Aerosol formation due to a dental procedure: insights leading to the transmission of diseases to the environment

Abstract

As a result of the outbreak and diffusion of SARS-CoV-2, there has been a directive to advance medical working conditions. In dentistry, airborne particles are produced through aerosolization facilitated by dental instruments. To develop methods for reducing the risks of infection in a confined environment, understanding the nature and dynamics of these droplets is imperative and timely. This study provides the first evidence of aerosol droplet formation from an ultrasonic scalar under simulated oral conditions. State-of-the-art optical flow tracking velocimetry and shadowgraphy measurements are employed to quantitatively measure the flow velocity, trajectories and size distribution of droplets produced during a dental scaling process. The droplet sizes are found to vary from 5 µm to 300 µm; these correspond to droplet nuclei that could carry viruses. The droplet velocities also vary between 1.3 m s^-1 and 2.6 m s^-1. These observations confirm the critical role of aerosols in the transmission of disease during dental procedures, and provide invaluable knowledge for developing protocols and procedures to ensure the safety of both dentists and patients.

Keywords: COVID-19; coronavirus; dental procedures; droplet velocity; experiments; size distributions.

Figure 1.

Experimental procedures to detect aerosol formation by a CUS. (a) Set-up schematic for the OFTV technique to detect the droplet velocity and Lagrangian path. Examples of the raw OFTV images in the (b) P₁ plane and (c) P₂ plane recorded with a high-speed camera at 7.6 kHz.

Figure 2.

(a) Schematic of the experimental set-up of backlight illumination for the shadowgraphy technique. (b) An example of backlight illumination at 0.08 s recorded using a high-speed camera at 7.6 kHz and two halogen backlights.

Figure 3.

The mean field of the (a) V (y-axis) velocity component, (b) U (x-axis) velocity component and (c) velocity magnitude for the plane parallel to the CUS tip, P₁, and the (d) V (y-axis) velocity component, (e) U (x-axis) velocity component and (f) velocity magnitude for the plane perpendicular to the CUS tip, P₂. The white arrows in the figures specify the velocity vectors.

Figure 4.

Temporal evolution of the velocity magnitude of the splatter for the P₁ plane at (a) 0.01 s, (b) 0.05 s and (c) 0.10 s and for the P₂ plane at (d) 0.01 s, (e) 0.05 s and (f) 0.10 s. The white arrows in the figures specify the velocity vectors.

Figure 5.

Sequences of splatter formation at (a) 0.04 s, (b) 0.08 s and (c) 0.12 s. Image samples were recorded using a high-speed camera at 7.6 kHz and two halogen backlights.

Figure 6.

Droplet trajectories representing 20% of the detected droplets for the (a) P₁ plane and (b) P₂ plane.

Figure 7.

(a) Histogram of the droplet size distribution. (b) The velocity distribution of the droplets. (c) The Rosin–Rammler curve fitted for our obtained experimental droplet size data with a 29.5 ml min⁻¹ flow rate.