Investigation of the Effectiveness of Infection Control Measures in the Dental Office

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Introduction
Aerosol transmission is an important mode of COVID-19 (SARS-CoV-2) infection [1]. Dentists and dental hygienists, who provide medical treatments and care in the oral cavity, may be exposed to expiratory aerosols from asymptomatic infected persons without their masks. Although there have been several detailed measurements of aerosol generation during dental procedures and oral cleanings [2,3], studies on specific measures to reduce the risk of aerosol transmission are limited.
The situation when providing dental care, wherein something near another person's face must be treated without their mask, can be similar to other situations, such as providing food to the elderly and caring for young children. Therefore, clarification of the mechanism of infection and evaluation of riskmitigation measures in dental treatment situations are expected to make a useful contribution.
In this study, we measured the change in the number of inhaled particles in the breathing zone of dentists to investigate the typical dental treatment situation of aerosol dispersion and clarify the effects of circulators, air purifying filters, and vacuum on reducing the risk of infection for dentists and hygienists.

Measurement site
The experiments were conducted in the operating room of the Nippon Dental University Hospital, as shown in Fig. 1. The size of the room was W4.1 m x D6.7 m x H2.6 m. The two dental units were placed in the operating room.
A thermal manikin for the patient and a manikin for the dentist were placed on the window side of the dental unit. The distance between the patient's face and dentist's face was approximately 0.5 m, assuming that the patient was undergoing a dental procedure. Thermal manikin as a patient was operated in the comfort mode, the manikin as the dentist was coiled with an electric wire heater, and heat generation was adjusted to 70 W.
The HVAC system of the operating room is a combination of a ventilation system using an outdoor air handling unit and an air conditioning system using a concealed packaged air conditioner. The air flow rate of the ventilation system is 5.6 ACH (air change per hour), and that of the air conditioning system is 14.7 ACH. Exhaust air was discharged from an adjacent disinfection room through a duct between the operating and disinfection rooms.

Measurement cases
In this study, ventilation rate measurements and aerosol dispersion measurements were conducted to investigate the effectiveness of circulators, air purification filters, and vacuums in reducing the risk of infection (in Table  1, Fig.2). Case a is the basic condition of conventional air conditioning, case b is the condition in which the circulator was placed at the center of the operating room and operated at high air flow mode, case c is the condition in which the circulator was placed behind the patient and operated at low air flow mode, case d is the condition in which the circulator blew the air from inside the room to the outside of the door in order to increase    The circulator, shown in Fig. 3, used in cases b, c, and d, is an experimental prototype to clarify the effectiveness of the circulator and air-purifying filters, and the filters can be detached. In this study, experiments were conducted with and without the filters. The prototype had two fans, the upper one stacked under one, to enable experiments with high air volumes. In cases b and c, both fans were operated in the high air flow mode, while in case d, only the upper fan was operated in the low air flow mode to avoid discomfort caused by the airflow. The detail position of the circulator and airflow rates in each case are summarized in Fig. 4 and Table 2, respectively.

Measurement method
Measurement items and locations are shown in Table 3 and Fig. 1.

Thermal environment
The outlet and inlet temperatures of the HVAC system and indoor thermal environment were measured continuously at 1 min intervals throughout the measurement period.

Ventilation Efficiency
For cases a-0, a-1, b-1, and d, air change rates were measured using the step-down method. CO2 was used as the tracer gas. CO2 concentration was measured at six locations, as shown in Fig. 1: C1 (the dentist's breathing zone), C2 (the patient's breathing zone), C3 (the Table 2. Airflow volume of circulator used in this study

Case name
Operation condition  position opposite the doctor across the dental unit, assuming the hygienist's breath zone), C4 (the chair of the adjacent dental unit), C5 (a corner of the room), and C6 (inlet of air path duct).

Aerosol concentration near the mouth of dentist
In this measurement, aerosols were sprayed from the mouth of the patient manikin, and the number of inhaled particles at P1 (the doctor's mouth, FL+1.55 m), P2 (the doctor's hand, FL+1.00 m), P3 (the hygienist's mouth, FL+1.55 m), P4 (the chair of the adjacent dental unit), and P5 (the inlet of the air path duct) were measured at 10 s intervals for 10 min after aerosol injection. In this study, based on the previous research that aerosols of 1 Pm or less play a significant role in the transmission of COVID-19 [4], we focused on the number of particles of 0.3 -0.5 Pm, the smallest particle size level that can be measured by the particle counter used for this measurement.
"Simulated saliva" [5], a mixture of water, salt, and protein, was used as the aerosol particles for simulating diffuse behavior of actual droplet nucleus. Aerosol particles were sprayed by applying 20 PSI pressure to a nebulizer (CH Technologies, 6 jet type) for 5 s.
The experimental schedule is shown in Fig. 4. The particle counter cannot distinguish between background dust and aerosols derived from the nebulizer; therefore, to reduce the background particle concentration, the air purifier was operated for 10 min after each measurement, and then the operating room was closed and unoccupied 5 min before the aerosol was injected. However, the background aerosol concentration was not zero; therefore, the average number of inhaled particles at each location during the minute before aerosol injection, as the background concentration at that location, was subtracted from the measured inhaled particles at each time. The incremental number of particles after aerosol injection was evaluated as aerosols derived from the patient's exhaled air. In case a-1, the peak number concentration of particles 0.3 -0.5 Pm near the dentist's mouth reached approximately 60 times the background number concentration.
In the case of unsteady injection conditions, such as a breath or a cough, the diffusion behavior of the aerosol is not stable owing to instantaneous differences in the flow field at the moment of aerosol injection. Thus, in this study, the same procedure was repeated three times for each measurement case, and the temporal changes in concentration during the evaluation period were ensemble-averaged to evaluate typical temporal changes in aerosol concentration for each case.

