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Catching Covid On A Train ‘is As Likely Wherever You Sit Or Stand’

Catching Covid on a train is just as likely wherever you sit or stand, according to new research.

There is no ‘safe spot’ that reduces risk as airborne diseases spread along the length of a carriage, and in the absence of effective ventilation systems commuters have nowhere to escape or hide, say scientists. The study found air flow is slowest in the centre – yet this has no affect on an individual’s vulnerability.

First author Rick de Kreij, of Cambridge University, explained: “If an infectious person is in the middle of the carriage, then they’re more likely to infect people than if they were standing at the end. However, in a real scenario, people don’t know where an infectious person is, so infection risk is constant no matter where you are in the carriage.”

The findings in the journal Indoor Air are based on computer simulations and controlled experiments in a real train carriage. They also showed masks are more effective than social distancing at preventing transmission – especially in trains that are not ventilated with fresh air. The results also demonstrate how challenging it is for individuals to calculate absolute risk.

Mr de Kreij called on train operators to improve ventilation systems to help keep passengers safe from respiratory infections – including flu. He said: “In order to improve ventilation systems it’s important to understand how airborne diseases spread in certain scenarios, but most models are very basic and can’t make good predictions. Most simple models assume the air is fully mixed, but that’s not how it works in real life.

“There are many different factors which can affect the risk of transmission in a train – whether the people in the train are vaccinated, whether they’re wearing masks, how crowded it is, and so on. Any of these factors can change the risk level, which is why we look at relative risk, not absolute risk – it’s a toolbox that we hope will give people an idea of the types of risk for an airborne disease on public transport.”

His team developed a one-dimensional model which illustrates how an airborne disease such as Covid can spread. It is based on a single train carriage with closing doors at either end and can be adapted to fit different types of transport – such as planes or buses.

The motion is linear – to the left or right only. It considers the essential physics for transporting airborne contaminants while being much cheaper than 3D models. It was validated using measurements of controlled carbon dioxide tests conducted in a full-scale railway carriage. CO2 levels from participants were measured at several points. The evolution of the gas showed a high degree of overlap with the modelled concentrations.

Many commuter trains in the UK have been manufactured to be as cheap as possible when it comes to comfort – getting the maximum number of seats per carriage. In addition, instead of pulling fresh air in from outside – which has to be heated or cooled at more cost – they recirculate it.

So, if it’s impossible for passengers to know whether they’re sharing a train carriage with an infectious person, what should they do to keep themselves safe? Mr de Kreij said: “Space out as much as you reasonably can – physical distancing isn’t the most effective method but it does work when capacity levels are below 50 percent. And wear a high-quality mask which will not only protect you from Covid – but other common respiratory illnesses.”

The researchers are now looking to extend their 1D-model into a slightly more complex, yet still energy-efficient, zonal version where cross-sectional flow is characterised in different zones. It could also be extended to include thermal stratification which would offer a better understanding of the spread of an airborne contaminant.

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