Whether you’re watching from afar, cheering in person or (girl can dream) competing in the Olympics, each event is thrilling from start to finish – but not as thrilling as waiting to see what illness you might contract from taking part. In fact, experts have warned that any athletes who compete in the murky waters of Paris's Seine river run the risk of getting sick afterwards – with diarrhoea extremely likely.
“I would never have selected the Seine,” Davey Jones, professor of Environmental science and public health at Bangor University, told BBC Science Focus. “I know it’s iconic, but it also runs through a hugely urbanised area which is always a recipe for disaster when it comes to potential exposure to chemicals or biological pathogens.”
So how bad could it be? According to Jones, we’re unlikely to see any illness soon after the open-water marathons on Thursday (the women's event) and Friday (the men's). “The risk will come the next day or the day after when they get diarrhoea," he says. "They’ll probably recover quite quickly within a week or two."
Several athletes have also shown concerned after Tuesday's training session in the Seine was called over concerns about levels of faecal matter in the water. The session had been cancelled five times previously because of pollution in the river.
"I think if anyone's saying they're not concerned at all, they’re probably lying," Austrian swimmer Felix Auböck told Associated Press. "I am concerned. I just hope and trust the organisation in the sense that they let us in when it’s safe enough to do so. But, of course, you're concerned because no one wants to get ill or sick."
A viral threat
According to Jones, part of the problem is that we’re not testing for the right things when we decide whether water is safe to swim in. Organisers have repeatedly cancelled training sessions and even postponed events because of unsafe levels of the bacteria E.coli being present in tests.
While E.coli is certainly a risk, Jones says: “Evidence suggests a lot of us get sick in water from viruses, and viruses behave very differently from E.coli. So if you just measure E.coli it provides no information whatsoever on the risk of viruses.”
In fact, Jones’ research team has observed ear infections as a classic post-swimming illness, as well as norovirus (the vomiting bug).
“If it’s failing on E.coli it’s likely to be failing on norovirus – based on the evidence we have from Wales and England,” Jones says.
“Those things are in the population all the time. Whenever there’s sewage overflow, for example, into the Seine, norovirus gets released into the water,” he added. “You only need, in theory, to ingest 1-10 viral particles to get sick. And when we’re sick, every time we go to the toilet, we emit trillions of these viral particles.”
These nasties spread through water directly from the sewage in our houses. While this waste is usually treated before being released into rivers like the Seine, treatment may not entirely eliminate the risk of disease. Even in the best UK treatment works, 0.1 per cent of the sewage still comes out. “And 0.1 per cent of a lot is still a lot,” says Jones.
Besides, these sewers are designed to overflow following heavy rain as the rainwater fills up their capacity. The effluent, known as ‘stormwater’, is a blend of rainwater and raw sewage, and it flows straight into the river. Heavy rain, of course, delayed the first outdoor Olympic swimming event this year.
Even though the Seine is a massive river, the dilution would have to be huge to eliminate the risk from these pathogens. The athletes’ body suits protect them to some extent, but they can’t keep their head above water all the time or stop their mouths opening.
According to Prof David Werner, environmental systems modelling expert at Newcastle University, it would be a very difficult job to clean up the Seine properly: “Clean rivers require major mitigating measures. In a city like Paris with a high population and mostly urban land use the challenge to achieve bathing water quality in an urban river will be huge.”
What’s more, Werner’s research has shown that drainage systems for roofs and roads also release enough pathogens into the water to make bathing water unsafe.
So would these experts swim in the Seine? “Intuitively I wouldn’t swim in the river where it flows through Paris after heavy rainfall,” says Werner.
“As a scientific experiment to get sick, I would,” says Jones. “But personally, I wouldn’t. I think there are better places to do it.”
But given the Seine has been selected and the open-water events are going ahead, Jones sees it as an opportunity for this “great scientific study to see how many people get sick.
“I wish them the best of luck.”
About our experts
Prof Davey Jones holds a Professorial Chair in environmental science and public health at Bangor University. He has advised UK and Welsh Government on their COVID-19, public health, agriculture, waste and climate change policies, including being part of the SAGE sub-committee on COVID-19 and the environment. His research has been published in the Journal of Ecology, Journal of Virological Methods, and Nature.
David Werner is a Professor of environmental systems modelling at Newcastle University. His research has been published in journals Water Science & Technology, Science of the Total Environment, and Environmental Pollution.
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