Ventilate, ventilate again and again: This is a key recipe in the fight against the corona pandemic in schools. In order to reduce the risk of infection, the air in the classroom should be exchanged three times an hour, advises the Federal Environment Agency. All windows should be opened wide every 20 minutes, also during the breaks.
Especially in the first to sixth grades, in which most children cannot yet be vaccinated, they should be ventilated regularly and consistently. But interim results of a study at the Munich University of Applied Sciences now suggest that this idea misses the point of reality. And other common ventilation concepts are therefore also unreliable.
In the research project “Safe Classroom”, scientists have been measuring the air quality in 233 classrooms at 52 schools in the greater Munich area and in Mainz since the summer. They record the CO₂ content in the air; this provides information about the proportion of exhaled air and thus also about the amount of aerosols through which the coronavirus can spread.
Scientists from the university, the Technical University of Munich, the Ludwig Maximilians University and the Mainz Max Planck Institute for Chemistry are involved. The project is still ongoing; In the meantime, however, they have evaluated the data from more than 7,000 teaching days, says project manager and medical computer scientist Christian Schwarzbauer from the Munich University of Applied Sciences.
The data show: The recommendation of the Federal Environment Agency is “hardly implemented in practice,” says Schwarzbauer. “The proportion of classrooms that were regularly ventilated every 20 minutes was less than eight percent.” Instead, the room is usually only aired between the hours or only during the breaks. The result: the limit values for carbon dioxide in the air were exceeded on more than every fourth day of class with classic window ventilation – and not in individual hours, but on a daily average.
Apparently, reliance on the devices was less ventilated
Measuring devices, so-called CO₂ traffic lights, changed little. If there were such devices in the room, the rooms were still not ventilated enough on 22.4 percent of the class days. Even in rooms with permanently installed ventilation and air conditioning systems, things didn’t go any better. Here the limit values were exceeded on 22.8 percent of the days. These systems are actually considered the gold standard for ventilation, says Schwarzbauer. But many are wrongly adjusted. And they are apparently so quiet that they can fail unnoticed. Then the windows stay closed, the air stands still.
The study showed that decentralized ventilation systems, such as those installed on the walls, and do-it-yourself systems, as conceived by the Max Planck Institute for Chemistry in Mainz, were significantly more effective. Here, hoses lead the exhaust air from the tables to the window, where it is blown outside by fans. The Michaeli-Gymnasium in Berg am Laim had recently tested such a system, but the city did not support an expansion. In rooms with such systems, the limit values were exceeded on only 4.6 percent of the class days.
On the other hand, most frequently, namely on 34.2 percent of the days, the CO₂ limit values were exceeded when a mobile air purification device was installed. Such devices filter aerosols from the air, no CO₂; so that doesn’t mean that the machines won’t work. Apparently, however, reliance on the devices was less ventilated.
In fact, the researchers have already calculated how quickly the virus concentration in the air would increase depending on the ventilation. The results underline how important it would be to really ventilate every 20 minutes. Ventilation systems and air purifiers also do well here. In the next step, the scientists now want to calculate how high the real risk of infection is in the participating classes.