Spread of COVID-19 in schools: Video instructions to make effective exhaust air system with materials from a DIY store

In the third year of the coronavirus pandemic, the data clearly show that effective ventilation, i.e. the exchange of indoor air for outdoor air, massively reduces the concentration of infectious particles in indoor air. At the same time, however, classrooms are usually ventilated too infrequently and too briefly.

Christina Hopfe and her team from the Institute of Building Physics, Services and Construction (IBPSC) at Graz University of Technology (TU Graz) are now sharing promising findings and information on this. In autumn 2021, the researchers, led by Robert McLeod (IBPSC), equipped two classrooms at the Sacré Coeur School in Graz with a low-cost and simply constructed exhaust air system in a pilot test based on a concept from the German Max Planck Institute. The system draws stale, aerosol-containing air via extractor hoods and expels it outdoors while at the same time providing a permanent supply of fresh air.

With commercially available components, most of which can be obtained directly from a DIY store, material and installation costs of around 500 to 700 euros are incurred per classroom. An explanatory video (in English and German) on the construction and installation of the system has now been published on the Institute’s website at www.coved.tugraz.at and on TU Graz's YouTube Channel, written DIY instructions will follow by mid-August. “With well-planned and installed exhaust fans, the risk of infection is about eight times lower. Every classroom and every group room in the kindergarten can be retrofitted with such an effective exhaust air ventilation system easily and cost-effectively. It would be a huge contribution to reducing the incidence of infections in schools and kindergartens,” says Christina Hopfe.

Aerosols out, fresh air in

The principle of the exhaust air system is simple. Warm air rises upwards, transparent extraction hoods under the ceiling draw this stale air including the infectious aerosols into exhaust pipes, and a fan transports this air outside. A tilted window, which can be sealed at the side in cold winter months to prevent draughts, ensures a permanent supply of fresh air. The use of extraction hoods in combination with an upward airflow helps to remove the aerosols directly above infected persons before they can circulate in the room.

“This is where our exhaust air system differs significantly from expensive air filtration systems that suck in air to clean it. Filter systems are not a ventilation variant. Aerosols are drawn to this unit in the room, which means that the virus is not extracted where it is, but can spread throughout the room before it is extracted. As a result, people in the room can be exposed (locally) to a greater aerosol concentration over time,” Hopfe describes and continues: “The systems are helpful, but in our opinion should only be used when no other ventilation option is available.”

Low-maintenance, effective and cheap

The exhaust air system has a number of other advantages in addition to those mentioned above. There is no need to interfere with the building structure, the system has no filters and thus requires practically no maintenance, it is much quieter and more cost-effective than air filtration systems and even more energy-efficient compared to natural ventilation. “And the bonus is that no one has to freeze during shock ventilation,” says Hopfe.

The researchers built and installed the exhaust air system together with students from TU Graz and discussed it with the pupils of the Sacré Coeur. “It was a very exciting project for the children,” says Christina Hopfe. Generally, this could also be done by the school’s respective domestic staff. “It is relatively simple to set up and install this system. The components are available in DIY stores, and we have described everything else in a video and documented it in a step-by-step guide that will soon be published online,” says the building physicist.

CO₂ concentration as indicator for aerosol concentration

In addition to the exhaust air system, the two classes were also equipped with a CO₂ measuring device including a traffic light system. CO₂ can neither be smelled nor felt, so bad air is often only noticed much too late, resulting in drowsiness and difficulty concentrating. In addition, the higher the CO₂ content in the room, the higher the aerosol concentration. The two school classes in Graz have therefore been equipped with a CO₂ sensor including a traffic light display that shows when more fresh air is needed with the appropriate colouring.

“The minimum in our eyes would be constant monitoring of the CO₂ concentration in schools, kindergartens and university buildings. CO₂ sensors don’t detect corona viruses, but they clearly indicate when it is time to ventilate. And as with taking a temperature, the same applies here: it does not have to be measured to three decimal places. And to stay with the comparison, whether a temperature of 39.7 or 39.8 degrees, it is almost the same – namely high fever. Similarly with the CO₂ concentration. If this rises into a certain range (from approx. 1000 ppm; parts per million), fresh air is needed. There is no point in getting lost in the question of measurement accuracy here,” emphasizes Christina Hopfe.

The crux of ventilation in schools

In Austria, schools usually have a natural ventilation strategy, which means that ventilation takes place through open windows. However, Christina Hopfe says that this usually does not work well in everyday life: “In winter, many people find it too cold for adequate shock ventilation, while in summer it is often too hot, and overall it is simply too noisy in many classrooms due to road traffic when the windows are open. In schools with ventilation and air-conditioning systems, on the other hand, we often found deficiencies in maintenance and operation. So ventilation is rarely adequate, and that was the case even before Covid.”

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