Experts believe that SARS-CoV-2-laden aerosol particles are one of the key contributing factors to the spread of the coronavirus pandemic. Ventilating rooms remains one of the most important preventative methods against infection with COVID-19. However, the distribution of aerosol particles carrying viruses within a room is not uniform and depends on many factors. Calculations performed by researchers at TU Berlin’s Hermann Rietschel Institute reveal that areas with higher viral concentrations can develop in interior spaces despite high levels of virus-free supply air.
“Our results reveal clear differences in the distribution of viruses within a room and thus show which areas represent a greater risk. Our new evaluation method makes it possible to assess the effectiveness of any ventilation measures and the different levels of risk in different areas of a room,” says Professor Dr. Martin Kriegel, head of the Hermann Rietschel Institute (HRI). Assessing the risk of infection with SARS-CoV-2 via aerosol particles requires knowledge of their distribution within a room. Indoor air technology makes clear that there are areas within rooms where aerosol particles congregate and that there are other areas which are better flushed. In addition, viruses lose their tenacity over time, in other words become less infectious.
Experimental analyses represent one option for determining which type of ventilation has a high level of containment removal effectiveness and where best to position air cleaning devices. However, such experiments are time consuming and the limited number of measurement points and sensors means that they are only partially effective. In many cases, real boundary conditions cannot be recreated in an experiment.
As an alternative, researchers at the HRI are using computational fluid dynamics (CFD) to create time-dependent simulations of the distribution of airborne particles. Their work takes account of both the vitality and sedimentation of the viruses to demonstrate the potential risk of a smear infection. CFD calculations are now state of the art and recognized in research and development. They make it very easily possible to include modified geometries and adapt room climate boundary conditions. Comparative examinations of experiments and CFD studies conducted at the HRI over a number of years reveal very close congruence.
Using this new approach, it is possible to evaluate the effectiveness of ventilation systems and air cleaners in combating the risk of COVID-19 infection. The simulation identifies places where there is a higher viral load in the air and on surfaces. “This method is also suitable for the simulation of other airborne pathogens,” explains Kriegel.
These calculations have now been combined with the infection risk model developed at the HRI. The infection risk model was developed using the current state of knowledge regarding SARS-CoV-2 and the findings of a retrospective analysis of 12 small to large outbreaks in Germany and other countries. It calculates the potential risk of infection via aerosol particles in inner spaces based on criteria such as the time spent in a room.
However, our previous model as well as the models developed by other researchers did not take account of the spatial distribution of viruses in a room, despite the fact that, depending on the situation, significant local differences are possible. These models only permit an approximate evaluation of the risk of infection,” says Kriegel. And this is not sufficient to recommend and implement measures. “If we view a room in detail, we can see variations in virus concentrations of up to 100 percent. By extension, this means that an increased risk of infection persists in badly ventilated areas of rooms even when there is a good overall supply of virus-free air,” Kriegel explains. Significant differences in viral loads are particularly to be expected in rooms retro-fitted with air-conditioning units and air-cleaning devices. “The complexity of three-dimensional and transient room air flows is generally underestimated. Using indoor air technology to protect people requires a lot of know-how, particularly from the area of clean room technology,” Kriegel warns.
“Detailed statements on the need for ventilation measures for a specific room can only be made by combining previous calculations with computational fluid dynamics, “ Kriegel concludes.
The Hermann Rietschel Institute (HRI) at TU Berlin is the world’s oldest scientific institute dedicated to ventilation and air-conditioning technology. The research conducted by the HRI has contributed significantly to developments in the field and has helped incorporate research findings in industry. The appointment of Professor Dr. Martin Kriegel in 2011 saw the introduction of an interdisciplinary research focus on the distribution of airborne contaminants in rooms, drawing upon research findings on indoor airflows and air quality. The HRI has excellent facilities for conducting experiments, including a number of air flow labs, an operating room and a clean room. Numerical simulations are carried out using the institute’s 500-core high-performance computer.