Background
In March 2020, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) outbreak emerged globally, marking the third outbreak following the 2003 Severe Acute Respiratory Syndrome and 2012 Middle East Respiratory Syndrome for human Coronavirus disease (COVID). SARS-CoV-2 caused varying degrees of fever and physical discomfort in young people. Moreover, elderly patients with underlying conditions experienced particularly severe symptoms and most succumbed to progressive respiratory failure. To curb the spread of the virus, governments worldwide implemented a series of social lockdown policies. The pandemic led to the collapse of medical systems in several regions and the closure of dentistry departments. Moreover, the global economies were hit hard.
SARS-CoV-2 is primarily transmitted through direct contacts by respiratory droplets and aerosols, with the upper air way and salivary gland serving as the early sites of infection [
1,
2]. Larger droplets carrying SARS-CoV-2 deposit heavily in the upper respiratory tract, while smaller aerosols can directly enter the lungs, leading to Lower Respiratory Tract Infections. To date, this virus has evolved a variety of variants, and there is currently no specific treatment for these mutants. Therefore, it is of utmost importance to effectively prevent the spread of the virus from contaminated sources.
In dental practice, dental handpieces have the potential to spread viruses by aerosols that disperse into the surrounding environment. Despite the implementation of various protective measures by clinical staff, such as wearing N95 masks, facial barriers, gloves, and disposable protective clothing, the close proximity between patients and clinical staff makes it difficult to completely prevent viral transmission through droplets and aerosols. This poses a heightened risk of SARS-CoV-2 infection in the clinical setting during an ongoing pandemic. Consequently, it becomes crucial to urgently address the challenge to minimize the generation of droplets and aerosols that may carry SARS-CoV-2 during dental practice.
Although a great deal of research has been conducted on protective measures to reduce the risk of SARS-CoV-2 infection in dental clinics [
3], there is limited evidence for source control strategies that aim to minimize aerosol production. In clinical practice, dental handpieces can generate varying droplets and aerosols under different dental handpiece rotational speeds and cooling conditions (air-water ratio). However, the dispersion pattern and viral load of these droplets and aerosols under different handpiece settings are still unknown. In this study, we used an advanced digital dental platform equipped with a precise controlling system to investigate the generation of droplets and aerosols during treatment process. By manipulating the rotational speed and cooling condition, we aimed to identify practical parameters that could effectively minimize the production of droplets and aerosols, thereby contributing to the control of SARS-CoV-2 transmission in clinical settings.
According to previous research studies, microbiological approaches are widely applied to mimic the transmission of viruses through droplets and aerosols. However, these studies have limitations. Bacterial particles do not fully represent the splash of viral particles as the transmission of these microbes differs from that of viruses in droplets and aerosols [
4‐
6]. The viral Spike (S) protein is crucial for SARS-CoV-2 transmission and pathogenesis, making it a significant area of research. To address the wild-type S protein’s instability issue hindering large-scale production, scientists developed a mutated version called HexaPro (S6P) to improve stability, heat tolerance, and expression level [
7,
8]. In this study, we used S6P as the model protein and spiked S6P-encoding plasmid in water to conduct a quantitative analysis of viral load at various clinical sites between patients and clinical staff. Furthermore, we evaluated the potential risk of infection based on the observed viral load. Based on the quantified results, we proposed practical strategies aimed at reducing the generation of droplets and aerosols during dental practice. These strategies could also contribute to reducing the risk of other viral infections, such as human immunodeficiency virus (HIV) and human papillomavirus (HPV) in dental setting.
Discussion
There are three main routes of transmission for infectious aerosols in dentistry, which include direct contact, surface contact, and instrument splashing [
10]. The U.S. Centers for Disease Control and Prevention (CDC) defines aerosol as a suspension containing tiny (< 5 μm) inhalable particles or droplets in the air [
11]. The World Health Organization (WHO) indicates that dental equipment could produce aerosols and droplets, increasing the transmission risk of infectious diseases [
12]. According to reports, most of the visual aerosol particles are generated by rotary dental handpieces, followed by ultrasonic dental scalers [
13]. Aerosols have the capacity to disperse through the air and readily adhere to various objects within the clinic, including tables, floors, computers, and medical staff. Unfortunately, although the large-capacity vacuum pump can reduce the propagation by more than 75%, particles smaller than 2.0 μm cannot be removed. In the context of aerosol mitigation strategies, Zhu et al.’s findings suggest that external aerosol scavenger units not only have a limited ability to eliminate aerosols but, more concerning, they may also redirect aerosol movement, thereby potentially heightening the risk of infection within the dental clinic [
14]. Supplemental internal evacuation systems prove effective only in specific situations, depending on the tooth being operated on. Therefore, it is challenging to completely eradicate the transmission of aerosols [
15]. According to reports, the most heavily contaminated areas with aerosols are the face and chests of medical staff, with no contamination observed beyond a three-meter distance from the source. Therefore, it would be meaningful to effectively manage and control the source of aerosol transmission.
