Airborne virus transmission occurs through droplets formed during respiratory events associated with the airflow through a network of nasal and buccal passages. The airflow interacts with saliva/mucus films where droplets are formed and dispersed, creating a route to transmit SARS-CoV-2.
A new study from the University of Central Florida aimed to understand the underlying ‘why’ of how far sneezes travel. This is the first study that identified physiological features that could make people super-spreaders of viruses.
Using computer-generated models, scientists numerically simulated sneezes in different types of people. They later determined associations between people’s physiological features and how far their sneeze droplets travel and linger in the air.
They found that people’s features, similar to a stopped-up nose or a full set of teeth, could increase their potential to spread viruses by affecting how far droplets travel when they sneeze.
Michael Kinzel, an assistant professor with UCF’s Department of Mechanical Engineering and study co-author, said, “Knowing more about factors affecting how far these droplets travel can inform efforts to control their spread.”
“We show that the human body has influencers, such as a complex duct system associated with the nasal flow that disrupts the jet from your mouth and prevents it from dispersing droplets far distances.”
The study suggests that- when people have a clear nose, such as blowing it into a tissue, the speed and distance sneeze droplets travel decreases. This is because a clear nose gives a path in addition to the mouth for the sneeze to exit. Yet, when individuals’ noses are congested, the area that the sneeze can exit is restricted, hence causing sneeze droplets expelled from the mouth to increase in velocity.
Similarly, teeth also restrict the sneeze’s exit area and cause droplets to increase in velocity.
Kinzel says, “Teeth create a narrowing effect in the jet that makes it stronger and more turbulent. They appear to drive transmission. So, if you see someone without teeth, you can expect a weaker jet from the sneeze from them.”
When they simulated sneezes in the different models, they found that the spray distance of droplets expelled when a person has a congested nose, and a full set of teeth is about 60 percent greater than when they do not.
It means, when someone keeps their nose clear, such as by blowing it into a tissue, that they could be reducing the distance their germs travel.
Scientists also simulated three types of saliva: thin, medium, and thick.
They found that thinner saliva resulted in sneezes comprised of smaller droplets, which created a spray and stayed in the air longer than medium and thick saliva.
For instance, three seconds after a sneeze, when thick saliva was reaching the ground and thus diminishing its threat, the thinner saliva was still floating in the air as a potential disease transmitter.
Kareem Ahmed, an associate professor in UCF’s Department of Mechanical and Aerospace Engineering and study co-author, said, “The findings yield novel insight into the variability of exposure distance and indicate how physiological factors affect transmissibility rates.”
“The results show exposure levels are highly dependent on the fluid dynamics that can vary depending on several human features. Such features may be underlying factors driving super spreading events in the COVID-19 pandemic.”
In future studies, scientists want to investigate the interactions between gas flow, mucus film, and tissue structures within the upper respiratory tract during respiratory events.
- D. Fontes et al., A study of fluid dynamics and human physiology factors driving droplet dispersion from a human sneeze, Physics of Fluids (2020). DOI: 10.1063/5.0032006