Cough Fantastique

Tang-Settles-JRSI-2009-Fig5 Cough Plumes Made Visible (courtesy of Gary Settles) Now that proper containment of your coughing and sneezing has become an even more urgent duty in the name of public health (into the crook of the elbow and not in my general direction please!), the unique views that fluid dynamics researcher Gary Settles has on what forcefully comes out of human facial apertures could prove especially valuable. Think of the wavy, fluidic appearance of the gas coming out of a truck’s exhaust pipe or the mirage-like shimmer of the air above hot pavement, and you are getting into the physics behind one of Settle’s techniques: schlieren optical analysis, whose roots go back to the 17th century microscopy pioneer Robert Hooke and whose name derives from the German word for streaks. Although the schlieren optical technique Settles deploys is usually more apropos for physics and engineering studies, it also has the potential to reveal chemical phenomena such as chemical plumes from concealed weapons (if they are volatile enough), he tells C&EN. “To understand chemical trace sensing you need to see how the air is moving, usually by way of thermal plumes," Settles says. "This naturally brings together fields such as analytical chemistry, fluid dynamics, and optics.” Tracing expelled air from coughing isn't exactly chemistry, but in these pandemically-minded days, it sure is timely. Using a one-meter government surplus telescope mirror that he purchased for $3000 in 1981 (new today they go for at least $80,000), Settles took a look at how medical masks affect the way a human cough—the carrier of minuscule infectious agents such as H1NI viruses and the ones that cause tuberculosis, chickenpox and measles—infiltrates the nearby airspace. The physics key to their schlieren optics study, Settles and his colleagues at Penn State and the National University of Singapore, write in the Journal of the Royal Society Interface (doi: 10.1098/rsif.2009.0295.focus), is that “it renders visible the optical phase gradients owing to real-time changes in air temperature.” Shown above is a set of images comparing the trajectories of normally invisible plumes coughs from a cougher without a mask, wearing a standard surgical mask, or wearing a higher-end version known as the N95 mask that has a tighter fit. The conclusions the researchers make from their images, including video records, have both intuitive and nonintuitive components. With no mask, a cough plume rushes outward at a downer angle of roughly 30º, producing a turbulent jet that does a good job of distributing whatever pathogenic payload the cougher might have to offer. The standard mask is best at thwarting the forward motion, but its loose fit means that the roughly 2 liters of expelled gas volume leaks well out of the tops and sides. The tighter N95 mask leaks out of the sides less, but pressure builds more inside, resulting in more of the ejected forcing itself straight through into a slow moving forward-moving front of potentially eminently avoidable air. In the latter case, the researchers point out, that forward moving front quickly gets swept up into “general motion of the cougher’s human thermal plume.” Compared to coughing, sneezing, breathing and talking, the researchers note that others have suggested that singing “may be a particularly effective means of transmitting airborne infection.” This perhaps serves up a hint to the State Department and Department of Homeland Security: watch for any odd singing-based gestures of international détente, say, an offer by the North Koreans to send over a ten-thousand person choral group. Also noted in the Interface paper is that “different languages may have different levels of risk for producing potentially infectious exhaled aerosols,” a conjecture that the researchers note could be investigated with “the real-time non-invasive schlieren imaging technique.”

Author: Ivan Amato

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