Editors' ChoiceInfectious diseases

Fluid dynamics of disease transmission

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Science Translational Medicine  02 Mar 2016:
Vol. 8, Issue 328, pp. 328ec36
DOI: 10.1126/scitranslmed.aaf3856

Coughs and sneezes can directly contribute to the transmission of communicable diseases, as these violent exhalations break up mucus and explosively emit pathogen-bearing droplets into the environment. The range of contamination of an unprotected sneeze can be quite substantial—up to 20 feet—with the smallest droplets carried the farthest in a turbulent cloud. Scharfman and colleagues set out to investigate what physical mechanisms set the droplet size distribution of the sneeze ejecta and therefore the range of contamination.

Using high-speed videography, the authors recorded the initial ejection of mucosalivary fluid from the mouth at up to 8000 frames per second, more than 300 times faster than regular video recordings. Rather than a spray containing different droplet sizes, the video revealed a complex cascade of bulk fluid emission at 35 meters per second (around 80 miles per hour), followed by fluid sheet formation, sheet destabilization, ligament formation, and finally droplet pinch-off. In particular, the authors observed elongation, twisting, and shortening of the ejected fluid ligaments, along with bead formation on the ligament. These observations suggest that the viscoelastic properties of the mucosalivary fluid play an important role in the fluid fragmentation process and droplet size selection during sneezing, and may therefore directly affect the range and concentration of contamination. These results also begin to fill an important gap in our understanding of how pathogens may be transmitted from peer-to-peer and how the properties of the fluid carrier medium might help shape and select the contamination profile while indoors. This knowledge can have important consequences on the redefinition of route of transmission for common infectious diseases, in particular the notions of airborne versus large-droplet transmission typically used in public health and infection prevention. The work by Scharfman et al. also raises the interesting question about whether pathogens might be able to interact with the fluid properties to enhance their dispersal during violent exhalations, begging further research into the fluid dynamics of disease transmission.

B. E. Scharfman et al., Visualization of sneeze ejecta: Steps of fluid fragmentation leading to respiratory droplets. Exp. Fluids 10.1007/s00348-015-2078-4 (2016). [Full Text]

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