Currently, the operating principle of clinically used dental turbine handpieces is driven by compressed air to rotate while avoiding overheating during the cutting process, equipped with an air-water spray cooling device [
16]. During the rotation driven by compressed air, the cooling water is splashed around. Previous studies demonstrated that the contamination level generated by high-speed handpieces is significantly higher than that by low-speed handpieces [
17,
18]. However, those studies only considered the single factor of handpiece speed and overlooked other factors that could influence the generation of aerosols. Annika Johnson et al. confirmed that traditional assistant-based irrigation and self-irrigating drills have no difference on the splashes produced, and the study also proved that hydrogen peroxide solution can produce a larger splash area than saline. However, the study did not consider the effect of different air-water ratio in the self-irrigating drills on the splash propagation [
19]. For instance, increasing the speed of rotary handpiece usually requires a higher volume of water and air, resulting in an increased potential source of contamination. Hence, compressed air and cooling water of the dental handpiece may play a role in generating aerosols. Furthermore, Han, Pingping, et al. utilized high-speed and low-speed air turbine handpieces from different brands, which might introduce inherent variations in design and performance, thus affect study outcomes [
18]. Also, previous studies have not yet discussed the optimal parameters for rotary speed and air-water ratios that can effectively minimize aerosol transmission [
17,
18]. In comparison, we utilized a digital dental chair (Planmeca Sovereign® Classic, Finland) at Columbia University College of Dental Medicine Center for Precision Dentistry. This advanced equipment allows for precise adjustments of the handpiece speed and air-water ratio. Throughout the study, we consistently used the same dental handpiece and modified its settings via the control panel. This consistent methodology enabled accurate assessment of the impact of the handpiece on aerosol and splatter generation within the dental operatory and provided practical strategies for reducing droplets and aerosols during dental treatment.
Previous studies have identified several methods for collecting aerosols and splashes, such as instrumental, optical [
12], filter paper [
20], and spectroscopic methods [
13,
21]. These techniques can analyze the collected splatter and aerosols using microbiological methods to determine their shape, size, and fluorescence intensity. However, instrumental methods can measure particle concentration but are limited to particles of a fixed size, making it challenging to discern viral particles [
22,
23]. Similarly, filter paper, optical, and spectroscopic methods are unable to measure particle concentration, providing only droplet counts [
9,
20,
24,
25]. Microbiological methods typically prioritize the detection of alpha-hemolytic streptococci or anaerobes, often neglecting viruses [
6,
10]. The latest research utilized state-of-the-art experimental fluid mechanics tools to detect the number and the transmission speed of aerosol droplets through the advanced high-speed imaging techniques and optical flow tracking velocimetry. The initial velocity of these droplets can be quite significant, typically ranging between 1 m/s and 2.6 m/s [
14,
26,
27].
Those studies concentrated on analyzing the properties of aerosol generation at a constant rotational speed, noting that the transmission path of aerosols varied with the turbine’s direction. In contrast, the purpose of our research is to find the best combination of generating less aerosol by precisely adjusting the rotation speed of the dental handpiece and the air-water ratio, so as to explore the methods of protecting oral hygiene professionals [
14,
26,
27]. In this study, a simulated clinical working environment was established, and the transmission of aerosols and droplets was observed from various directions and angles on patients and medical staff. A significant concentration of aerosols and droplets was observed within 60 cm of the contamination source, with the highest concentration observed within 30 cm. Limited aerosols and droplets were found at 30–60 cm from the source. A previous study also demonstrated the greatest level of contamination from a dental procedure within a 1-ft radius of the source, diminishing at 2 ft [
28]. Furthermore, our findings indicate that the operator and assistant positions displayed a higher degree of contamination when compared to other positions. Additionally, there was a notably greater level of contamination observed around the operator in comparison to the assistant. In the operator area, the maximum contamination was on the operator’s face shield, followed by the left arm, and the right arm of the operator exhibited minimal contamination. In the assistant area, the maximum contamination was on the left arm of assistant, followed by the right arm. The face shield of the assistant exhibited minimal contamination. Veena et al. reported that the right arm of the operator displayed the most contamination [
28], while the assistant area was consistent with our results. However, a recent study has presented contrasting findings, revealing that the highest concentration of deposited splatters is primarily located on the patient’s chest, followed by the assistant’s face shield [
19]. While there is no unanimous agreement regarding the exact location with the highest contamination within the operatory, it is evident that operators, assistants, and patients all face potential exposure to splatters and aerosol contaminants. Additionally, Zhu et al. revealed that the use of barriers can significantly reduce aerosol and splatter levels [
14]. These findings altogether underscore the importance and effectiveness of implementing personal protective equipment (PPE) in dental settings.
