In August 2020, the world health organization (WHO) guidelines recommend delaying routine nonessential oral health care amid COVID-19 pandemic and call for more research on indoor aerosol generated by dental procedures. The reason is that dental professionals, staff, and patients in dental offices are exposed to aerosol droplets, particles, and pathogenic microorganisms in the saliva and blood of the infected patients. The infectious microorganisms transmitted from saliva and nasopharyngeal secretions include pneumonic plague, Legionella pneumophila, tuberculosis, influenza viruses, herpes viruses, SARS virus (a form of coronavirus), pathogenic streptococci and staphylococci, HIV, and hepatitis viruses [1, 2] . airborne and spread to the room. Extraoral high volume evacuators (EHVE) can also be used to remove the aerosol particles near to the area of particle generation [1] ; however, its performance depends on the volumetric rate of evacuation and particle generation rate. In addition, using extra devices around the dental unit causes a restricted environment and inconvenience to the dentists. Recent COVID-19 outbreak has resulted in increased use of portable air purifiers in dental offices, despite the scarcity of published research on their performances in dental offices [26, 27] . Further research on the protective effectiveness of air purifiers in dental clinics was recommended [28] . The portable air purifiers can be located at the corners in the dental offices, and they cause much less inconvenience during dental operations than extra-oral high evacuators do. In addition, these portable air purifiers do not require modification to existing ventilation systems.
Despite earlier research on number concentrations for micron [29, 30] and nano-size particles [31] [32] [33] related to the dental processes, to the best of our knowledge, no research has been done on the dispersion or transport of airborne particles lingering in different parts of the office. The nature of the extensive surface area in dental offices may enhance the losses of particles onto various surfaces. Furthermore, research on the effects of air purifiers is needed to develop guidelines and protocols to reduce waiting time between patients and ensure the safe operation of dental offices.
The objective of this study is to understand the spatial and temporal concentration-distribution of airborne particles generated from dental procedures in dental offices. The remainder of this paper is presented as follows. Section 2 presents the experimental design of concentration measurements in the dental office.
Section 3.1 reports the number concentration distribution of particle under the effects of operating conditions during the generation; Section 3.2, the spatial and temporal change of particle concentrations distribution under the effects of operating conditions at the generation zone; Section 3.3, at the corner of the office. The results reveal the effective removal mechanisms that depend on particle size. Finally, Section 5 summarizes the entire work. Results in this paper are deemed valuable to the best practices for particle removal from dental offices.
The concentrations of micron and submicron particles were measured on May 15, 2020 in a dental operation room on the second floor of the dental clinic in Toronto, Ontario, Canada. Figure 1 shows the schematic of the operatory and layout of the instruments. This typical dental operatory room is 3 m wide, 3 m long, and 4 m high; it has one central dental unit. The mechanical ventilation system was turned off and the window was closed throughout the test. The temperature and relative humidity of the room air were 13.4 0 C and 88%, respectively. range of 0.5-20 µm in diameter and those smaller than 0.5 µm. The APS was located on the left-hand side of the doctor, to prevent any inconvenience for the doctor during dental operations. A stainless-steel sampling tube, which is 1/4-inch of inner diameter and 0.3 m long, was connected to the inlet of the APS for sampling air 10 cm away from the operation area (i.e., the patient's mouth). Both OPCs were running continuously. One OPC was located beside the APS, and another OPC was 1.8 m away from the source.
Both OPCs report particles with diameters of 0.3, 0.5, 1, 2, 5, 10 µm. The first OPC is calibrated against the APS.
Before the operation, the room was unoccupied for 15 hours before the background concentrations were measured at the source without air purification. As seen in Figure 2 , all particles in the background air were less than 10 #/cm 3 and those larger than 1 m in diameter were less than 1 #/cm 3 . Airborne particles were generated over 5 min of continuous drilling operation (high-speed handpiece) using a pig jaw. Pig teeth are commonly used for dental studies because of similarities between the structure of human and pig enamel and dentin [34, 35] . The particle number concentrations were measured during 5 min of continuous dental operation and afterward until the number concentrations reached the background.
