Estimating the risk of airborne SARS-CoV-2 transmission in a train carriage

In a recent Indoor Air journal study, researchers in the United Kingdom evaluated the risk of airborne transmission of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in a mechanically ventilated inter-city train carriage in the United Kingdom.

Study: An evaluation of the risk of airborne transmission of COVID‐19 on an inter‐city train carriage. Image Credit: Blue Planet Studio / Shutterstock.com

Background

During the coronavirus disease 2019 (COVID-19) pandemic, traveling through public transport was thought to increase the risk of exposure to SARS-CoV-2, especially during long journeys, due to reduced ventilation and the inability to enforce social distancing. While mask-wearing and disinfection of surfaces were encouraged to reduce viral transmission through large droplets and fomites, reducing airborne transmission was a challenge.

Various studies exploring the transmission of airborne pathogens in trains have recommended using personal protective equipment (PPE), increasing the distance between passengers, and opening windows to increase ventilation. However, train carriages with mechanical ventilation optimized for energy efficiency and thermal comfort do not have the flexibility to modify the airflow.

About the study

The present study used computational fluid dynamics (CFD) simulations and in-service measurements of carbon dioxide (CO₂) and salt aerosol concentrations to determine the risk of airborne SARS-CoV-2 transmission in a U.K. inter-city train carriage. Laboratory tests were also performed to assess the efficacy of the heating, ventilation, and air conditioning (HVAC) filter material used in the train to remove particles smaller than 10 micrometers (µm) in diameter.

CFD simulations of the airflow were used to understand the airflow direction within the carriage and areas of recirculation and stagnancy. In addition, salt aerosol droplets sprayed from a nebulizer inside the empty train carriage during driver training exercises were measured to determine aerosol particle concentrations. The flow rate of the nebulizer was fixed to be within the range of the average human breathing rate.

CO₂ sensors that function on the non-dispersive infrared principle were used to determine CO₂ concentrations based on the proportional absorptance of infrared light while the carriage was in service. CO₂ concentrations were considered a proxy for rebreathed air. The number of passengers and their locations were manually noted.

The probability of one infected individual was calculated using the infectivity rate based on the number of passengers and the quanta generation rate. The quanta generation rate depends on multiple factors, such as the viral load and activity level of the infected individual, the SARS-CoV-2 variant, and the vaccination status of the passengers.

Study findings

Assuming a lower infection rate of one in 500, the absolute risk was very low at 0.01%, even with 136 passengers in the carriage, which would have 48 passengers standing. A higher infection rate of one in 100 increased the transmission risk; however, it remained low at 0.05%.

The highest quanta generation value gave an infection probability value of 4%. With the carriage nearly at capacity with 136 passengers, the number of passengers with secondary infections was estimated to be 5.6, which dropped to 1.4 when the carriage had only 34 passengers.

When the risk estimates for scenarios in the train carriage were compared to those from other typical situations, the infection risk from a one-hour-long train journey with one infected person in the carriage was six times lower than that from a full day spent in a well-ventilated office room, and 10 to 12 times lower than that from a full day spent in a poorly ventilated office with an infected person. However, the relatively lower risks are a factor of the duration of stay in an office, with the risks still not low enough to ignore.  

Furthermore, the cumulative risk over extended periods is more challenging to estimate, as they depend on factors such as repeated exposures, variants in circulation, and time spent next to an infected person. In an office, however, the likelihood of multiple exposures is higher since the same people visit the office every day, while the probability of sharing a carriage with the same person is lower, and the time spent next to them is shorter.

Conclusions

An hour-long train journey with one SARS-CoV-2-infected person in a mechanically ventilated inter-city carriage in the U.K. had a six times lower infection risk than a full day spent in a well-ventilated office and a 10 to 12 times lower risk than a full working day in a poorly ventilated office with one infected person.

The risk of potential reinfections is significant if the train carriage has an exceptionally infectious passenger. Given the speculative nature of these results due to the estimated quanta generation values, it is vital to continue to adhere to disease mitigation behaviors to reduce transmission risks.