The complex sneeze, caught on tape

However commonplace it may be in human life, the sneeze remains somewhat of an enigma to science, and we are still a long way from understanding the simple sneeze in all its phlegm-flam glory.

We know that sneezes can spread infectious diseases like measles, influenza or SARS, suspending viruses in droplets that may be inhaled or deposited onto surfaces and later picked up on the hands of some unsuspecting passerby. But we don't understand exactly how far a sneeze can spread or if and why some people spread sickness through sneezing more effectively than others.

"This is a major blind spot when designing public health control and prevention policies, particularly when urgent measures are needed during epidemics or pandemics," said Lydia Bourouiba, the Esther and Harold E. Edgerton Assistant Professor and head of the Fluid Dynamics of Disease Transmission Laboratory at the Massachusetts Institute of Technology (MIT) in Cambridge.

"Our long term goal is to change that," she added.

Last year Bourouiba and her collaborators described in a paper how sneezes are complex, turbulent, highly variable, multiphase flows that can suspend and spread potential pathogen-carrying droplets much further than ever suspected -- with the smaller droplets spanning the size of a room and reaching ventilation ducts at ceiling heights within a few minutes. (See reference at the end of this release.)

This month during the American Physical Society's 68th Annual Meeting of the Division of Fluid Dynamics, held Nov. 22-24, 2015, in Boston, Mass., these researchers will present new work that shows how droplets are formed within a high-propulsion sneeze cloud -- a critical piece of the puzzle that has so far been missing.

"Droplets are not all already formed and neatly distributed in size at the exit of the mouth, as previously assumed in the literature," she said. Instead, they undergo a complex cascading breakup that continues after they leave the lungs, pass over the lips and churn through the air.

How the Work was Done

The team's experiments involved capturing high-speed videos of two healthy subjects sneezing about 50 times over the course of several days at different times of the day. They coupled sophisticated data extraction algorithms with new, state-of-the-art 3-D visualization techniques developed by Bourouiba's collaborators Professor Alexandra Techet and Dr. Barry Scharfman at MIT to tease out the features of the sneezes from the videos.

The visualizations show how in a sneeze the mucosalivary fluid fragments from sheets to ligaments to droplets outside of the respiratory tract -- something that has never been reported before in respiratory flows. This will be reported in a paper titled, "Visualization of sneeze ejecta: steps of fluid fragmentation leading to respiratory droplets," by Scharfman B. E, Techet A. H., Bush J. W. M. and Bourouiba L. (2015), which is currently in press in the journal, Experiments in Fluids.

Bourouiba said the goal of this work in particular, and of her Fluid Dynamics of Disease Transmission Laboratory at MIT in general, is to ground infectious disease prevention firmly in physical understanding to help guide national and international public health policies and develop effective mitigation technologies.

Toward that end, their future work will study phlegm breakup in sick people sneezing and explore how different pathogens spread in a sneeze.