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.

Children tend to be at higher risk of falling sick from wild swimming than adults, partly because they swallow more water

"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".