particles detected within 100 pre-set 'bins' of particle size, ranging from 0.0062 to 9.6 µm, at a sampling frequency of 1 Hz. Air samples were acquired at six locations ( Figure 1, Table 1). Each position was measured relative to the phantom head on which the dental AGPs were performed. Air samples were directed to the ELPI+ via 2 m lengths of silicone tubing (Tygon®"; internal diameter 12.7 mm, external diameter 17.5 mm; Cole-Parmer Instrument Co, Illinois, USA: Figure S1). Each tube was individually connected to the particle sizer for a period of 30 seconds before being replaced with a tube from the next sampling location to enable a serial analysis of all six air sample locations within a 3 min cycle. A pilot study demonstrated that the tubing had no discernible effect on particle size measurements (see supplementary data: Annex A, Figures S1-S3). All nonexperimental air-conditioning equipment was turned off during the experimental work, and the average room temperature and relative humidity were recorded at 27 C and 67% respectively.
Each experiment comprised a three-minute baseline period, followed by a series of six aerosol generating procedures (AGPs) carried out over 18 minutes with a post-procedural duration of 18 minutes to monitor aerosol decay ( Figure 2). Each experiment was performed using one of four treatments ( Table 2). The technical specifications of each aerosol removal system are described in Table 3. Each treatment was performed in triplicate. The AGPs incorporated the serial use of six commonly used dental preparation instruments each of which were operated for three minutes within the phantom head, in the upper and lower anterior sextants, in the following order: (I) Air turbine hand-piece, (II) Electric contra-angle hand-piece, (III) Air turbine hand-piece, (IV) Three in one syringe, (V) Ultrasonic scaler and (VI) Ultrasonic scaler (Table 4).
Total particle concentration (calculated as the sum of particle concentrations over the 0.0062 to 9.6 µm bin range) did not consistently exhibit a Gaussian (normal) or log-normal distribution and so excluded the use of parametric statistical tests. The low sample number (n=3) precluded non-parametric analyses. Therefore, descriptive statistics were used and all particle concentration data are expressed as median values. Area under curve (AUC) calculations were performed using GraphPad Prism (v7.0e for Mac OS, GraphPad Software, La Jolla California USA). The AUC calculations reflect the total "dose" of aerosol (units of mL cm -3 min). The AUC calculations were used to assess the overall efficiency of each treatment and were expressed as the median value ± minimum/maximum. Estimation of fallow time in the control treatment group was performed by linear regression of particle concentrations at each sample location following cessation of AGPs and was calculated as the time at which the extrapolated particle concentration decreased below the upper baseline particle concentration.
Aerosol generated under the control conditions ( Table 2, intervention group A (LVS only)) was observed at all locations within the surgery and remained detectable at 15 min ( Figure  3, t=36 min) from the end of the last procedure (instrument VI at t=21 min). The most persistent particles were in the range 0.012 to 0.025 µm. Particle concentrations decreased with increasing distance from the phantom head, with a notable, time-related decrease of particles in the range 0.054 to 0.236 µm diameter. Particles > 0.05 µm persisted at low concentrations (~25 x 10 3 cm -3 ) for the duration of the study.
The particle size distributions generated during the use of all instruments and applying interventions B to E ( Table 2) were like those in the control but with markedly reduced concentrations ( Figure 3). Compared with control conditions all interventions produced a remarkable decrease in the number and distribution of particles detected in the extra-oral space (Location 2: 20 cm) and more distal locations. Following the end of the sequence of procedures (t=21 min) there was infrequent detection of low concentrations of aerosol particles from beyond the extra-oral space, and particles > 0.05 µm were generally at the baseline level (Figure 3).
In the control group, total particle counts remained elevated above the baseline range for the duration of the experiment at all locations ( Figure 4, and Figure S4). Therefore, for the control group linear regression was used to calculate the time needed for the total particle concentration at each location to return to baseline levels ( Figure 10). This produced an estimated median time of 26 min (range 25 -31 min) from the end of the sequence of procedures (t=21 min). In the case of experiments using either the HVS(IO), or the HVS(IO) combined with the ACS (Table 2, intervention groups B and C) the concentration of particles returned to within the baseline range at the end of the procedures (t=21 min) ( Figures 5,  and 6 respectively). However, the total number of aerosol particles remained marginally above the baseline for interventions which included the HVS(EO) (Figures 7 and 8).
