Engineers at the University of Minnesota are studying how the coronavirus that causes COVID-19 floats in the air and where hot spots for infection risk exist in spaces as small as elevators or as large as orchestra halls.

Simulating the flow of particles — big enough to carry concerning levels of virus, but small enough to hang in the air — the researchers found that ventilation and the places in which people sit or stand can increase transmission risks in different indoor environments. The research also assessed the amount of virus that could be spread by people simply talking or breathing — rather than coughing or sneezing.

The results will help in planning as more people return to office buildings and as students go back to class, said Jiarong Hong, a U associate professor of mechanical engineering leading the research.

"Could be in an elevator, could be in a classroom, or a supermarket," Hong said. "A person will be emitting aerosolized particles through breathing and speaking, and we're simulating how those particles are being transported."

The researchers also are consulting at the request of the Minnesota Orchestra to determine indoor risks at rehearsals or performances.

The potential for airborne transmission of SARS-CoV-2 is a source of dispute among researchers and public health leaders. The virus commonly spreads when infected people direct large droplets at anyone within 6 feet of them. But experts differ on whether smaller aerosols can linger in the air and carry enough virus to pose threats to others in a wider radius.

The measles virus has demonstrated that capability. Minnesota researchers conducted groundbreaking research on airborne transmission of that highly infectious virus — finding in 1991 that it carried due to ventilation patterns all the way from the surface of the Metrodome during a Special Olympics event to fans in the upper deck. But the size of the COVID-19 virus and the amount needed to cause an infection has left doubts about that mode of transmission in the current pandemic.

The airborne risk for COVID-19 can't be dismissed, though, when there are published studies of outbreaks with few other plausible sources of transmission, said Dr. Frank Rhame, a virologist with Allina Health in Minneapolis. Those include a choir rehearsal in Washington state and an outbreak of people sporadically seated around an infected person in a restaurant in China.

"It's hard to imagine some of these transmissions didn't happen without airborne particles," he said.

The lack of spread of COVID-19 among people involved in mass protests following the May 25 police killing of George Floyd undercuts the airborne transmission risk a bit, Rhame said, at least for people in the outdoor air that can diffuse the virus. But he worries about the stale air of a closed environment such as an elevator.

"If you don't talk and you're wearing a mask, I'm largely protected and I don't mind someone getting on the elevator with me," Rhame said. "What I really worry about is the guy who got on that elevator five minutes ago who was not wearing a mask."

The elevator scenario is where a U team led by Hong and Suo Yang, an assistant professor of mechanical engineering, started with their research, which will soon be published. First, they measured in eight volunteers how far and wide aerosolized particles traveled from their mouths when they talked and breathed. Then they used that data for computer simulations of the spread of aerosols in different indoor environments and under different levels of ventilation.

In the elevator scenario, the research showed that stronger ventilation removed more of the particles capable of carrying the virus, but also had a jet-stream effect that concentrated the buildup of particles to hot zones. Risks also were much greater if the infected person spoke during a simulated elevator ride. Lower ventilation levels by contrast left more of the particles in the elevator that eventually settled on the floor and walls.

Solutions suggested by this research could include signs telling people not to speak on elevators and markers on the floors identifying the low-risk spots to stand.

Ventilation cleared more aerosols when U engineers ran simulations for a small grocery store, because the infected shopper was moving around and not emitting high concentrations of particles to any one spot, Hong said. The positioning of the cashiers relative to the ventilations sources is important, he added.

Simulations of a small classroom were more problematic when assuming that a teacher was infected and was standing in one spot during a lecture and projecting particles at students.

The location of the ventilation in the room created different hot zones — with ventilation at the far back corner, away from the teacher, promoting the spread of particles to students in the back, Hong said. Ventilation above the teacher concentrated aerosols in the front of the room, but appeared to protect students more than 3 yards away.

Ventilation from a single source on the ceiling removed few of the problematic aerosols from the room, which instead tended to end up on the side walls and on the floor in the corners.

The findings from the research can be used to reorient classrooms so that teachers and students are in low-risk spots relative to ventilation sources, and to identify the spots on walls and floors that need frequent cleaning, Hong said.

"We know these are hot spots, right, so we can actually change, for example, the sitting position of the students," he said. "Maybe we can spread them or naturally position them to avoid this hot spot."

Open windows to add ventilation sources to classrooms could help as well, he added.

Others nationally have studied these scenarios and reached similar conclusions. Portland State University's Richard Corsi assessed how many aerosols capable of carrying the virus would be left when someone stepped on an elevator after an infected person without a mask had ridden in it for 10 floors.

Corsi also has conducted airflow risk assessments of 100 classrooms on his campus and modified his computer simulations of those rooms until the number of people likely to be infected during a typical class period is below one.

Some classrooms didn't appear safe with any modifications, but others became low-risk environments when installing higher-quality filters in the air-circulation systems, spreading students out, and installing a portable air purifier on a table in the middle of the room, he said. (Putting the purifiers on the floor risked pushing virus-carrying particles that landed on the floor back into the air, he noted.)

"It ends up being a pretty small number" of students who can be in rooms that meet his safety threshold, he said. "Often times there might be a classroom of 48 people and we can have six."

More research is needed to help improve these airflow models, he said, including about the amount of virus needed to cause infection.

Linsey Marr, a civil and environmental engineer at Virginia Tech, has conducted research in this area as well and said the risk of airborne transmission for COVID-19 is underappreciated because the World Health Organization initially believed it wasn't a significant factor.

Marr through her research has become a strong advocate for mask-wearing in public, even if standard masks only provide partial protection.

"Masks for the general public do not need to meet the same standards as in healthcare," she tweeted last week. "Your mask 50% effective + my mask 50% effective = 75% reduction in exposure. I'll take that over nothing any day."

Hospitals in Minnesota have made renovations due to the risk of airborne transmission that could occur when patients undergo aerosolizing procedures such as the placement of breathing tubes that cause them to gag and cough.

Rooms were converted with negative airflow capabilities at Fairview's Bethesda Hospital in St. Paul — which only treats COVID-19 patients — and Allina's Abbott Northwestern Hospital in Minneapolis. Other U researchers also published test data last week for a portable hood with a filter on top that could be placed over patients to prevent the spread of any particles they emit during such procedures.

The role of airborne transmission is unclear with respect to the COVID-19 pandemic so far in Minnesota, where more than 36,000 people have suffered lab-confirmed infections and more than 1,400 people have died. However, the state has not experienced any mass-spreader events such as a choir rehearsal by which airborne transmission was responsible for a large number of cases, said Kris Ehresmann, state infectious disease director.

Hong said he hopes his team's research can help with planning to lower infection risks. Ongoing analysis for the Minnesota Orchestra could help the organization with its reopening plans, for example, and produce interesting findings with respect to airflow around musicians.

"It's different from people just talking and breathing," Hong said. "They might be producing higher amount of aerosols, for example, playing wind instruments."

Jeremy Olson • 612-673-7744