Aerosol Transmission of Coronavirus: Classrooms Need More Ventilation




Dirk Müller



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1,000 students wearing face masks in a densely packed lecture theater – unimaginable in times of the coronavirus pandemic. But the risk of aerosol transmission in large, modern lecture theaters is lower than in many classrooms, according to calculations by RWTH researchers. The data also allow conclusions to be drawn about family celebrations.


In a comparative analysis, researchers from RWTH and their partners have assessed the risk of transmission of coronavirus through aerosols in classrooms. They found that compared to other types of room, classrooms pose a higher risk if they are not mechanically ventilated and if there are no guidelines in place to ensure sufficient ventilation.

This is due to the relatively high occupancy rate and the fact that teachers and pupils tend to spend long periods of time in classrooms, as Professor Dirk Müller from the Institute for Energy Efficient Buildings and Indoor Climate explains. The study compared the of risk of infection in various types of room. According to the study, classrooms and sports halls pose a greater risk than fully occupied lecture theaters with a capacity of 1.000, for example.

Classrooms, lecture halls, open-plan offices, and sports halls were evaluated in comparison to a reference situation, in which 25 individuals spend one hour in an average-size, automatically ventilated classroom with an air exchange rate of 4.4 times per hour. In the researchers’ simulation model, this reference situation defines a relative risk of one. Measured against the reference, the researchers consider the risk of infection in lecture halls and open-plan offices to be relatively low. The only situation that is even more critical in terms of infection risk than the classroom situation is the use of sports halls – under physical strain, individuals emit larger amounts of small airborne particles.

Larger Gatherings at Home Pose a Risk

"Data has now confirmed that entertaining larger gatherings at home can be much riskier than events in a public setting. In the private sphere, with window ventilation alone, the air exchange rate is often so low that the transmission of the virus via aerosols works well," says Müller. The risk of infection would be much lower in many public buildings equipped with ventilation systems. Well-ventilated rooms such as modern lecture theaters would also be much less of a problem, even with high occupancy rates.

Classrooms without mechanical ventilation might pose a higher risk of infection, especially in winter, when the windows are not kept open long enough to secure sufficient ventilation: often it is too loud outside to keep the windows open, or school students are sitting in freezing classrooms. As a result, there is insufficient ventilation, despite the regulations in place. According to Müller, recent studies have shown that window ventilation in classrooms often results in insufficient air exchange rates, as indicated by high carbon dioxide concentrations.

In a classroom holding 35 persons, the risk of infection may be almost twelve times higher than in the reference classroom. Even with a reduced occupancy of 18 persons, the air in the room would have to be exchanged 3.3 times per hour. This corresponds to an outdoor air volume flow of 660 cubic meters per hour.

Ventilation Technology Helps Protect Against Infection

How often do the windows have to be opened to achieve sufficient ventilation? The Aachen researchers and their partners are involved in formulating ventilation on behalf of the German Environment Agency. These also include guidelines on how to find out whether ventilation is sufficient: According to Müller, the carbon dioxide concentration in the room can be determined with the help of a simple measuring device.

An important factor is what the pupils are actually doing in a room: is it mainly the teacher who is talking, are the pupils engaged in group work activities, or are they engaged in physical activities? Sports activities indoors should only take place with a reduced numbers of participants; outdoor sports are to be preferred.

According to the calculations of the researchers, the risk of infection in large lecture halls is quite low; nevertheless, they recommend wearing face masks. Although the density of people in the lecture hall is comparable to that in a classroom, there is a much larger vertical column of air for each individual present as well as mechanical ventilation. An air exchange rate of two to three times per hour, which is typical for these buildings, is sufficient not to increase the relative risk of infection. The situation in open-plan offices equipped with ventilation systems is also considered unproblematic. In accordance with occupational health and safety regulations, the available office space and its movement areas are so generously dimensioned that even when fully occupied, the risk of infection through aerosols is relatively low.

“From my point of view, we should get a more rounded view of the situation before deciding on measures,” says Müller. This also applies to the obligation to wear of face masks in some pedestrian areas: “From a ventilation technology perspective, it can be assumed that virus transmission is not possible as long as the physical distancing rules are observed there.”