Keep your distance: A new video from the Bauhaus-Universität Weimar illustrates how germs can spread through the air.
What happens when you cough? With the help of a so-called schlieren mirror, researchers at the Building Physics Professorship make visible what is normally hidden from the naked eye: Air currents in a room. The short film impressively shows why we should follow the World Health Organization’s (WHO) recommendations on how to prevent the spread of the coronavirus.
The silhouette of a man is shown, first breathing normally and then beginning to cough heavily. At start, he demonstrates without anything covering his mouth. Then, he uses his hand, the crook of his elbow, and various types of masks. »When coughing without covering the mouth, it is especially clear how easily the air we breathe can permeate a room«, explains Prof. Conrad Voelker, Professor of Building Physics. For this reason, it is essential to cover your mouth when coughing according to the experiment. »The best method is to cover your mouth with the crook of your arm and to keep your hands clean to avoid spreading viruses and other pathogens via body contact or surfaces«, adds Prof. Voelker. The use of respiratory masks also shows an improvement in slowing the spread. Besides that, respiratory masks filter aerosols transporting viruses, but there is no 100% effective method to prevent droplet infection.
The schlieren imaging for the visualization of indoor airflow
MSc. Amayu Wakoya Gena, a DAAD-scholar in Building Physics at the Bauhaus-Universität Weimar, led the experiment, which was carried out for the current situation. Within his doctoral thesis, he uses the so-called schlieren imaging, which helps to visualize and measure indoor airflow. The centerpiece of the measuring devices is a concave and highly polished mirror with a diameter of about one meter. With the help of this schlieren mirror, even the smallest air currents are visible. »The principle is like when a road overheats in summer and the air above the asphalt shimmers«, explains Prof. Voelker. Just like the air above the asphalt, warm, moist air that we breathe has a different density than the cooler air in the room. This difference in density leads to a deflection of light, which then appears as dark spots in a schlieren photo or video. These density differences, however, are extremely small in indoor airflows. They cannot be seen with the naked eye, but the schlieren mirror allows them to be visualized.
There are currently only four large schlieren systems in the world. Each one is used in a different research field. In Weimar, the focus is on building physics measurements. At the Bauhaus-Universität Weimar, the large schlieren imaging system is used primarily to investigate indoor airflow and determine the influence of indoor climate on the occupants. The goal is to develop individual solutions that optimize the energy efficiency of rooms. In 2017, researchers at the Bauhaus-Universität Weimar were the recipients of the award for »Deutschland - Land der Ideen« competition. The state of Thuringia provided around 400,000 euros to fund the schlieren mirror. The project was co-financed by European Union funds through the European Regional Development Fund (ERDF).
The film starts by clicking on the picture or can be viewed here:
Addendum from April 6, 2020
In view of the current debate about the use of face masks, we explicitly point out that the experiment was carried out with a dust mask and a simple surgical mask. One can see the visualizations of the indoor air currents, i.e. No medical statements can be made based on the video regarding the effectiveness of mask protection. Whether and how droplets diffuse through the material must be tested in further investigations. Our experts are already in contact with medical experts in this regard. However, it cannot be predicted when reliable quantitative data are available.