Airing or air purification? Research and science confirm:

Most rooms, especially classrooms, only have windows on one side of the room, so that only shock ventilation is possible in these rooms. However, the exchange of air with the outside air takes place much more slowly with shock ventilation than with cross ventilation – and only if there is a sufficiently large temperature difference between the inside and outside air or if the wind is strong enough.

If the temperatures are almost identical and if there is no wind, there is almost no air exchange. This means that window ventilation in the summer and in the comparatively mild autumn only has a protective effect in case of continuous airing! In the winter, with a higher temperature difference, there may initially be a good air exchange, but only for a short time and with decreasing effect. Due to the cold air supplied from outside, the interior space gradually cools down so that the required temperature difference becomes increasingly smaller.

Most of the time, especially in the winter, the room is aired too briefly in order to prevent it from getting too cold. However, the amount of air thus exchanged is far from enough. In this context, Eberhard Bodenschatz, Director at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, commented on the subject of ventilation in the news magazine "Der Spiegel" : "You just have to find out the temperature difference between indoors and outdoors. If the temperature outside the door is 10 °C and inside the room 20 °C, you will have renewed 50 % of the air as soon as you measure 15 °C inside the room after airing. Areas where ... the temperature does not change when airing should better be avoided".

Regular ventilation increases the energy consumption

Regardless of the effectiveness, open windows are a pure waste of energy when outside temperatures are low. Airing several times an hours cools down interior spaces, the precious heat energy escapes unused through the window – while the people inside the room are freezing. Additionally, the air supplied from outside must be heated again and again before being replaced by cold air again during the next ventilation interval. This not only contradicts the efforts of national climate policy, it also inevitably leads to immense additional costs.

Airing causes room temperatures below the minimum values required by law

Both employers and schools have a duty of care to their employees or pupils. They have to make sure that they do not suffer any harm from too high or too low temperatures. The Workplace Regulation A3.5 specifies room temperatures that are beneficial to health as follows:

However, shock ventilation in the winter does not only lower the room temperature by a few degrees as the UBA states in its infographic "Airing properly in everyday life". The room temperature drops to values around 15 °C in next to no time and is therefore below the values required by law.

Airing is "noisy" and disturbs lessons

According to the UBA, road traffic has long been the dominant source of noise in Germany. The agency states that 2.3 million people in Germany are exposed to noise levels exceeding 65 dB(A) all day. "Road traffic noise disturbs or annoys more than half of the German population", says the UBA website. And this noise enters classrooms through open windows. The lessons become more susceptible for disturbances from outside and this in turn leads to a lack of concentration, teachers therefore point out.

CO₂ is not an indicator of the virus load in the room air

The CO₂ emissions of people are approximately the same when breathing at rest, speaking or singing/shouting. The amount of aerosols emitted, however, depends on the number of infected persons inside the room and their activity. When speaking or singing, a lot more aerosol particles are exhaled than when breathing without exertion. Consequently, the virus load in a room can increase much faster than the CO₂ level. These examples show why the measured CO₂ value does not allow any reliable conclusions to be drawn about the aerosol concentration.

The UBA, too, writes in its "Ventilation recommendation for schools": An increased CO₂ concentration does not permit any conclusions about aerosols containing viruses, but it does indicate that there has been no ventilation for too long and therefore the risk of infection may also be increased.

This is quite simple and logical. With 30 healthy pupils in a classroom, the CO₂ concentration increases at a certain rate depending on the airing behaviour. If none of the pupils is infected, the concentration of virus-laden aerosol particles is low or even zero. But if several of these 30 pupils were infected with SARS-CoV-2, then the room could have a virus concentration causing an infection after only a short stay despite a low CO₂ concentration in the room.

The CO₂ content also increases in relation to the number of people present. If it took, for example, 30 minutes to reach 1000 ppm in a fictitious classroom with e.g. 20 people, it would take approx. 60 minutes if only 10 people were present. But still, one of these 10 people could be infected. This is why there might be a high infection risk even with a low CO₂ level under these circumstances.

Experts warn: airing conveys a supposed sense of security at the expense of health

Despite massive criticism from renowned scientists, many decision-makers rely on the UBA recommendations, which are negligent from a scientific point of view, and thus endanger the health of those who have to work or learn with open windows every day. Yet numerous studies prove that the effectiveness of ventilation in the fight against the spread of the coronavirus is doubtful.