For more than a year we have become accustomed to seeing stickers on the floor of numerous establishments that tell us where to position ourselves to maintain a two-meter separation and reduce the risk of SARS-CoV-2 transmission. However, this measure, if not accompanied by other prevention strategies, is effective in avoiding contagion through aerosols, those small droplets that travel through the air and we release when we speak or cough.
These are the conclusions of a study published in the journal Sustainable Cities and Society that explores the air transport of virus-laden particles released by infected people inside buildings. “We investigated the effects of building ventilation and physical distancing as control strategies for indoor exposure to airborne viruses,” explains Gen Pei, first author and PhD student in architectural engineering at Pennsylvania State University. (USA).
The researchers examined three factors: the amount and rate of air ventilated through a space, the indoor air flow pattern associated with different ventilation strategies, and the aerosol emission mode of breathing versus speaking. They also compared tracer gas transport, normally used to test leaks in airtight systems, and human respiratory aerosols ranging in size from one to ten microns. Aerosols in this range can carry SARS-CoV-2.
The mask is necessary
“The results of our study reveal that virus-laden particles released by an infected person without a mask can rapidly travel to another person’s breathing zone within a minute, even from a distance of two meters,” explains Donghyun Rim, another of the authors. “ This trend is manifested in rooms without sufficient ventilation. The results suggest that physical distance alone is not sufficient to prevent human exposure to exhaled aerosols and should be implemented with other control strategies such as a mask and adequate ventilation ”.
The researchers found that the aerosols traveled farther and faster in displacement-ventilated rooms, where fresh air continually flows from the floor and pushes old air into a ventilation shaft near the ceiling. This is the type of ventilation system that is installed in most residential homes and can result in a concentration of viral aerosols in the human breathing zone seven times higher than mixed-mode ventilation systems. Many commercial buildings use mixed-mode systems, incorporating outside air to dilute the indoor air, resulting in better air integration and medium aerosol concentrations, the researchers say.
“This is one of the surprising results: the likelihood of airborne infection could be much higher in residential settings than in office settings ,” Rim said. “However, in residential settings, the operation of separate mechanical fans and air filters can help reduce the likelihood of infection.”
According to Rim, increasing ventilation and air mixing rates can effectively reduce transmission distance and the potential build-up of exhaled aerosols, but ventilation and distance are just two options in an arsenal of protection techniques.
“Airborne infection control strategies, such as physical distancing, ventilation and the use of masks, must be considered together for layered control,” Rim said.
Currently, the team of researchers is applying this analysis technique to other types of closed environments, such as classrooms and public transport systems.
Text: Pennsylvania State University
