Sewage, rat pee and toxic algae: Open water swimming is booming, but is it bad for your health?

Swimming in rivers and lakes is booming, but heavy rain, sewage overflows and farm runoff can lead to disease outbreaks. Here's how to spot when the risks to health are highest.

On a heat‑bleached Saturday, a line of swimmers waits by a London slipway, dry robes flapping, phones glowing in their hands. A pollution app shows an amber warning for the tidal Thames River. Some people shrug and put their phones aside before wading into the water despite the waiting sewage, joking about getting "a bit of vitamin S".

Others, more cautious, pause. They sniff the river, think about the storm that hammered the city the night before, and decide to sit this one out.

Open‑water swimming, also known as wild swimming, have boomed in countries from the UK and the US to Australia in the past decade, fuelled by social media, urban "blue space" campaigns and a search for low‑cost ways to feel better in difficult times. Studies suggest that spending time in or near natural water is  associated with better mental health and lower stress, even after accounting for exercise and income. (Read more about the surprising benefits of blue spaces.)

But with health authorities regularly issuing warnings about disease and pollution lurking in lakes and rivers, there are health risks associated with going for a dip too. So how safe is wild swimming really? And are there ways to judge for yourself whether it's a good idea to get in?

Sewage, rats and toxic algae

Almost two centuries ago, hot weather combined with a seriously polluted Thames to create a stench so nauseating it became known as the "Great Stink". The public outrage over the smell led to reforms including a new sewage system for London.

These days, the river is busy with the bright tow‑floats and bobble hats of swimmers. Similar scenes play out along revived rivers from the Seine to the Danube and the Hudson. In Basel, Switzerland, some companies even give new employees "wickelfisch" – a fish shaped swim bag that locals use while swimming in the Rhine.

Rivers, once treated as open sewers, are now increasingly seen as clean and swimmable. Paris has invested about €1.4bn (£1.2bn/$1.6bn) in cleaning up the Seine, building a vast underground basin at Austerlitz that can hold about 50,000 cubic metres – roughly the volume of 20 Olympic pools – of stormwater instead of dumping it straight into the river.

Meanwhile London's Tideway "super sewer", a 15-mile-long tunnel (25km) that traces the Thames, can store around 1.6 million cubic metres of combined sewage and rainwater before sending it for treatment. Governments are designating more official bathing waters each year.

Yet a summer storm can undo weeks of clear test results as water quality is often hit by weather events. In the US, stormwater runoff and sewage overflows are among the main sources of pollution leading to public warnings and beach closures along lakes or coastlines.

"After heavy rain and storm events, urban rivers typically become microbiologically unsafe for swimming for at least 24 to 72 hours, mainly due to sewage overflows, runoff and disturbed sediments," says Isabel Douterelo Soler, a senior lecturer in environmental microbiology at the University of Sheffield in the UK.

"When there is too much rain, these systems can become overloaded," she adds. When that happens, water companies release sewage-polluted water directly into rivers to prevent it backing up into homes or overwhelming treatment plants. Those overflow outlets into rivers are known as "combined sewer overflows".

Substantial outbreaks of infectious disease have been reported among wild swimmers, with risk linked to swallowing river water. Even when routine monitoring seems reassuring, river swims can trigger........

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