Then we measured the airborne particle concentrations under six scenarios. conditions. For all scenarios, the smaller the particle size, the higher concentration is.
In closed-door scenarios, by comparing the scenario that no air purifier is running (Figure 3 .a) with the scenario that the air purifier is running at the beginning of operation (Figure 3 .b), it can be observed that particles have a wider distribution in Figure 3 .b, which means particles are growing to the larger sizes. For instance, the concentration of higher than 200 #/cm 3 is observed for 0.5-1.3 m particles in Figure 3 .a, while, this range of concentration is observed for 0.5-1.5 m particles in Figure 3 From this observation, it can be inferred that running the air purifier from the beginning causes air circulation in the room. The air circulation can enhance the interaction between airborne particles leading to agglomeration in the area that particles are generated [36] . Thus, the particles may grow to the larger ones when the air purifier was on at the beginning of the operation. Growing to larger sizes is preferable in terms of particle removal. Removal by HEPA filter is size dependant; the larger sizes, the more probable filtration is. The filtration of micron particles is due to interception and impaction [37].
growing particles to larger sizes during the first 5 min while the air purifier was running from the beginning of the operation. The concentration of higher than 200 #/cm 3 is observed for 0.5-1 m particles in Figure   3 .a (air purifier off), while, this range of concentration is observed for 0.5-1.4 m particles in Figure 3 .d.
Moreover, the concentration of 200-70 #/cm 3 is detected for 1-2.2 m particles in Figure 3 .c, however 1.4-2.5 m particles have this concentration range in Figure 3 .d.
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The particles generated in the 5-min long operation gradually spread in the room, and their concentrations were decreased by different mechanisms. They are introduced in the next sections. Table 3 summarizes the times it takes for the number concentrations to reach their background levels (removal times) for all six scenarios. In the worst-scenario scenario, when the door is closed and no air purifier is running in the room, it takes 95 min for 0.5 m particles to return to the background level. air purifier. Figure 4a shows the lowest particle concentrations in the room when the high-speed air purifier is running from the beginning of the operation. However, the removal time is almost the same for all these 3 scenarios: low-speed air purification after the dental operation, high-speed air purification after the dental operation, and high-speed air purification from the beginning of the operation. It can be inferred that particles were captured with the HEPA filter and Activated Carbon Filter installed in the air purifier. In addition to filtration, enhancing air circulation in the room by the air purifier leads to faster particle settlement on the surface areas. These results suggest that air purifier has a crucial role in removing airborne contamination of dental offices in the generation zone. is recommended as a short term solution for the dental offices without air filtration systems.
The particle removal time varies with particle size although the air purifier and open door help reduce the concentration of all-size particles in the generation zone. The next section elaborates on the size dependency of particle spread and removal because smaller particles probably carry more infectious microorganisms because the concentration of smaller particles is higher than the larger ones. Table 3 . At the beginning of the dental operation 8
There are several mechanisms of particle removal from the air including settling, air circulation, and air filtration. First, all particles in a closed-door room without air circulation or filtration settle down because of gravity. It is well-known that the larger particles have higher gravitational settling velocity and that their removal times are shorter than the smaller particles. Figure 5a further confirms this mechanism. For example, 2.5-m particles disappeared faster than those that were smaller. Second, air circulation leads to the dispersion of particles and their subsequent removal by settling on the surface areas or exiting the room or both. The drag force on a particle is also size-dependent. It usually takes a longer time for a larger particle to disperse than the smaller ones do. Figure 5e indicates that air circulation through the open door expedited the particle removal, although the air purifier was off. In addition, Figure 5e shows expedited removal of smaller particles and confirms that air circulation is the dominant mechanism in this scenario. Third, the filtration efficiency is also size dependant and it increased with the particle size for micron particles [37] . Moreover, Figure 5 .b, 5.c, and 5.d show that the removal times do not vary with particle size. Therefore, a combination of settling, air circulation, and air filtration all play roles in particle removal for these scenarios. Comparing these scenarios with that in Figure 5f demonstrates the strong effects of air circulation
due to the open door.
