Risk assessment of rehearsal rooms for choir singing regarding aerosols loaded with virus [original]

Hermann-Rietschel-Institut | Fachgebiet Gebäude-Energie-Systeme | Marchstr. 4 D-10587 Berlin

Risk assessment of rehearsal rooms for choir singing regarding aerosols loaded

with virus

Authors: Anne Hartmann1, Dirk Mürbe2, Martin Kriegel1, Julia Lange1, Mario Fleischer2

1 Technical University of Berlin, Hermann-Rietschel-Institut

2 Charité Universitätsmedizin Berlin, Department of Audiology and Phoniatrics

DOI: http://dx.doi.org/10.14279/depositonce-10388

Introduction:

Regarding the current knowledge aerosols are one of the transmission ways of SARS-CoV-2-

viruses [RKI2020]. During speaking and singing aerosols are generated too, because the

airways, besides ventilation, are also the place of origin of voice source and spoken

language. Reports of high infection rates during choir rehearsal in closed spaces

[Hamner2020] indicate an increased aerosol production during singing, which could be

originated by characteristic mechanisms of voice physiology and the higher continuity of

vowel production over the time. Current investigations regarding the emission of particles

during speaking [Hartmann2020] and singing [Mürbe2020] showed an increase in the

aerosol production in both situations. Compared to breathing through the nose an average

increase of about factor 10 was described for speaking and for singing an average increase of

about factor 30 was described compared to speaking. A noticeable difference between the

different persons was found, whereas the role of “high-emitting persons” has to be kept in

mind for the transmission of the virus.

For the assessment of the risk of an infection with SARS-CoV-2-viruses during singing and for

an improved risk management in case of a restart of rehearsals and performances of choirs

besides the evaluation of the current prevalence of the disease and besides the general

hygiene and distance rules, other factors play an important role as well. These factors

include the number of Singers, the duration of rehearsal or performance, but particularly the

room situation and the air condition systems. Especially, the last two factors have been

addressed in an unsatisfactory manor in the current risk management and should therefore

be investigated in this paper through an analytical risk management based on the measured

particle concentration and typical room situations.

Background and methods

The longer a person stays inside a room, the further the aerosol concentration is raising.

Most important for the progression of this increase is the emitted concentration of the

contamination (number of infected persons and their particle emission rate), the air change

rate (supplied fresh air in relation to the room volume) as well as the effectiveness of the

ventilation system (ventilation effectiveness). Based on well-known equations to calculate

the concentration of contamination in internal spaces under the influence of different air

change rates, a risk assessment for different scenarios has been performed. The details of

the procedure can e.g. be found in [Kriegel2020].

Hermann-Rietschel-Institut | Fachgebiet Gebäude-Energie-Systeme | Marchstr. 4 D-10587 Berlin

Investigated scenarios

To assess the risks three typical application scenarios for choir rehearsals and performances

as well as a reference case have been selected:

 choir rehearsal room

 concert hall I

 concert hall II

 reference case: office

The calculations has been performed for the rehearsal room of the Berlin Philharmonic Choir

(assembly hall Clara-Grunwald-Schule Berlin), the Main hall of the concert house Berlin

(concert hall I – format “Schuhkarton” (“shoebox”)) as well as the concert hall of the

Dresden Philharmonic(concert hall II – format “Weinberg” (“vineyard”)). The number of

singers has been assumed basing on a radial distance of 2 m and the size of the stage area.

One infected singer in the room has been assumed and only the aerosols emitted by this

person have been classified as potentially critical, because they could carry viruses. An

average emission rate for singing of 2000 P/s has been used [Mürbe2020]. The number of

visitors applied to the current hygiene concepts of the two investigated concert halls. The

boundary conditions for the different scenario can be found in table 1.

Table 1: boundary conditions of the investigated scenarios

room

volume

in m³

number of

persons

choir

Number of

persons

visitors/

office

activity

emission

rate

in P/s

air change

in 1/h

ventilation

system

re-

hearsal

room

2,250

singing

2.000

rehearsal:

0.05

break:

window

concert

hall I

18,000

350

singing

2.000

2.2

mechanical

concert

hall II

22,500

498

singing

2.000

2.9

mechanical

office

speaking

130

2.4

mechanical

office

speaking

130

work:

0.05

break:

window

Results of the risk assessment

The progression of the aerosol concentration, of the potentially infectious aerosols (emitted

by the infected person), during 30 min of singing with a constant number of persons and a

constant air change rate in both concert halls as well as with missing ventilation in the

rehearsal room can be found in figure 1. For all three cases as well as the reference case a

break for ventilation of 15 min after the 30 min in which all persons have left the room has

been assumed. Finally, the concentration after another rehearsal of 30 min is listed. The air

change rate has been calculated as constant for both concert halls over the whole time,

Hermann-Rietschel-Institut | Fachgebiet Gebäude-Energie-Systeme | Marchstr. 4 D-10587 Berlin

whereas the ventilation through windows just has been performed during the breaks in the

rehearsal room as well as the office.

In addition, another case with a rehearsal of 60 min, followed by a 15 min break and another

rehearsal of 60 min has been investigated the result can be seen in figure 2.

Figure 1: progression of the aerosol concentration during a rehearsal of 30 min with a break of 15 min in the

middle for the three investigated rooms as well as the reference scenario office

Figure 2: progression of the aerosol concentration during a rehearsal of 60 min with a break of 15 min in the

middle for the three investigated rooms as well as the reference scenario office

Discussion

With simple analytical equations and simplified boundary conditions the increase of the

concentration of potentially infectious aerosols has been investigated. Nevertheless, in real

environments the results can be significantly different. A reason therefore is the real

ventilation effectiveness. There will be positions in the room with a higher concentration of

virus and other positions with a lower concentration. Typical positions with different

concentrations are near persons as well as above persons, because of thermal buoyancy

effects (the person is warmer than the surrounding air). Because of the fact that aerosols are

ideally airborne, they will be dispersed with the room air flow.

