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).

Surgical Face Masks Worn by Patients with Multidrug-Resistant Tuberculosis

“…We do not believe that higher-level respirator use by patients with TB is warranted for this purpose due to cost and discomfort (38). Moreover, whereas a better face seal can reduce leakage around the filtration piece of a respirator during inspiration, it is unlikely to substantially resist the air pressure generated by cough, and air is likely to escape around either a surgical mask or respirator, as illustrated by others using real-time optical imaging of air flow patterns during cough (39). The purpose of a surgical mask, like a hand or tissue, is not to contain the cough but to stop the exit of a portion of the larger droplets otherwise destined to become droplet nuclei.…”

Violent expiratory events: on coughing and sneezing

“…Figure 7 illustrates the physical picture that emerges from a synthesis of our flow visualizations, prior work on cloud visualization using schlieren (e.g. Tang et al 2009) and droplet sampling (e.g. Morawska et al 2009b).…”

Airborne transmission of disease in hospitals

“…Although Tang et al (2009) have shown with Schlieren airflow imaging that wearing both surgical and N95 masks should be effective in containing infectious aerosols produced by the wearer, there are many complex issues surrounding mask-wearing. First, there is the actual physical filtering effectiveness of the mask in blocking the passage of bacteria and viruses.…”

“…Natural means include breathing, talking, sneezing, singing and, in particular, coughing. In this volume, coughing is explored using an artificial cough machine by Pantelic et al (2009) and using human volunteers by Xie et al (2009) and Tang et al (2009). Artificial means of producing potentially infectious aerosols are abound in hospitals, particularly when taking high-risk respiratory samples S698 I. Eames et al Airborne transmission of disease like nasopharyngeal aspirates or when using respiratory assist equipment, such as nebulizers (Hui et al 2009), ventilators (Hui et al 2006a,b) or oxygen masks (Hui et al 2006a(Hui et al ,b, 2007Ip et al 2007) for patients in respiratory distress.…”