Airflow dynamics of human jets: sneezing and breathing - potential sources of infectious aerosols

Abstract

Natural human exhalation flows such as coughing, sneezing and breathing can be considered as 'jet-like' airflows in the sense that they are produced from a single source in a single exhalation effort, with a relatively symmetrical, conical geometry. Although coughing and sneezing have garnered much attention as potential, explosive sources of infectious aerosols, these are relatively rare events during daily life, whereas breathing is necessary for life and is performed continuously. Real-time shadowgraph imaging was used to visualise and capture high-speed images of healthy volunteers sneezing and breathing (through the nose - nasally, and through the mouth - orally). Six volunteers, who were able to respond to the pepper sneeze stimulus, were recruited for the sneezing experiments (2 women: 27.5±6.36 years; 4 men: 29.25±10.53 years). The maximum visible distance over which the sneeze plumes (or puffs) travelled was 0.6 m, the maximum sneeze velocity derived from these measured distances was 4.5 m/s. The maximum 2-dimensional (2-D) area of dissemination of these sneezes was 0.2 m(2). The corresponding derived parameter, the maximum 2-D area expansion rate of these sneezes was 2 m(2)/s. For nasal breathing, the maximum propagation distance and derived velocity were 0.6 m and 1.4 m/s, respectively. The maximum 2-D area of dissemination and derived expansion rate were 0.11 m(2) and 0.16 m(2)/s, respectively. Similarly, for mouth breathing, the maximum propagation distance and derived velocity were 0.8 m and 1.3 m/s, respectively. The maximum 2-D area of dissemination and derived expansion rate were 0.18 m(2) and 0.17 m(2)/s, respectively. Surprisingly, a comparison of the maximum exit velocities of sneezing reported here with those obtained from coughing (published previously) demonstrated that they are relatively similar, and not extremely high. This is in contrast with some earlier estimates of sneeze velocities, and some reasons for this difference are discussed.

Figure 1. Experimental set-up for the shadowgraph imaging of the human respiratory airflows described in this study (reproduced from Tang et al. 2012).

Figure 2. Illustration of the parameters digitised frame-by-frame from the high-speed airflow images captured from each volunteer: the maximum visible propagation distance (max-X) and the maximum visible 2-dimensional (2-D) area (max-A).

Figure 3. Sneezing airflow parameters.

A: Measured visible propagation distances and derived velocities; B: Measured 2-dimensional (2D) areas and derived expansion rates.

Figure 4. Nasal breathing airflow parameters.

A: Measured visible propagation distances and derived velocities; B: Measured 2-dimensional (2D) areas and derived expansion rates.

Figure 5. Mouth breathing airflow parameters.

A: Measured visible propagation distances and derived velocities; B: Measured 2-dimensional (2D) areas and derived expansion rates.

Figure 6. Reanalysed coughing airflow parameters for comparison.

A: Measured visible propagation distances and derived velocities; B: Measured 2-dimensional (2D) areas and derived expansion rates.