In summary, an air purifier running at high fan speed may ensure the removal of 0.5 to 3 m particles, while air circulation is more effective for smaller particles. Since the door of dental offices might be open frequently, an air purifier with a strong fan may help prevent cross-contamination from one room to the other through the door. Nonetheless, our study herein does not undermine the effectiveness of external highvolume evacuation (EHVE) and suction, which are often used near to the generation zone.
However, it does not mean that the room is completely cleaned even when the particle concentrations in the generation zone dropped back to the background. The particles may be transported from the source to the rest of the room. Dental staff walks around in the same room, and they often remove their masks for a short break at the corner, where there is little air circulation. It is necessary to investigate the spread of particles by analyzing the concentration at the corner of the room, and the results are presented in the next section. These results indicate the effectiveness of high-speed high-efficiency air purification. Generally, it can be inferred that the peak is observed in the corner when the rate of particle settlement and removal from the air is lower than particle transport to the corner. Table 4 indicates that it took 6 min for the concentration peak to reach the corner when The travel time of the concentration peak and peak concentration ratios are close to each other for the three closed-door scenarios including air purifier off (Figure 6a ), low-speed air purifier running after the operation (Figure 6b) , and high-speed air purifier running after the operation (Figure 6c) . Thus, the same fraction of particles reaches the corner at the same time for these scenarios. This is surprising because these results imply that the air circulation result from the air purifier has little impact on the air movement to the corner of the room (See Figure 1 ). removal mechanisms. Second, the peak is observed in the corner when the rate of particle settlement and removal from the air is lower than particle transport to the corner. Thus, a fraction of 1, 2.5 m particles, which is not removed from the air, traveled to the corner. 
The following conclusions can be drawn from the results of this study:
• In the worst-scenario scenario with no protection system in the closed-door office and continuous high-speed drilling, it takes 95 min for 0.5 m particles to return to background level and that it takes a shorter time for particles larger than 0.5 m to be removed from the air. In the real operations with the patient, which usually is less than five minutes, air may be cleaner because of other measures like suction from the source (i.e., the mouth).
• There are three size-dependent mechanisms for particle removal: gravity settling, air circulation, and air filtration. Technologies that combine all of them are the most effective in air cleaning. The air purifier expedited the removal time at least 6.3 times faster than the scenario with no air purifier in the generation zone. Running high-speed air purifier at the beginning of the operation is the most effective scenario in reducing airborne particle concentrations. The air purifier at one corner could not eliminate the concentration peak in the other corner of the room except for the scenario when the door was closed and the air purifier was running at the highest speed from the beginning of the operation.
• 
In the second part of this study, the number concentrations were measured for three dental operations with real patients. The air ventilation system was blocked, and the door was closed, however, it was opened several times during the operations. The first operation was conducted in 2 parts, shown in by patterned area. The higher concentrations entered the room from outside. After closing the door, the number concentration was reduced by the air purifier. Moreover, the concentration peaks were observed, in the moments that the door was open. The major fraction of particles was generated in the second part of the operation. During this time, the air purifier was running at low speed in 7 min and turbo speed in 7 min. In the first 7 min, the removal rate was 0.28 (#/cm 3 min) and the second 7 min was 1.14 (#/cm 3 min), 4 times faster than the time with low speed.
The second operation was conducted in a single part, and no considerable particles were measured. Similar to the fist operation, the number concentration of outside was higher than inside.
The number concentration in the third operation was higher than the first two operations. The third operation was conducted in 2 parts. Higher values of concentration coming from outside are observed in this operation comparing to the first two because APS was closer to the door in 3 rd operation.