Airflow studies reveal strategies to reduce indoor transmission of COVID-19

Social distance flow

Simulation of pedestrian counter-current (red and pink particles) confined within a corridor (blue border), in conditions of weak social distancing. Credit: Kelby Kramer and Gerald J. Wang

Scientists studying the aerodynamics of infectious diseases share steps to curb transmission during indoor activities.

Wear a mask. Stand six feet away. Avoid large gatherings. While the world awaits a safe and effective vaccine, checking the COVID-19 the pandemic depends on widespread compliance with these public health guidelines. But as colder weather forces people to spend more time indoors, stopping the transmission of disease will become more difficult than ever.

At the 73rd annual meeting of the American Physical Society’s Fluid Dynamics Division, researchers presented a series of studies investigating the aerodynamics of infectious diseases. Their findings suggest strategies for reducing risk based on a rigorous understanding of how infectious particles mix with air in confined spaces.

Research at the start of the pandemic focused on the role played by the large, rapidly falling droplets produced by coughing and sneezing. However, documented super-shedding events have suggested that the airborne transmission of tiny particles from daily activities can also be a dangerous route of infection. Fifty-three of the 61 singers in Washington state, for example, were infected after a 2.5-hour choir rehearsal in March. Of the 67 passengers who spent two hours on a bus with a COVID-19 infected individual in Zhejiang Province, China, 24 tested positive later.

William Ristenpart, a chemical engineer at the University of California, Davis, found that when people talk or sing out loud, they produce significantly more micron-sized particles than when they use a normal voice. The particles produced during screaming, they found, far outweigh the number produced during coughing. In guinea pigs, they observed that the flu can spread through contaminated dust particles. If the same goes for the SARS-CoV-2the researchers said, so objects that release contaminated dust, such as textiles, can pose a risk.

Abhishek Kumar, Jean Hertzberg and other researchers from the University of Colorado, Boulder, focused on how the virus could spread during musical performances. They discussed the results of experiments designed to measure aerosol emission by instrumentalists.

“Everyone was very concerned about flutes in the beginning, but it turned out that flutes don’t generate that much,” Hertzberg said. On the other hand, instruments such as clarinets and oboes, which have wet vibrating surfaces, tend to produce abundant aerosols. The good news is that they can be controlled. “When you put a surgical mask over the bell of a clarinet or trumpet, the amount of aerosol is reduced to the levels of a normal tone of voice.”

Engineers led by Ruichen He at the University of Minnesota studied a similar risk reduction strategy in their study of the flow field and aerosols generated by various instruments. Although the level of aerosol produced varied by player and instrument, they rarely moved more than a foot away. Based on their findings, the researchers devised a pandemic-sensitive seating model for live orchestras and described where to place filters and audience members to reduce risk.

While many former office employees continue to work from home, employers are exploring ways to safely reopen their workplaces while maintaining sufficient social distance between individuals. Using two-dimensional simulations that modeled people as particles, Kelby Kramer and Gerald Wang of Carnegie Mellon University identified conditions that would help avoid crowding and jamming in confined spaces such as corridors.

Traveling to and from office buildings with passenger cars also poses a risk of infection. Kenny Breuer and his collaborators at Brown University performed numerical simulations of how air moves through car cabins to identify strategies that can reduce the risk of infection. If air enters and leaves a room at locations away from passengers, it can reduce the risk of transmission. In a car, they said, that means strategically opening some windows and closing others.

WITH mathematicians Martin Bazant and John Bush have proposed a new safety guideline based on existing models of airborne disease transmission to identify maximum levels of exposure in a variety of indoor environments. Their guideline depends 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 maximum depends on the size and ventilation rate of the room, the occupant’s face lining, the infectivity of the aerosolized particles and other factors. To facilitate easy implementation of the guideline, the researchers worked with chemical engineer Kasim Khan to design an online app and spreadsheet that people can use to assess transmission risk in a variety of settings.

As Bazant and Bush wrote in an upcoming article on the work, standing six feet away “offers little protection from pathogen-carrying aerosol droplets small enough to be continuously stirred across an interior space.” A better understanding, based on flow dynamics, of how infected particles move through a room could ultimately produce smarter strategies for reducing transmission.

Highlighted abstracts

Singing, dust and airborne disease transmission

Influenza transmission in the guinea pig model is insensitive to the velocity of the ventilation airflow: evidence of the role of aerosolized formulas

Aerosol in performance

Risk assessment of airborne disease transmission during wind instrument reproductions

The Flow Physics of Social Distancers: Uncovering Patterns in Pandemic-Era Pedestrian Flows Using Particle-Based Simulations

Air flows inside passenger cars and implications for airborne disease transmission

A guideline to limit indoor airborne transmission of COVID-19

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