HVAC condition and thermal environment
The operational conditions of the HVAC system and thermal environment during the entire measurement period are shown in Figs. 6 and 7. The supply air temperatures were unstable for both the air conditioner  Table 4 shows the air change rate evaluated from the time variation of CO2 concentration at each location, and Fig. 8 shows the time variation of CO2 concentration at the doctor's mouth and at the inlet of the duct between the operating room and disinfection room in case d. The air change rate was about 0.6 ACH when the HVAC system was turned off (case a-0) and 1.7-1.8 ACH during the normal air conditioning (case a-1). The measured air change rate was lower than the designed air change rate, but this may be because the disinfection room was open during the measurement. The air change rate in case b was also 1.7 to 1.8 ACH as in normal air conditioning (case a-1), and the circulator was not effective in terms of ventilation efficiency under the present measurement conditions. The air change rate in case d was 9 ACH, and the air change rate was about 5-times higher than in normal air conditioning (case a-1), However, air backflow was observed from the duct between the operating room and the disinfection room.

Aerosol concentration near the mouth of dentist
The incremental number of particles during the 5 min after aerosol injection at locations P1, P2, P3, and P4 is shown in Fig. 9 for each case.

Case a-1 (Basic case)
At P1 (dentist's mouth), the number of inhaled particles increased and fell rapidly within approximately 1 min after aerosol injection. Thereafter, the number of inhaled particles remained nearly constant during the evaluation period. The peak after aerosol injection occurred because the dense droplets reached the doctor's mouth, and the constant number of inhaled particles after the peak indicates that the aerosol diluted by airflow drifted around the dental unit. The number of inhaled particles at P2 (doctor's hand) was smaller than that at P1 (doctor's mouth). This result indicates that the droplets diffused upward from the patient's mouth owing to the thermal plume generated by the human body.
The temporal change in the number of inhaled particles at P3 (hygienist's mouth) showed the same trend as that at P1 (dentist's mouth).
No peak was observed at P4 (the chair of the adjacent dental unit), but the number of inhaled particles increased gradually approximately 30 s after the aerosol injection. This indicates that the droplets from the patient were diluted and spread to the adjacent dental unit.

Case b (The circulator placed in the center of the room)
In both cases b-0 and b-1, compared to basic case a-1, the time required for the peak of inhaled particles caused by aerosol injection to decay was reduced at P1 and P3 (dentist's and hygienist's mouths), which means that dentists are exposed to a smaller amount of dense droplets. In case b-1, in the case of the filter, the number of inhaled particles after the peak decreased gradually, whereas the inhaled number of particles after the peak remained almost constant in case a-1.

Case c (The circulator placed behind the patient)
Similar to case b, the time required for the peak of the number of inhaled particles to decay was shortened at P1 (doctor's mouth) compared to case a-1. The peak was not observed at P3 (hygienist's mouth), which may be due to the location of the circulator in a position where the hygienist was upwind of the patient. In addition, in case c-1, the case with the filter, a gradual decrease in the number of inhaled particles after the peak was observed, but this was not as obvious as in case b-1, because the air flow rate of case c-1 was approximately 1/3 of that of case b-1.

Case d (The circulator blows air from inside the room to outside)
The peak in the number of inhaled particles at P1 (dentist's mouth) was not as large as in other cases, except for case 3, and the number of particles inhaled at P3 (hygienist's mouth) decreased faster than that in a-1. However, since there was air exchange between the operating room and outside in this case, it is impossible to determine the source of the inhaled particles after the peak passed, whether the particles observed were generated aerosols or dust from the outside.

Case e (Extraoral vacuum)
There was almost no increase in the number of inhaled particles at any measurement location.

Case f (Neck fan)
The number of inhaled particles at P1 (dentist's mouth) decreased faster than in case a-1. There was no significant difference in the change in the number of inhaled particles compared with Case a-1 at the other measurement locations. Fig. 10 shows the average wind velocity and standard deviation (S.D.) for a total of 30 min for the three evaluation times in each case. In case b, when the circulator was placed in the center of the room, the wind velocity and S.D. increased at a height of the patient's mouth, and in case c, the case where the circulator was placed behind the patient, the wind velocity and S.D. were also increased at the height of the doctor's mouth. In case d, when the circulator was facing out of the window, both the wind velocity and S.D. did not increase at any location. In addition, in case f (extraoral vacuum) and case e (neck fan), both the wind velocity and S.D. did not change at the measurement locations, and the ranges of influence of these devices on airflow were limited to a close area.

Conclusion
Several measurements were conducted to clarify the effects of the circulator, air purifying filter, and vacuum, on reducing the infection risk to dentists and hygienists. The following conclusions were drawn from this study.
x Under these measurement conditions, in a normal air-conditioned case, the droplet cloud from the patient remained near the dentist and hygienist for approximately 1 min. x When an extraoral vacuum was placed at 30 cm from patient's mouth, the number of inhaled particles was almost zero, suggesting that the risk reduction effect was significant. x In the three cases, where circulator was placed at different positions in the room, and in the case where a neck fan was attached to the dentist, the time exposed to the droplet cloud was reduced compared to the case with normal air conditioning. x The use of air purifying filters could remove the diluted droplets remaining in the room, but the effect was not significant when using a filter fan unit with a low air flow rate.
x The results with the circulator indicate that the increase in the background wind speed and its fluctuation contribute to the rapid dilution of the respiratory droplet cloud, leading to mitigation of the risk of infection by aerosols.