COVID-19: understanding the aerodynamics of infectious disease

COVID-19: understanding the aerodynamics of infectious disease
© iStock/Furtseff

Countries across the globe have implemented measures such as social distancing and mask wearing in order to reduce COVID-19 transmission, but as the weather gets colder and people spend more time indoors, reducing the spread of the virus may become more difficult.

Researchers have presented a range of studies, at the 73rd Annual Meeting of the American Physical Society’s Division of Fluid Dynamics, investigating the aerodynamics of infectious disease, that provide strategies to help reduce indoor transmission of COVID-19 based on understanding how infectious particles mix with air in confined spaces.

Aerodynamics of infectious disease indoors

Coughing and sneezing have garnered the most attention from researchers looking to discover the dangers of COVID-19 transmission, however, activities such as singing have also shown to be dangerous routes of infection.

Chemical engineer at the University of California, Davis, William Ristenpart, conducted research that demonstrated that when people speak or sing loudly, the micron sized particles they produce are dramatically larger than when they use a normal voice, and the particles produced when shouting greatly exceed the number produced during coughing.

Furthermore, in animal models, using guinea pigs, the researchers demonstrated that influenza can spread through contaminated dust particles, and note that if the same is true for the SARS-CoV-2, then objects, such as tissues, which release contaminated dust may pose a risk.

Are musical instruments super spreaders?

Researchers from the University of Colorado, Boulder, including Abhishek Kumar, Jean Hertzberg, and others, looked at how the virus might spread during music performance, discussing results from experiments designed to measure aerosol emission from instrumentalists.

Hertzberg said that instruments like clarinets and oboes, which have wet vibrating surfaces, tend to produce copious aerosols, but that this can be controlled: “Everyone was very worried about flutes early on, but it turns out that flutes don’t generate that much. When you put a surgical mask over the bell of a clarinet or trumpet, it reduces the number of aerosols back down to levels in a normal tone of voice.”

Another study from engineers at Ruichen He at the University of Minnesota investigated a similar risk-reduction strategy in their study of the flow field and aerosols generated by various instruments. The findings demonstrated that aerosols produced by musician and instrument varied, but rarely travelled further than one foot.  

Based on these findings, the researchers devised a pandemic-sensitive seating model for live orchestras and described where to place filters and audience members to reduce risk.

Airborne transmission in the workplace

In order for workers to eventually return to office spaces, employers must find ways of safely reopening without putting employees at risk, and office space often means social distancing can be difficult.

Kelby Kramer and Gerald Wang from Carnegie Mellon University used two-dimensional simulations that modelled people as particles and identified conditions that would help avoid crowding and jamming in confined spaces like hallways.

Many office workers also carpool to limit their impact on the environment, however, travelling to and from workplaces may now prove much more difficult, as both public transport and shared cars pose a risk. To identify the risks associated with car cabins, Kenny Breuer and collaborators at Brown University performed numerical simulations of how air moves through the cabins, finding that if air enters and exits a room at points far away from passengers, then it may reduce the risk of transmission, which, in a passenger car, means strategically opening some windows and closing others.

Cumulative exposure time

Spending more time indoors can lead to increased exposure to COVID-19 if there are particles in the air, so MIT mathematicians Martin Bazant and John Bush have proposed a new safety guideline built on existing models of airborne disease transmission in order to identify maximum levels of exposure in a variety of indoor environments.

Based on a metric called “cumulative exposure time”, which is determined by multiplying the number of people in a room by the duration of the exposure, the guideline offers information depending on the size and ventilation rate of the room, the face mask of its occupant, the infectiousness of aerosolised particles, and other factors.

Bazant and Bush noted that staying six feet apart: “offers little protection from pathogen-bearing aerosol droplets sufficiently small to be continuously mixed through an indoor space,” and highlight that a better, flow-dynamics-based understanding of how infected particles move through a room may ultimately yield smarter strategies for reducing transmission.

To facilitate easy implementation of the guidelines, the researchers have designed an app and online spreadsheet, with chemical engineer Kasim Khan, which can be used to gauge the risk of transmission in a variety of settings.


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