Droplet size distribution, atomization mechanism and dynamics of dental aerosols

Abstract

Since the outbreak of COVID-19 pandemic, maintaining safety in dental operations has challenged health care providers and policy makers. Studies on dental aerosols often focus on bacterial viability or particle size measurements inside dental offices during and after dental procedures, which limits their conclusions to specific cases. Fundamental understanding on atomization mechanism and dynamics of dental aerosols are needed while assessing the risks. Most dental instruments feature a build-in atomizer. Dental aerosols that are produced by ultrasonic or rotary atomization are considered to pose the highest risks. In this work, we aimed to characterize dental aerosols produced by both methods, namely by Mectron PIEZOSURGERY® and KaVo EXPERTtorque™. Droplet size distributions and velocities were measured with a high-speed camera and a rail system. By fitting the data to probability density distributions and using empirical equations to predict droplet sizes, we were able to postulate the main factors that determine droplet sizes. Both dental instruments had wide size distributions including small droplets. Droplet size distribution changed based on operational parameters such as liquid flow rate or air pressure. With a larger fraction of small droplets, rotary atomization poses a higher risk. With the measured velocities reaching up to 5 m s^-1, droplets can easily reach the dentist in a few seconds. Small droplets can evaporate completely before reaching the ground and can be suspended in the air for a long time. We suggest that relative humidity in dental offices are adjusted to 50% to prevent fast evaporation while maintaining comfort in the office. This can reduce the risk of disease transmission among patients. We recommend that dentists wear a face shield and N95/FFP2/KN95 masks instead of surgical masks. We believe that this work gives health-care professionals, policy makers and engineers who design dental instruments insights into a safer dental practice.

Keywords: Airborne transmission; COVID-19; Dental aerosols; Dental drill; Droplet size distribution; PIEZOSURGERY.

Fig. 1

Experimental systems. a) The Mectron PIEZOSURGERY® with the camera and the rail system, b) the Mectron PIEZOSURGERY® hand piece, c) the KaVo EXPERTtorque™ Lux E680 with the camera and the rail system, d) the hand piece of the KaVo EXPERTtorque™ Lux E680.

Fig. 2

Number-weighted probability density (pd) distribution of droplet diameters of Mectron PIEZOSURGERY® at irrigation levels (a) 1, (b) 3, and (c) 6. Volume-weighted pd distributions of droplet diameters of Mectron PIEZOSURGERY® at irrigation levels (d) 1, (e) 3, and (f) 6. pd represents probability density. The x axis is on log-scale. The lines show the predictions of the probability density function.

Fig. 3

The formation of droplets at the tip of the Mectron PIEZOSURGERY®. The arrows showing z and y directions point towards the positive directions. The positive values of the x direction are pointing away from the reader. The origin point was at 3 cm below the tip of the dental instrument as illustrated on the figure. (The position of the origin point is not to scale.) The instrument tip moves in the y-direction to cut the tissue. The irrigation level was 6.

Fig. 4

Aerosol number concentration of the Mectron Piezosurgery® in the x-direction for irrigation levels a) 1, b) 3, and c) 6. Aerosol number concentration in the y-direction for irrigation levels d) 1, e) 3, and f) 6.

Fig. 5

Probability density (pd) distributions of velocities in the y-direction of the Mectron PIEZOSURGERY® for irrigation levels a) 1, b) 3, and c) 6. Pd distributions of velocities in the z-direction for irrigation levels d) 1, e) 3, and f) 6. Pd distribution of average velocities for the irrigation levels g) 1, h) 3, and i) 6. Average velocities are reported based on the sign of the z-direction without taking into account the sign of the y-direction.

Fig. 6

Probability density (pd) distribution of KaVo EXPERTtorque™ at different air pressures and the liquid flow rates: a) the number-weighted distribution, b) the volume-weighted pd distribution. The x-axis is on log-scale. The lines show the predictions of the probability density function.

Fig. 7

The formation of droplets at the tip of the KaVo Expert Torque™. The arrows showing z and y directions point towards the positive directions. The positive values of the x direction are pointing away from the reader. The origin point was at 3 cm below the tip of the dental instrument as illustrated on the figure.

Fig. 8

Aerosol number concentration of KaVo EXPERTtorque™ a) in the x-direction, and b) in the y-direction.

Fig. 9

Probability density (pd) distributions of velocities of KaVo EXPERTtorque™ a) in the y-direction, b) in the z-direction, c) average velocity. Average velocities are reported based on the sign of the z-direction without taking into account the sign of the y-direction.

Fig. 10

Wells evaporation-falling curve of droplets for freely falling pure water droplets under different relative humidities (RH). Initial droplet temperature is 33 °C and room temperature is 18 °C. Falling times were calculated based on 2 m distance. The figure is redrawn based on (Xie et al., 2007). Copyright (2007) Wiley. Used with permission from X. Xie, Y. Li, A.T.Y. Chwang, P. L. Ho and W.H. Seto. How far droplets can move in indoor environments – revisiting the Wells evaporation-falling curve. Indoor Air 17: 211–225. (2007).