How Safe is Singing Under Pandemic Conditions? - CO₂-Measurements as Simple Method for Risk Estimation During Choir...

The SARS-CoV-2 pandemic has forced choirs to pause or at least to restrict rehearsals and concerts. Nevertheless, an uncertainty about the risks of infection while singing remains, especially with regard to distances, duration of singing, number of singers and their positions in the room, size of the room as well as ventilation strategies. Based on the assumption that CO₂ is a suitable indicator for the exhaled aerosols in a room, it is the aim of this study to deduce recommendations for a choir rehearsal with a minimum risk of infection.

During two choir rehearsals in a typical, nonventilated classroom, we installed 30 CO₂ sensors, which allow spatial and temporal evaluation of the CO₂ dispersion during singing. Various singing and ventilation phases were applied and the rates of CO₂ increase during singing as well as its decrease during ventilation phases were evaluated and compared for different scenarios.

The measurements reveal a linear relation between the duration of singing, size of the room and number of persons. For our size of the room of 200 m³ the average CO₂ increase is 1.83 ppm/min per person. Masks or pure breathing without singing do - in contrast to aerosol dispersion - not influence the rate of CO₂ increase. CO₂ disperses fast and homogeneously on horizontal planes. However, a vertical layering with a maximum CO₂ concentration is observed near the ceiling. Shock ventilation shows the largest CO₂ decrease within the first 5 min, after 10 min of ventilation the outside base concentration of 400 ppm is reached again.

The evaluated relations allow to calculate safe singing times for a defined number of singers and size of the room until a critical threshold of 800 ppm is reached. Furthermore, in order to monitor the actual CO₂ concentration during choir rehearsal, just one CO₂ sensor is representative for the air quality and CO₂ concentration of the whole room and thus considered sufficient. For an early warning, it should be installed near the ceiling. Direct singing into a sensor should be avoided. A ventilation time of just 5 min is recommended which represents a compromise between strong CO₂ reduction and still sufficient room temperature during winter time.