With CO2 sensor and air exchange: How choir rehearsals can be carried out safely during pandemics

Scientists at the measurement campaign
In order to investigate how aerosols are distributed in space as possible carriers of viruses during a choir rehearsal, research teams from TU Bergakademie Freiberg and the University of Leipzig have carried out measurements of choir rehearsals in recent months.

In the journal "Journal of Voice", the researchers now report on their results. Accordingly, choir rehearsals can be carried out thanks to regular air exchange and special sensors in such a way that the CO2 concentration and thus also the aerosols in the room air remain below the critical range for an infection.

Aerosols are very small droplets that float in the air for a long time and can thus also transmit viruses. "Exhaled air contains CO2 in addition to the aerosols. Aerosols accumulate particularly in areas with a high CO2 concentration. If the aerosols contain viruses, the risk of their transmission through the air also increases in these areas, ”explains Prof. Rüdiger Schwarze from the Chair of Fluid Mechanics and Turbomachinery at the TU Bergakademie Freiberg.

Based on the measurement results, the team has now provided a kind of formula for safe choir rehearsals: In a 200 cubic meter room, each singing person is responsible for the increase in the CO2 concentration by around 1.8 ppm (1 ppm corresponds to one ten thousandth of a percent) per minute. This makes it possible to determine the time that a choir group can rehearse without an increased risk of infection. "If 15 people rehearse in a classroom, a critical CO2 concentration of 800 ppm is reached after 15 minutes, while 10 people can sing for 22 minutes without ventilation. If the room is regularly shock-ventilated for five minutes at these intervals, or if a room ventilation system is used, the CO2 concentration drops rapidly and it can be safely rehearsed, ”explains Dr. Lennart Heinrich Pieper from the Centre for Musicians' Medicine at the University of Leipzig. Prof. Dr. Michael Fuchs, Director of the Center, adds: “Of course, CO2 measurement is only one component in the context of all hygiene measures to reduce the risk of infection. But a very practical one for choirs – and this applies to the corona virus as well as to other viruses in possible future pandemic situations."

Further recommendations for choirs

The researchers also recommend installing a CO2 sensor near the ceiling of a sufficiently large rehearsal room: "Since the CO2 concentration is highest in the upper area of a room, a conventional sensor can warn choir groups at an early stage if the CO2 concentration reaches the threshold value," says Prof. Rüdiger Schwarze. It is also recommended if singers perform the usual warm-up exercises of their voice alone at home or in the car  instead of together. "Because during the movement-intensive vocal exercises, a particularly large number of aerosols are exhaled, swirled and the CO2 concentration rises very quickly," explains Dr. Lennart Heinrich Pieper.

Measure aerosol propagation

In order to record the CO2 content and thus the aerosol propagation during the choir rehearsal, the team installed a sensor field in the rehearsal room with ten measuring stands and three measuring probes each, with the help of which they continuously checked the air quality on three different levels – at hip height, mouth height and above the head. For this purpose, the air was flushed between the individual measurement runs; the value of the CO2 concentration is brought back to the same starting point. The team then placed the measurements next to each other and compared them. In order not to influence the measurements, the scientists spent the entire period outside the room and received the data via Bluetooth in real time on their monitors. All the singers involved had given their consent for the measurements to be carried out.

About the research project

Since the beginning of the pandemic in 2020 the TU Bergakademie Freiberg and the University Medical Center Leipzig have been researching aerosols and the associated virus spread – especially regarding singing in closed rooms. Together, measurements were carried out with several choir groups in rooms of different sizes, such as the Collegium Musicum of the TU Bergakademie Freiberg, the Universitätschor Leipzig and the Thomanerchor Leipzig.

Original publication: Katrin Bauer, Robert Hardege, Sebastian Neumann, Rüdiger Schwarze, Michael Fuchs, Lennart Heinrich Pieper: How safe is singing under pandemic conditions? - CO2 -measurements as simple method for risk estimation during choir rehearsals. Journal of Voice.

Contact persons

Flow simulation and pure particle propagation: Prof. Dr. Rüdiger Schwarze, Tel.: +49 (0) 3731 392486, E-Mail: Ruediger [dot] Schwarzeatimfd [dot] tu-freiberg [dot] de  
Medical questions: Peggy Darius, Tel.: +49 (0) 341 9715 798, E-Mail: presse-mfatmedizin [dot] uni-leipzig [dot] de