Onstage at Orchestra Hall, Michael Gast lifted his French horn and, as a metronome clicked, played a scale loudly. Once, twice, three times.

His forehead red, his cheeks deflated, Gast turned to his audience: six University of Minnesota researchers.

Two peered at a screen before nodding. They had been watching, more than listening, measuring not the sound from Gast's horn but the particles. A Ph.D. student adjusted the funnel in the horn's bell.

Once more. Gast took a deep breath.

U mechanical engineers are working with the Minnesota Orchestra to study the strength and concentration of aerosols that emerge from brass and woodwind instruments, trying to assess how risky they might be in the spread of the coronavirus that causes COVID-19.

The first goal is to identify the risks in Orchestra Hall of virus-carrying particles flying between and among players and to figure out how to capture or disperse those particles — paving the way for safe, in-person concerts again. But the results could reverberate beyond the performing arts and play to a national audience of scientists and policymakers who are only beginning to understand the role of aerosols in fueling the pandemic.

"The findings are very significant, with potential impact beyond orchestra performance," said Jiarong Hong, an associate professor of mechanical engineering and a co-leader of the research.

The U.S. Centers for Disease Control and Prevention this fall stated that aerosols can "sometimes" play a role in virus transmission, but that larger droplets are still the primary means when they are projected by infected people toward others nearby. The droplet risk prompted social distancing strategies to prevent viral transmission by having people stay 6 or more feet apart.

SARS-CoV-2 might not present the aerosol risk that exists with the measles virus. Even so, little state or federal guidance suggests what to do about reducing this murky aerosol threat, which could magnify as winter drives Minnesotans indoors.

The U team's first results, published in the Journal of Aerosol Science, found that certain instruments generated more aerosols than others — a small trumpet more than a large tuba.

The risk of the instruments projecting virus-carrying aerosols horizontally into the crowd wasn't as bad as feared. Aerosols tapered off by 30 centimeters, or about a foot, Hong said.

The next stage of their research involved airflow measurements in Orchestra Hall and strategies to contain aerosols. Results have yet to be published, but the orchestra is putting them to use.

"The degree of aerosol coming out of a wind instrument or a brass instrument is actually less than we thought it might be," said Dr. Jon Hallberg, who has been advising the Minnesota Orchestra on how to mitigate risk as its players perform a revised fall season, for broadcast only, to an empty hall.

"Realizing there isn't this enormous difference between breathing and playing the instrument in terms of aerosol production, velocity and spread — that became very reassuring," he added.

Hallberg, a U faculty member, connected the orchestra with the researchers, who already were running simulations of how aerosols might spread under different ventilation levels in everyday environments such as elevators and classrooms.

A vast concert hall presented different challenges. Early experiments in Europe measured the droplets launched by instruments, but not the aerosols. A few outbreaks had the performing arts world worried. A choir practice in Washington state in March turned into a superspreader event when an infected singer projected the virus to as many as 52 of 61 participants. Two singers died.

While string players and percussionists can play wearing masks, wind and brass musicians blow air into a diverse set of instruments, some featuring metal mouthpieces, some reeds. Some tubes are straight, others coiled.

Trumpeter Charles Lazarus said he was happy to play for the U researchers and feels fortunate to be part of an orchestra relying on science to take steps back to the stage.

"This study not only gives us information about how to stay safe," he said, "it also allows us to share this information with musicians all over the world."

In front of a special mirror that visualizes airflow, Lazarus played as softly as he could, then as loudly as he could.

"It was a workout," he said. "You don't often hear a conductor ask: Play as loudly as possible for three minutes."

Among the 10 instruments they analyzed, U researchers found that the trumpet, oboe and bass trombone generated the most aerosols, while the tuba was less hazardous than someone talking or breathing.

"You put a lot of gas in the tuba, so you'd think, oh, that's going to produce a lot of particles," Hong said. "But … the tubing is much longer than other instruments, so it gets deposited along the way."

The amounts also varied by the musician, the pattern of notes, the loudness of the trumpet and the pitch of the woodwinds.

The trumpet is "most problematic," Hong said. So in recent weeks, the researchers experimented by putting one, two or even three filters across the trumpet's bell to see if they could tamp down aerosols without affecting the sound quality.

"More layers of course would cut down aerosol productions," Hong said. "At the same time, the trumpet becomes harder and harder to play."

Three layers cut particles by 92% but hurt sound quality, whereas two layers blocked 75% with only a slight reduction in quality.

While other orchestras have halted performances, the Minnesota Orchestra has found ways to keep making music. Over the summer, musicians hosted chamber concerts outside on Peavey Plaza. This fall, they're broadcasting Friday night performances on Classical MPR and Twin Cities Public Television.

Two weeks before rehearsal, players start light quarantines. A week out, they get tested for COVID-19. Backstage has been cleared of trunks so musicians have space to set up. Onstage, players are spaced apart. No one shares music stands.

"The NBA can spend $300 million to make a bubble in Orlando. Performing arts groups can't do that," Hallberg said, "So this is like a bubble-lite."

Hallberg, medical director of the University of Minnesota Physicians Mill City Clinic, has acted as the orchestra's touring physician, traveling with the band to Cuba and South Africa, so he knows what it takes to put on a concert.

An orchestra cannot eliminate risks inherent to playing together, but Hallberg said it can minimize them with best practices, built by studies like this one.

"I also want to see them make art," he said, "and provide solace that we all so desperately need."

At Orchestra Hall, the U researchers were surprised to find that the major concern wasn't the horizontal projection of aerosols by the musicians. Instead, temperature differences between warmer body heat and colder ambient air caused aerosols to rise in a plume a few meters above the performers.

"We were expecting that flow coming from the instruments was going to be dominant … but it turned out to be not the case," Hong said. "We had to redesign our experiment along the way."

An upcoming report will offer suggestions on how to address any risks created by the plume, including a filter above the musicians that could entrap 95% of the particles, and a reduction in temperature that could drive the particles farther upward and out of harm's way. Hong said it will have broader implications and recommendations for the use and positioning of air filters to capture particles in other indoor locations. CDC researchers have been in contact with the U team and might use their findings in their updated guidance to the nation on COVID-19 risks.

"The thermal plume is extremely important," Hong said. It's something a restaurant needs to consider, too, when deciding where to install filters, he said. A plume could have the power to lift particles up and over a plexiglass partition between tables.

A CDC case report about an outbreak in a restaurant in China showed how people sitting at three tables suffered COVID-19 cases that originated from one infected person in the center of the dining room. Some restaurateurs in Minnesota have reacted by adding high-quality air filters in their ventilation systems.

Thanks to a 2014 renovation, the air in Orchestra Hall is fully exchanged and filtered eight times an hour.

Learning that onstage, instruments don't move aerosols very far is "hopeful news," said Mele Willis, artistic operations manager.

Performers who played this summer only in quartets, or with others in the same instrument sections, have gained confidence from the U findings to start playing onstage in larger groups and different combinations. As the group grows, the stage will too, helping the musicians maintain 6-feet distances.

But, Willis noted, "We won't have a full Mahler symphony onstage for a while."

Jenna Ross • 612-673-7168