Physical distancing ‘not enough’ when indoors

Virus particles from an infected person can travel to another person’s breathing zone within one minute, even with a distance of two meters.

Dave Yasvinski 3 minute read September 16, 2021
vector image of two people talking

Safety measures are still required for safe indoor gathering. GETTY

A new study says six feet of separation doesn’t stop the spread of COVID-19 indoors without the use of additional control measures.

The study, conducted by the Penn State Department of Architectural Engineering and published in the journal Sustainable Cities and Society, examined three factors: the amount and rate of air moving through an indoor space, the airflow pattern associated with different methods of ventilation and the aerosol emission mode of breathing versus talking.

“We set out to explore the airborne transport of virus-laden particles released from infected people in buildings,” said Gen Pei, first author of the study and a doctoral student in architectural engineering at Penn State. “We investigated the effects of building ventilation and physical distancing as control strategies for indoor exposure to airborne viruses.”

The team also compared the movement of tracer gas, commonly used to test air-tight systems, against respiratory aerosols small enough to carry COVID-19.

“Our study results reveal that virus-laden particles from an infected person’s talking — without a mask — can quickly travel to another person’s breathing zone within one minute, even with a distance of two meters,” said Donghyun Rim, corresponding author of the study and an associate professor of architectural engineering.

“This trend is pronounced in rooms without sufficient ventilation. The results suggest that physical distance alone is not enough to prevent human exposure to exhaled aerosols and should be implemented with other control strategies such as masking and adequate ventilation.”

Respiratory aerosols were found to travel faster and further in residential homes, which typically use displacement ventilation — a method of circulation that pumps fresh air from the floor while pulling older air out through exhaust fans attached to the ceiling. This type of ventilation is capable of producing a concentration of viral aerosols seven times higher than the mixed-mode method typically found in commercial buildings. Mixed-mode systems dilute indoor airflow by adding outside air into the mix, leading to better air integration and a lower concentration of aerosols.

“This is one of the surprising results: Airborne infection probability could be much higher for residential environments than office environments,” Rim said. “However, in residential environments, operating mechanical fans and stand-alone air cleaners can help reduce infection probability.”

While improving ventilation and the mix of air indoors can reduce the speed and distance at which the virus travels, researchers said it is vital to incorporate other safety measures now familiar to pandemic-weary populations. “Airborne infection control strategies such as physical distancing, ventilation and mask wearing should be considered together for a layered control,” he said.

The team is following up its findings by testing their methods on a variety of indoor areas, such as classrooms and methods of transportation, to assess the risk and need for additional precautions in an attempt to stem the spread of the virus and its variants.

Dave Yasvinski is a writer with