When the aerosol concentrations are expressed as dose (mL cm -3 min) all interventions reduced total aerosol exposure ( Figure 9). Intervention group B ( Table 2, HVS(IO) with LVS) reduced the median dose by 80%, while intervention group E (HVS(IO)+HVS(EO)+ACS with LVS) reduced the median dose by 90%. However, HVS(IO) was noticeably less effective than intervention groups C, D and E in controlling the range of (maximum-minimum) of the dose.

Discussion
The results of this study demonstrate that all the aerosol management interventions evaluated were relatively effective in controlling aerosols generated by dental handpieces. Most particles produced by our sequence of AGPs were < 0.3 µm. The use of either the HVS(IO), or the HVS(IO) combined with the ACS was enough to reduce the fallow time to 0-min.
During AGPs the concentration of particles in the range 0.05 to 0.15 µm diameter range is increased substantially. This size range corresponds to the reported size range of the SARS-CoV2 virus (0.05 to 0.15 µm). 22 Within the working micro-environment (Locations 3-4, <50 cm) the presence of active aerosol management interventions substantially reduces the concentration of airborne particles in this range but does not eliminate them. Thus it is important for dental workers to utilise both appropriate and properly fitted respiratory protective equipment such as FFP3 masks in combination with aerosol management interventions. 24 In the absence of aerosol management interventions, particles in the range 0.05 -0.236 µm, remained at elevated concentrations within the macro-environment (Locations 5-6, >50 cm) for longer than the experimental period. Our control study estimated that it may take at least 28 to 34 minutes after cessation of AGPs for the total particle concentration to return to baseline levels. Intervention groups B and C, which included the addition of HVS(IO), or HVS(IO) with ACS, both had the effect of returning particle concentrations to within the baseline range by the end of the sequence of procedures i.e. no additional fallow-time was required before particle concentrations returned to baseline levels. In the case of interventions D and E, which included HVS(EO), particle concentrations remained marginally above the baseline which is in agreement with previous work. 19 Interventions B and C reduced particle concentrations in the macro-environment (Locations -5-6, >50 cm) to within the baseline range during AGPs. Intervention C, (HVS(IO) in combination with an ACS) was effective in controlling both the median and the range (maxmin) of the aerosol dose at all locations. In a dental surgery of the size used in this study (35 m 3 ), and in the context of SARS-CoV-2, it provides further evidence to support a reduction in fallow time below the current recommend period of 10 minutes 24 in agreement with other recent studies. 25 The use of a phantom head is a clear limitation of this study: the presence of saliva and other biological materials within the oral cavity may conceivably affect the particle size distribution of AGPs and so further, confirmatory research should be performed using patients. Such work should incorporate different size surgeries to validate the scalability of aerosol mitigation interventions. It should also be noted that a locally moist and warm atmosphere within a "turbulent gas cloud" allows the contained continuum of droplet sizes to evade evaporation for much longer time periods than occurs with isolated droplets, from a fraction of a second to minutes. 26 This may explain why the most persistent particles measured in our study were within the smaller, 0.012-0.025 µm range. Therefore, a patientorientated study is needed to confirm the nature of the fine particle aerosols containing mixtures of saliva, coolant, and pathogens. This may provide further evidence to support the use of antiviral disinfectants in coolant solutions.

Conclusions
Dental AGPs produce aerosols characterised by particles < 0.3 µm in diameter. Although, aerosol suppression treatments such as HVS(IO) alone or in combination with an ACS may rapidly reduce particle concentrations to within background range, they do not eliminate exposure during AGPs and so the use of appropriate respiratory protective equipment by dental practitioners is essential. HVS(IO) combined with the ACS was enough to reduce the fallow time to 0 minute, and to control the median and range of the aerosol particle dose at all areas in the surgery. The ACS used in these experiments was set to deliver 24 air changes per hour in an 35m 3 surgery which was close to maximum and further experimental work is needed to optimise the location and setting of equipment of this type.
In the absence of ventilation within a modest sized (35 m 3 ) surgery, particles associated with dental AGPs may persist for approximately half an hour. There appears to be scope for a reduction in fallow time from the current guideline of 10 minutes when effective aerosol management system(s) are used.
understand dental AGPs, so that dental hospitals, practices, labs and associated dental supply chain smaller businesses can remain open and operate safely through any future viral pandemics.