When the air-water ratio was fixed, increase in dental handpiece speed significantly raised the precipitation amounts of droplets and aerosols. However, the air-water ratio variations did not have a significant effect on the overall amount of collected splatters and aerosols. One plausible explanation might be that an increase in water proportion enhances the weight of the splash droplets, leading to a reduced travel distance. As splatters were only captured at a distance of 30 cm or more, the heavier, shorter-traveling droplets might fall closer to the source. This missed capture of droplets could explain why changes in the water proportion do not appear to impact the measured quantities of splatter and aerosols. The detection limit, therefore, may not represent the full scope of splatter distributions, particularly for droplets that fall within a shorter radius due to increased weight from higher water content. When dental handpiece speeds were fixed at 10,000, 30,000, and 40,000 rpm, the precipitation amounts of droplets and aerosols increased with decreasing air-water ratios at the position of the operator. Therefore, besides the proper protection, it is necessary to use a large suction aspirator at the position of the operator. However, at the assistant side, the precipitation amounts of droplets and aerosols increased with decreasing air-water ratios only when the turbine handpiece speed was at 10,000 rpm. These results demonstrate that reducing the turbine handpiece speed and increasing the air-water ratios can effectively reduce the precipitation amounts of droplets and aerosols in clinical setting.
In addition to the commonly used capturing fluorescein dye with filter papers, we innovatively replaced the filter paper with petri dish to collect splattered liquid drops containing S6P-encoding plasmid to quantify viral load. However, in previous aerosol research methods, S6P plasmids were not introduced into the experiments [
14,
19,
26,
27]. We then analyzed the copy number of the S6P plasmid through quantitative PCR, enabling the assessment of viral concentration and transmission. In prior research, the assessment of aerosol contamination has primarily centered on bacteria [
6,
10]. However, bacteria are relatively large so they can only provide an indication of the extent of droplet splatter rather than the finer aerosol particles. Virus can also potentially have a wider transmission range compared to bacteria due to its smaller size [
10]. Zemouri et al. revealed that the bacteria-containing aerosols generated by dental treatments were mainly distributed around patient’s head, which align with our results on aerosol distribution [
25].
During the COVID-19 outbreak, aerosols containing SARS-CoV-2 virus particles were a major source of contamination in dental clinics. The size of SARS-CoV-2 is 0.005–0.2 μm [
29]. The spread of virus particles is different from that of larger bacterial particles, as they can be easily transmitted via aerosols and remain suspended in the air for several hours. SARS-CoV-2 can survive for up to 72 hours on the surface of stainless steel and plastic, up to 24 hours on the surface of cardboard, and up to 4 hours on the surface of copper. Moreover, it can survive up to 1 day on clothes, up to 7 days on the outer layer of surgical masks, and up to 2 weeks at low temperatures [
30,
31]. Similar to the SARS virus, SARS-CoV-2 recognizes human ACE2 protein by viral spike protein, internalizing into cells.
According to previous study, medical staff could be exposed to 0.014–0.12 μl of saliva after 15 minutes of treatment with a high-speed dental handpiece [
32]. The saliva of infected patients contains the SARS-CoV-2 virus, with a median viral load of 3.3 × 10
6 copies/mL [
33]. In this study, the viral load was significantly different at various distances and directions.
Recent computer simulations by Jonathan Komperda et al. provides valuable insight into the field [
34]. Nonetheless, our study was carried out in a single operatory, providing a more direct and realistic reflection of actual clinical conditions as opposed to those derived from simulations set within the complex environment of a large dental clinic. Moreover, computer or numerical simulation software necessitates specific parameters for calculations. The omission of crucial parameters, including the rotational speed of dental turbine handpieces, air-water ratio, and aerosol splashing distance, may significantly impact the study outcomes. The primary objective of our study was to closely replicate actual clinical procedures, thereby enabling the experimental results and findings applicable to dental practice and instrumental in determining necessary parameters to for future modeling efforts.
In summary, this study thoroughly examined the impact of dental chair operating parameters, including rotational speed, air-water ratio, and distance and angle from the pollution source, on the distribution of SARS-CoV-2 aerosol, providing practical insights for dental treatment. Our findings indicated the transmission capacity of aerosols containing SARS-CoV-2 virus decreased with reducing rotational speed, under any fixed air-water ratios tested. However, this study still has some limitations. Firstly, the study concentrated exclusively on maxillary central incisors, but variations in tooth position and consequent adjustments in operator position may influence splatter patterns and outcomes. Secondly, the influence of treating duration on viral load remains unknown. Although we simulated the dental treatments by aerosol splash experiment, the effectiveness and accuracy of the aerosol splash experiment still need to be further confirmed in real clinical settings.