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04/05/2024 | Campus-Meldung

Bus and train safe even during waves of infection

A research team led by Kassel transport scientist Prof. Dr.-Ing. Carsten Sommer has investigated the objective and subjective perception of safety on public transport during the coronavirus pandemic. The result: buses and trains are well-ventilated spaces, FFP-2 masks and ventilation effectively reduce the risk of infection to almost zero.

Image: Müller von der Grün.
A bus in Frankfurt at night.

Buses and trains were safe means of transportation during the coronavirus pandemic and are also safe during waves of infection caused by other airborne diseases. This is one of the findings of a research project at the University of Kassel on public transport during the coronavirus pandemic. The scientists led by Prof. Dr.-Ing. Carsten Sommer show that the risk of infection from viruses in the air we breathe on buses and trains differs fundamentally from that in enclosed spaces.

The coronavirus pandemic is over, says Sommer. But the experience gained from the recent pandemic should be used for future pandemics and epidemics. Regardless of this, seasonal infectious diseases such as colds or influenza continue to occur, which are also transmitted by aerosols and therefore through the air. "Public transport is an important pillar of sustainable mobility. It must and can be maintained even during waves of illness. With our study, we are highlighting infection protection measures that can help those responsible to ensure that public transport can still be used safely for passengers even during periods of high infection rates," Sommer addresses politicians, public transport stakeholders and vehicle manufacturers.

Science shows options for action for better public transport

During the coronavirus pandemic, Sommer's research group worked on the EMILIA project (Development of measures for pandemic-resistant local public transport) to determine the objective and subjectively perceived health risk in local public transport, survey the behavior and usage changes of passengers in local public transport and derive more than 60 options for action for local public transport from the findings, which are to be published in the second half of 2024.

Numerous partners from all over Germany and the federal government are working together

Numerous public transport stakeholders from several federal states took part in the research project, which was funded by the Federal Ministry for Digital and Transport (BMDV) with 1.3 million euros over three years. In addition, there were three project partners who helped the University of Kassel's research. You can read more about the partners here: https: //

Combination of FFP-2 mask and optimal ventilation is the best protection

"Public transport is not a driver of infection," says Sommer. As in all everyday situations, infections can of course also occur on public transport. However, the risk of infection can be effectively reduced with optimally adjusted ventilation systems. Masks work even better and should be considered as an additional protective measure, especially in times of high incidence. Sommer therefore advises a combination of FFP-2 masks and optimized ventilation.

Even with high incidences, the exchange of stale and fresh air through the optimal setting of the ventilation system and the use of FFP2 masks can reduce the risk of infection to well below 0.1 percent. You can read more about reducing the risk of infection here: https: //

Subjective perception and objective risks are two different things

"The subjective perception of a risk and the objective, actual risk of infection may well contradict each other," says Natalie Schneider, research assistant. Generally opening doors during the pandemic has been assessed as an important measure in public transport and has thus become a tool for improving the subjective perception of safety, reports the scientist. However, the results of the aerosol simulation in the vehicles showed that air exchange via the open doors at stops only made a small contribution. Continuous ventilation in the vehicle proved to be much more effective.

Politicians advise against using public transport despite scientific findings

In the early days of the pandemic, politicians advised against using public transport due to the presumably high risk of infection, reports Sommer. During the pandemic, a passage was included in the Federal Infection Protection Act stating that public transport should only be used to 50% of its capacity, even though it is the task of public transport to bundle traffic. According to the Kassel researchers, this was in contrast to the results that scientists had already published in initial studies on the risk of infection from the coronavirus Sars-CoV-2 in public transport. You can read more about the studies here: https: //

People avoid public transport during the pandemic

The presumed higher risk of infection in public transport, contrary to scientific knowledge, had an effect. Anna Helfers, research assistant and doctoral student in the field of risk perception in psychology, points out that in the first year of the pandemic, 57% of respondents reported that they had reduced or stopped using public transport. In the third year of the pandemic, 32% still avoided public transport. Public transport was perceived as a dangerous place by the representative sample of respondents from Kassel (between 788 and 1,081 people per wave) during all three survey waves in 2021, 2022 and 2023. Subjectively, using public transport was perceived as much "riskier" than being at work. The respondents named the compulsory wearing of masks, the general opening of doors at every stop and the ventilation of vehicles as the most important measures in their subjective perception to reduce the risk of infection. In times when masks were compulsory, monitoring compliance with these regulations was important for the respondents.

