A public health emergency intimidated the world in 2019, soon with a pandemic announced by the World Health Organization (WHO) with the emergence of an abstruse virus [18], SARS-CoV-2. COVID-19 caused by SARS-CoV-2 has had the most remarkable impact on dental treatments and education in living memory. Operators are among the most exposed health professionals, considering the high biological risk of contamination through saliva, blood, aerosols and/or droplets produced during dental procedures [19, 20]. Furthermore, dental treatments are diverse and can expose not only dentists, but also attending assistants and other co-medical personnel. The present study was conducted to examine these important issues, and highly useful and novel results were obtained.

Since the spread of COVID-19 infections worldwide, a large number of studies have been conducted regarding devices for protection from aerosols generated during dental procedures. In one of them, Noordien et al. reported extra-oral dental aerosol suction device demonstrated a 50% reduction in the contamination of the dental care worker wrists and a 30% reduction in chest contamination [21]. However, although these studies describe the usefulness of EOV, they do not provide a comparison with the use of IOV or a detailed analysis of the angles of EOV. The present study investigated the effectiveness of suction devices used inside and outside the oral cavity, as shown in Fig. 4. The range of aerosol and droplets diffusion was found to be decreased when both the EOV + IOV were used, as previously reported [22,23,24,25], while the droplet range became wider when only the IOV was used alone. Aerosols and droplet amounts were quantitatively measured by analyzing the scattered light intensity of aerosols generated in four different areas of the face of the mannequin, as shown in Fig. 2. In areas 1 and 2, droplet volume was significantly reduced with the use of an EOV, suggesting its usefulness. The decrease in droplet volume noted in area 3, located in the perinasal area, due to the collection of aerosols and droplets in the direction of the lower face owing to the EOV, which may prevent patient infection. Some authors have suggested that extra-oral motor-driven suction devices are useful tools to reduce aerosols during dental treatment [25,26,27], which was confirmed by the present findings. It is speculated this is because angle and direction of the IOV prevented adequate suction, causing the aerosols and droplets to scatter in upward direction. Furthermore, it is suggested that the droplets range differs depending on mode of operation of the vacuum by the assistant.

One of the originalities of this study is that it focused on the patient's body position. Dental treatments are not always performed with the patients in a horizontal position in the dental chair. For those who are bedridden patients or those with back pain, the chair may be set at 45 or 90 degrees for treatment. In addition, dental treatment is often performed under adverse conditions during a home visit, thus it is important to verify whether position of patient has effects on the range of aerosol and droplets exposure.

One of the unique results obtained in this study is that the range of aerosols and droplets differed depending on the angle of the body when positioned for dental care. The luminance increased in areas 3 and 4 when the patient's body position was 45 degrees, suggesting that the droplets range differs according to body position of the patient. In addition, use of both IOV and EOV in area 3 amplified the droplet volume. These results suggest that the range of aerosols and droplets vary depending on the patient body position, which is especially important when using IOV and EOV.

　Water-sensitive paper is a simple and effective tool that can be employed by clinicians to check the diffusion range of aerosols and droplets. To determine the area of exposure, water-sensitive papers were placed on the face of mannequin and of the dentist. Even when an EOV was used, areas of exposure were confirmed. Although the EOV is effective, it would still be necessary to cover face of the patient with a towel or something similar.

The water-sensitive paper placed on the face of operator showed no heavy staining in any of the body positions tested. Surprisingly, use of an EOV at 45 degrees increased the water-sensitive area, suggesting the need to use an appropriate EOV device according to the patient position. Thus, the present findings suggest that an EOV is useful for oral surgery, though, it is necessary to consider both the patient and the dentist positions when operating the vacuum. They are also very useful for understanding the area of infection risk when providing dental treatment to patients with a COVID-19 infection. EOV may be one of the dental tools useful for decreasing the spread of pathogens included in aerosols and droplets, such as viruses and bacteria, thus it is recommended that dentists use an appropriate device for dental clinic staff for prevention of COVID-19 and other type of infections. Furthermore, the droplet diameter was also high at about 200–300 μm in all situations, suggesting that the increase in droplet volume is not directly related to the size of the droplet molecules.

Sinjari et al. described that being dental pulp inflammation one of the most frequent performed treatments during lockdown [28]. Sometimes cutting instruments such as an air turbine must be used when acute inflammation occurs, resulting in aerosols. The current guidelines suggested the use of airborne spreading devices should be minimized in order to reduce the production of PM particle in the situation of no mechanical ventilation. Based on this, Our results suggest that aggressive use of EOV and IOV during dental treatment can reduce aerosol generation.

This study is considered to be highly original because quantitative and qualitative analysis was conducted using a high-speed camera, and quantitative analysis was conducted using water-sensitive paper at the same time. Specifically, a comparison of aerosol and droplets generation during medical treatment, patient position, and angle of EOV can be made. We believe that the findings of this study will contribute to infection control during dental treatment in the future.

One of the limitations of this study is that aerosols and droplets are difficult to distinguish, and the precise particle size cannot be determined using the images obtained. Furthermore, the high-speed camera only captured images from a single direction, thus it was only possible to evaluate the droplet volume in two dimensions, which may have affected the accuracy of the evaluation. A future study that includes imaging from other directions or analysis in three dimensions. Also, it is impossible for IOV and EOV devices to aspirate all the aerosols and droplets generated in the room. So, it is necessary to take measures regarding remaining aerosols that cannot be aspirated by use of a vacuums. Kun-Szabó et al. recommended ventilation for at least 15 min after aerosol generation [29]. One of this solution is the use of air purifiers with high-efficiency particulate air (HEPA) filters or wearing proper personal protective equipment (PPE) [30]. Additionally, factors such as humidity, temperature, and air velocity are affected by air conditioners and humidifiers in the room, thus the results of the experiment may differ. Related research of aerosol and droplets should focus on humidity and temperature. Finally, this study did not use a particle counter to verify particle size and it will be necessary to compare the results with the splash range noted with water-sensitive paper.