A schlieren optical study of the human cough with and without wearing masks for aerosol infection control

Abstract

Various infectious agents are known to be transmitted naturally via respiratory aerosols produced by infected patients. Such aerosols may be produced during normal activities by breathing, talking, coughing and sneezing. The schlieren optical method, previously applied mostly in engineering and physics, can be effectively used here to visualize airflows around human subjects in such indoor situations, non-intrusively and without the need for either tracer gas or airborne particles. It accomplishes this by rendering visible the optical phase gradients owing to real-time changes in air temperature. In this study, schlieren video records are obtained of human volunteers coughing with and without wearing standard surgical and N95 masks. The object is to characterize the exhaled airflows and evaluate the effect of these commonly used masks on the fluid-dynamic mechanisms that spread infection by coughing. Further, a high-speed schlieren video of a single cough is analysed by a computerized method of tracking individual turbulent eddies, demonstrating the non-intrusive velocimetry of the expelled airflow. Results show that human coughing projects a rapid turbulent jet into the surrounding air, but that wearing a surgical or N95 mask thwarts this natural mechanism of transmitting airborne infection, either by blocking the formation of the jet (N95 mask), or by redirecting it in a less harmful direction (surgical mask).

Figure 1.

(a) The profile of a typical ‘single forced cough’ in terms of expelled airflow rate versus time, adapted from Khan et al. (2004). (b) Schlieren image of a cough directed downward by a 25-year-old male subject, revealing the character of the cough as a turbulent jet of air.

Figure 2.

Schematic top view of the 1 m aperture schlieren optical system of the Penn State University Gas Dynamics Laboratory, as used in this study (Settles 2001).

Figure 3.

Standard N95 (left) and surgical (right) masks used in this study.

Figure 4.

Quantitative results from high-speed video of a cough by a 57-year-old male volunteer. (a) Enlarged schlieren frame no. 300, exposed for 1 µs at 0.1 s from cough onset; (b) instantaneous velocity-magnitude contours and vectors averaged across the cough in the direction perpendicular to the page, obtained by ‘schlieren PIV’ processing of video frames 300 and 301 (figure 4b was first shown by Tang & Settles, 2008–reproduced here with permission from the New England Journal of Medicine).

Figure 5.

Schlieren images of two volunteers facing one another. The subject on the right is masked as a precaution. The volunteer on the left coughs in the direction of the other subject first without wearing a mask (a), then while wearing a standard surgical mask (b), and finally while wearing an N95 mask (c). This demonstrates the importance of wearing a mask in reducing the potential for airborne transmission of infection over this distance. Note that in (a) the jet of air produced by the cough plume is directed downwards at roughly a 30° angle, whereas in (b) the expelled air of the cough plume is split into upward and downward components as it exits from the top and bottom edges of the surgical mask. There is also leakage around the side edges of the mask out of the plane of the page, which is shown in the next figure. In (c), the tighter seal of the N95 mask against the face forces more of the expelled cough through the mask. A low-velocity expelled turbulent air mass can be seen just in front of the mask, which does not cross the distance to the other volunteer. The different behaviours of these coughs, sketched in (d), can also be seen more clearly in the accompanying video footage (see the electronic supplementary material Video S1).

Figure 6.

Side and frontal-view schlieren images of coughs by a 26-year-old volunteer while wearing standard surgical mask (a,b) and N95 mask (c,d). The sketch (e) portrays the salient point of this sequence: that massive air leakage occurs around the sides and top of the surgical mask during a cough. The airflows shown here are more clearly seen as dynamic phenomena in the accompanying video footage (see the electronic supplementary material Video S2).

Figure 7.

Schlieren images of a supine volunteer, quietly breathing and coughing. These images demonstrate the effectiveness of standard surgical and N95 masks in containing potentially-infectious exhaled plumes while the subject is lying down (perhaps resting or sleeping) in a supine position. Note the vertical extent of the exhaled breath without a mask in (a). In (b) the effect of the standard surgical mask in the supine position can be seen in splitting the cough, some air passing through the mask and some leaking at its top edge. In comparison, the cough is relatively well contained by the N95 mask in (c). The airflows shown here are more clearly seen as dynamic phenomena in the accompanying video footage (see the electronic supplementary material Video S3). The phenomena in (a) and (b) are also sketched in (d) for clarity.