How do wind instruments spread aerosols indoors and out?

In March 2020, the Skagit Valley Chorale in Mount Vernon, Washington, made headlines after a 2.5 h choir practice transformed into a COVID-19 super-spreader event. Since then, it’s become clear that SARS-CoV-2, the virus that causes COVID-19, spreads primarily via respiratory particles called aerosols and that singing increases transmission.

In the wake of the outbreak, a multi-institution team headed by air quality expert Shelly L. Miller of the University of Colorado Boulder got to work investigating risks and devising ways to keep musicians safe. Wind instruments, in particular, may pose a transmission risk because the musician’s breath is forced through the instrument. The team’s new study shows that clarinets generate aerosol plumes comparable to singing, which can be reduced by placing a mask over the end of the instrument (ACS Environ. Au 2021, DOI: 10.1021/acsenvironau.1c00007).

The study participants, undergraduate and graduate students who tested negative for COVID-19, sang or played wind instruments for 4–5 min alone in a small room with limited airflow. The researchers used imaging techniques to track the flow of aerosols emanating from each subject’s mouth or instrument and measured the velocities, lengths, and widths of those plumes. The researchers also took a unique approach by using measurements of CO₂ concentration to provide additional information about how the plumes spread out with distance.

The authors focused on a test case comparing just one clarinet player and one singer. The concentration of airborne particles at the base of the clarinet was similar to that measured in front of the singer’s mouth, and both scenarios produced more particles than speaking. However, simply securing a surgical mask with a rubber band to the end of the clarinet was enough to decrease emissions by about 50%.

Using the experimental data, computational models from coauthors at the University of Maryland simulated the spread of SARS-CoV-2 from an unmasked singer and clarinet over 30 min and 60 min time windows, in indoor and outdoor environments. The researchers found that a 30 min or shorter indoor performance posed less than a 10% risk of infection to people in a 4-by-4.5-by-3.5 m room. Outdoor performances of 60 min or shorter also showed low risk.

Masking the singer and the end of the clarinet would further reduce the aerosols in the room, says Tehya Stockman of CU Boulder, lead author of the study and a clarinetist herself. The aerosols would also disperse more quickly and not be as highly concentrated directly in front of the performer.

Although ballpark time estimates like these can be useful, Jiarong Hong, an expert in flow imaging from the University of Minnesota Twin Cities who was not involved with the study, cautions against making general recommendations regarding activity duration during the pandemic. These recommendations rely on estimates of infection risk, which vary greatly depending on numerous factors such as the infectious dose of a virus, indoor ventilation, positions of the individuals, and wind conditions in outdoor spaces.

Hong’s group has investigated particle emissions from wind instruments played in the Minnesota Orchestra Hall, which facilitates different airflow patterns than smaller spaces (J. Aerosol Sci. 2021, DOI: 10.1016/j.jaerosci.2021.105797). He noted that different instruments, musicians, pitches, volumes of sound, and note durations produce highly variable flow and aerosol concentrations (J. Aerosol Sci. 2021, DOI: 10.1016/j.jaerosci.2020.105669). “If you want to reproduce a real, practical scenario, you have to consider the variability among these factors,” he explains.

Stockman says a multilayered approach that includes masking, vaccines, social distancing, and good ventilation will help keep musicians safe. “There are easy techniques to mitigate your risk and still be able to play music,” she explains.