For this study, researchers created a computer model of a classroom with students and a teacher, then modeled airflow and disease transmission, and calculated the risk of airborne transmission.
The results indicate that masks and proper ventilation may be the key to enabling greater capacity in schools, businesses and other indoor spaces.
A new study from the University of Central Florida suggests that masks and a good ventilation system are more important than social distancing in reducing the airborne spread of COVID-19 in classrooms.
The study, published recently in the journal Physics of Fluids, comes at a critical time when schools and universities are considering resuming face-to-face classes in the fall.
"This research is important because it provides insights into how we understand safety in indoor environments," says Michael Kinzel, an assistant professor in UCF's Department of Mechanical and Aerospace Engineering and co-author of the study.
"The study shows that aerosol transmission pathways do not require a social distance of two meters when masks are required," he adds. "These results highlight that with masks, the likelihood of transmission does not decrease with increasing physical distance, which underscores how mask mandates may be key to increasing capacity in schools and other locations."
"The results suggest exactly what the CDC is doing, which is that ventilation systems and mask use are most important to prevent transmission and that social distancing would be the first thing to relax."
In the study, researchers created a computer model of a classroom with students and a teacher, then modeled airflow and disease transmission, and calculated the risk of airborne transmission.
The classroom model was 709 square feet with 9-foot-high ceilings, similar to a smaller university classroom, Kinzel says. The model had masked students - each of whom could be infected - and a masked teacher at the front of the classroom.
The researchers examined the classroom in two scenarios - a ventilated classroom and an unventilated classroom - and using two models, Wells-Riley and Computational Fluid Dynamics. The Wells-Riley model is commonly used to assess the probability of transmission inside a building and the Computational Fluid Dynamics model is often used to understand the aerodynamics of cars, airplanes and underwater motion.
The masks have proven beneficial in preventing direct exposure to aerosols because the masks provide a small puff of warm air that causes the aerosols to move vertically, preventing them from reaching nearby students, Kinzel says.
In addition, a ventilation system combined with a good air filter reduces the risk of infection by 40 to 50 percent compared to a classroom without ventilation. This is because the ventilation system creates a constant airflow that moves much of the aerosols through a filter that removes some of them, whereas in the no-ventilation scenario, the aerosols collect on top of the people in the room.
These results support recent guidance from the U.S. Centers for Disease Control and Prevention that recommends reducing the social distance in elementary schools from six feet to three feet when masking is universal, Kinzel says.
"If we compare the probabilities of infection when wearing masks, a social distance of three feet did not indicate an increase in the probability of infection compared to six feet, which may provide evidence for schools and other businesses to operate safely during the remainder of the pandemic," Kinzel says.
"The results suggest exactly what the CDC does, that ventilation systems and mask use are most important to prevent transmission and that social distancing would be the first thing to relax," the researcher adds.
Comparing the two models, the researchers found that Wells-Riley and Computational Fluid Dynamics generated similar results, particularly in the no-ventilation scenario, but that Wells-Riley underestimated the probability of infection by about 29% in the ventilated scenario.
As a result, the researchers recommend applying some of the additional complex effects captured in Computational Fluid Dynamics to Wells-Riley to better understand the risk of infection in a space, says Aaron Foster, a doctoral student in UCF's Department of Mechanical and Aerospace Engineering and lead author of the study.
"While the detailed computational fluid dynamics results provided a better understanding of the relationships between risk variation and distance, they also validated the more commonly used Wells-Riley models by showing that they capture the majority of the benefits of ventilation with reasonable accuracy," Foster says. "This is important because they are publicly available tools that anyone can use to reduce risk."
This research is part of a larger global effort to control airborne disease transmission and better understand the factors associated with being a superpromoter. Researchers are also testing the effects of masks on the transmission distance of aerosols and droplets. This work is funded in part by the National Science Foundation.