Last March, a person with mild COVID-19 symptoms went to a two-and-a-half-hour chorale practice in Skagit Valley, Washington. In the weeks that followed, more than fifty people — nearly everyone who attended the rehearsal — contracted the disease, and two people died.
Shelly Miller, a mechanical engineering professor at the University of Colorado Boulder, led a recent study of that rehearsal, one of the nation’s first super-spreader events. The study states that singing indoors and unmasked can swiftly spread COVID-19 via microscopic airborne particles known as aerosols.
In the study, Miller explains that although members of the chorale took precautions, such as using hand sanitizer, avoiding touching each other or surfaces and propping doors open, they didn’t wear masks. Members of the chorale were interviewed as part of the study, and researchers concluded that there simply were not enough opportunities for droplets and infected surfaces, known as fomites, to transmit the virus to the number of people who fell ill afterward.
Instead, poor ventilation in the indoor space led to a build-up of aerosols produced by the singers, and heat produced by the singers themselves mixed the air within the room.
Miller, who was also part of a study with scientists and musicians involving aerosols from woodwind instruments, says super-spreader events happen indoors when there's not enough ventilation and too many people are releasing aerosols.
As for wind instruments, Miller says they do release aerosols, particularly through the bell of the instrument. That has the potential to carry the virus.
Miller says the aerosol plumes emitted from the bells of instruments are more predictable than those released through singing.
Jean Hertzberg, associate professor of mechanical engineering at CU, has made videos about air flow from instruments using flow visualization. “You can see when the plume comes out,” Miller says. “The plumes that are released around the mouth and the keyholes are much more variable depending on what you're doing.".
Miller was surprised that researchers didn't see very much aerosol released by flutes, while they noticed a lot of aerosols released by instruments such as trumpets and oboes, which require pressure to make sound.
“They seem to be generating a little bit more aerosol,” Miller says.
To keep musicians and audiences safe, Miller recommends that brass and wind players use some sort of bell covering on their instruments. She also encourages singers and musicians to wear masks, and recommends performing in a room with the windows open to allow for ventilation.
Finally, Miller advises musicians to practice social distancing and limit their playing times to thirty minutes at the most, with breaks in between.
"All of those are recommendations based on our experience with COVID and the literature and the science, and also our measurements,” she notes.