Hermann-Rietschel-Institut | Fachgebiet Gebäude-Energie-Systeme | Marchstr. 4 D-10587 Berlin

It becomes clear, that window ventilation may not be enough in some circumstances to

reduce the risk of infection through aerosols significantly and therefore a deceptive security

may occur. In general, a user does not know how much fresh air is introduced into the room

through an opened window. This volume flow mainly depends on the temperature

difference between inside and outside as well as the velocity of the wind. Solely mechanical

ventilation systems will be able to reduce the risks of infection with a high predictability.

Furthermore, large rooms have a positive effect on the increase in the aerosol

concentration. Whereas in the office (60m³room volume/infected person) with window

ventilation the concentration of potentially infectious aerosols has started to increase

immediately, in the rehearsal room without mechanical ventilation system

(2270m³room volume/infected person) it can be assumed that the increase will be a lot slower,

because the potentially infectious aerosols will be dispersed in the whole room in a lower

concentration at first.

With an increasing number of infected persons in the room the increase will be significantly

steeper. Even with three infected persons of the choir (dashed, blue line) in the rehearsal

room (without ventilation, 757m³room volume/infected person) this curve has exceeded the

curve of the office with mechanical ventilation fast. On the other hand, if for the concert hall

it has been calculated with further infected persons in the audience the concentration in the

room after 30 min just has been increased by 10 % and is therefore still significantly below

the concentration in the office room.

In addition, it has been found that in the rehearsal room even if the windows have been

opened, just a little decrease in the aerosol concentration can be found, which will raise

above the level before the break pretty soon after the singer has entered the room again.

Summary and recommendations for the risk management

The transmission of SARS-CoV-2-virus mainly occurs via the airways in the shape of droplets

and aerosols. During singing droplets as well as aerosols are generated in a higher number,

whereas a higher risk during choir singing can justified.

The previous measures to reduce the risk have focused on the protection against droplets,

which is also used efficiently for singing with a distance of 2 meters. This distance is also

reducing the risk of transmission through aerosols in the local area, but no assessment of the

further dispersion of particles in the room is possible. For the evaluation of the aerosol

concentration in the room important factors have been included in the present calculations

of the increase of the concentration of potentially infectious aerosols, which compares

typical situations for choir singing with an office as reference scenario.

A possibility to reduce the risk lies in a shortening of the duration of rehearsal, whereas in

the current example two parts of rehearsal of 30 min each and a compulsory ventilation in

the middle should be a feasible compromise from an artistic view, which has been applied in

the calculations. To increase the air volume per person large rooms should be selected for

choir rehearsals. The number of singers has to be reduced, whereas already because of the

distance necessary to protect against droplets the number of persons in the room is limited.

An important factor for the risk reduction lies in the existing ventilation options, whereas

window ventilation and mechanical ventilation have been compared. Especially an adequate

mechanical ventilation has a significant potential to reduce the risk and therefore this

possibility should be used. Window ventilation may help to reduce the risk as well, but it

mainly depends on the weather conditions und is therefore difficult to assess. If there are

just short breaks, it has to be kept in mind, that window ventilation may not be enough to

reduce the aerosol concentration.

Hermann-Rietschel-Institut | Fachgebiet Gebäude-Energie-Systeme | Marchstr. 4 D-10587 Berlin

The investigations have shown, that under given circumstances and with an optimal use of

different possibilities choir as well as ensemble singing may be possible, even when a certain

remaining risk of infection has to be addressed. The number of singers as well as their

positions, the room size and the ventilation system have been found to be effective

measures to the reduce the risk of infection in addition to the basic hygiene and distance

rules. Based on the importance of singing for culture and education a best possible risk

management is of special importance, to make, in cooperation with private and public

decision makers, individualized choir rehearsals and performances possible.

Literature:

[Hamner2020] Hamner, L., Dubbel, P., Capron, I., Ross, A., Jordan, A., Lee, J., Lynn, J., Ball, A.,

Narwal, S., Russell, S., Patrick, D., Leibrand, H. (2020): High SARS-CoV-2 Attack Rate

Following Exposure at a Choir Practice – Skagit County Washington, March 2020, in: MMWR

Morbidity and Mortality Weeky Report, Early Release 69, p. 606-610

https://dx.doi.org/10.15585/mmwr.mm6919e6

[Hartmann2020] Hartmann, A., Lange, J. , Rotheudt, H. , Kriegel, M. (2020): Emission rate

and particle size of bioaerosols during breathing, speaking and coughing, in: Preprint,

http://dx.doi.org/10.14279/depositonce-10331

[Kriegel2020] Kriegel, M., Hartmann, A. (2020): Risk assessment for internal spaces regarding

aerosols loaded with virus, in: Preprint, http://dx.doi.org/10.14279/depositonce-10344.2

[Mürbe2020] Mürbe, D., Fleischer, M., Lange, J., Rotheudt, H., Kriegel, M. (2020): Aerosol

emission is increased in professional singing, in: Preprint,

http://dx.doi.org/10.14279/depositonce-10375

[RKI2020] Robert-Koch-Institut (2020): SARS-CoV-2 Steckbrief zur Coronavirus-Krankheit-

2019 (COVID-19),

https://www.rki.de/DE/Content/InfAZ/N/Neuartiges_Coronavirus/Steckbrief.html#doc1377

6792bodyText1, letzter Zugriff: 22.06.2020, 12:00Uhr

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