Herne draws conclusions: Staggered school start times increase comfort and safety

Johanna Koch from the EMILIA research team, together with WVI Prof. Wermuth Verkehrsforschung und Infrastrukturplanung GmbH, examined the impact of lower passenger numbers using an example from Herne. Johanna Koch reports that the city of Herne has staggered school hours to reduce the risk of infection during the pandemic. Specifically, this reduced the utilization of vehicles in Herne by 44% on average, thereby reducing the risk of infection by 18% without increasing vehicle requirements or costs. In other words: on a single morning without staggered school start times, 63 infections per 10,000 people could be expected. With staggered school start times, this would only be 52 infections per 10,000 people. It should be noted that the calculation assumes that no masks are worn.

Johanna Koch suggests developing concepts for staggered school times throughout Germany in good time before a new pandemic or, even better, staggering the start of lessons in general and also outside of a pandemic in order to increase comfort through reduced vehicle utilization and perceived safety for all passengers and to relieve public transport of the cost-intensive traffic peaks. You can read more about school time staggering here: https: //

Methodology and results of the study:

With the support of the ESI Group, the team of researchers from Kassel simulated the distribution of aerosols in a standard scheduled bus and a train. The team calculated the risk of infection under various conditions. "Wearing an FFP2 mask significantly reduces the risk of infection. In combination with effective ventilation in a public transport vehicle, the risk of infection is reduced even further," says Prof. Dr. Carsten Sommer, summarizing the key result. The lower occupancy of the vehicles with passengers reduces the risk of infection, albeit not as much as wearing a mask in combination with suitable ventilation. You can read more about the methodology here: https: //

The best tools for reducing the risk of infection: masks, ventilation, capacity utilization

The risk of infection in public transport changes with every change in environmental conditions. In order to be able to assess the effectiveness of individual protective measures, the researchers defined some basic conditions and examined the effectiveness of the individual measures by changing individual parameters in the simulation.

Basically, they made a very unfavorable assumption in terms of infection technology (worst case) and assumed an official incidence of 100 (100 infected persons per 100,000 inhabitants) and a dark figure of 3 (actual incidence 300) for a journey of 20 minutes in a city bus that is 80 percent full with only low ventilation and in which no passenger is wearing a mask. Under these conditions, there is an average risk of infection of 0.8 percent. This means that, based on these assumptions, 80 out of 10,000 people will become infected. In this worst-case scenario, the scientists have now changed individual assumptions in order to determine the protective effect of individual measures.

Wearing an FFP2 mask offers the best protection against virus infection from aerosols, even on public transport. If the other worst-case conditions continue to apply, but everyone wears an FFP2 mask, the risk of infection is reduced to almost zero.

The more efficiently the ventilation in the vehicle works, the more the risk of infection decreases. Under unchanged worst-case assumptions, but with optimal ventilation settings alone, the risk of infection drops to 0.06 percent. This means that 6 out of 10,000 people are infected on this journey.

Reducing the occupancy rate in the bus - under otherwise identical worst-case conditions - from 80 to 20 percent reduces the risk of infection from 0.8 to 0.3 percent. This means that in a bus with a low occupancy rate, 30 out of 10,000 passengers become infected; in a fully occupied bus, it is 80 out of 10,000 people.

"Any reduction in the risk of infection is a gain in safety," says Lea Fouckhardt, "but wearing FFP2 masks and optimal ventilation far outweigh other measures in terms of their positive effect." According to the researcher, if the three protective measures mentioned (FFP2 mask, optimal ventilation, low capacity utilization) are combined, the risk drops to almost zero. In this case, 5 out of 10,000,000 people would be infected.



Univ.-Prof. Dr.-Ing. Carsten Sommer
University of Kassel, Department of Transport Planning and Transport Systems
Phone: +49 561 804-3381
Mail: c.sommer[at]uni-kassel[dot]de

Dipl.-Ing. M. Sc. Natalie Schneider
University of Kassel, Department of Transport Planning and Traffic Systems
Phone: +49 561 804-3279
Mail: n.schneider[at]uni-kassel[dot]de

M. Sc. Lea Fouckhardt
University of Kassel, Department of Transport Planning and Traffic Systems
Phone: +49 561 804-3279
Mail: l.fouckhardt[at]uni-kassel[dot]de