What are the risks of swimming in water that is not designed for swimmers?

It's a first since 1923: starting Saturday, July 5, Parisians can legally swim in the Seine. Until August 31, three sites are open free of charge each day, as part of the Paris Plages operation. Presented as a legacy of the Paris 2024 Olympic and Paralympic Games, the authorization to swim in the capital's river is part of an approach of "adaptation to climate change, but also of quality of life," explained the mayor (PS) of Paris, Anne Hidalgo, during a press conference in mid-May.

The water quality of the Seine will be closely monitored daily using sensors and signaled by a flag system. In the event of pollution, particularly after heavy rain, swimming will be prohibited. Enhanced analyses of fecal bacteria, such as Escherichia coli ( E. coli ) and enterococci, will be conducted by the regional health agency, according to current health thresholds.

However, the temptation to swim outside of authorized areas, in Paris as elsewhere in France, can be strong, especially during heatwaves. What exactly are the risks of exposing yourself to water that's unsuitable for swimming?

There are three main categories of hazards:

• topological, linked to the current and the configuration of the places;
• bacteriological, associated with contamination by harmful microorganisms;
• chemicals, linked to different types of pollution.

Beware of currents undetectable from the surface

The topological risk is the most obvious. It depends directly on the configuration of the site (water depth, layout of the banks, strength of the current, etc.) and can lead to fatal accidents. According to Santé publique France , accidental drownings are responsible for around 1,000 deaths each year (around a quarter of which occur in rivers and bodies of water), making it the leading cause of death by everyday accidents among people under 25. Obviously, "it is strongly recommended not to swim if there is a strong current," says Marc Valmassoni, campaign coordinator for the non-governmental organization (NGO) Surfrider, which monitors recreational waters and campaigns for their improved quality.

Some rivers, which are otherwise prohibited for swimming, are particularly dangerous. This is the case of the Loire , which has significant variations in flow, friable sand and whirlpools that can trap swimmers. Tragedies are also frequent in the Rhône and the Saône, in Lyon, due to powerful underwater currents that are unsuspected from the surface. Generally speaking, swimming should be avoided at all costs near engineering structures such as bridges, which create whirlpools, or around dams and locks, where boat maneuvers cause strong currents that can trap swimmers.

At sea, and particularly on the southwest coast, rip currents—depressions on the beach that create strong currents with the tide—pull swimmers out to sea. At the risk of becoming exhausted very quickly, one should not try to swim against the current, but allow oneself to be carried out to sea and swim parallel to the beach when the current eases, in order to escape. Inland, the risk is also increased in gravel pits and quarries, where collapses in the seabed can create whirlpools that suck swimmers under.

Bacteriological risks leading to poisoning

The second risk category is microorganisms that proliferate in water. "We are much more demanding for drinking water and swimming pool water than for bathing water," warns Bernard Legube, professor emeritus at the University of Poitiers, a specialist in water physicochemistry and water treatment.

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Their concentration in rivers increases after heavy rains. They can cause wastewater treatment plants to overflow and cause wastewater to overflow and surface water and waste runoff. The Huveaune beach in Marseille, near the mouth of the coastal river of the same name, is even nicknamed "Epluchures Beach" . "As soon as it rains, it creates gullies, it's filthy. You should avoid swimming in a coastal river after the rains," advises Marc Valmassoni.

Furthermore, algae or microalgae multiply during extreme heat, a phenomenon exacerbated by global warming. The consequences vary depending on the microorganisms and the exposure time.

It is most often caused by bacteria present in fecal matter, such as Escherichia coli and enterococci, which are released into river waters, particularly after heavy rains. This is why many training sessions were cancelled ahead of the open water swimming events at the Paris 2024 Games, as major storms deteriorated the quality of the river water.

The Ministry of Health maintains a website where you can find information on the quality of bathing water, based on the results of the latest analyses carried out there.

• Swimmer's itch, or "duck flea"

Annoying but benign itchy skin rashes can occur after summer swimming in a body of water home to waterfowl and water snails. They are caused by the larvae of the cercariae, a parasite found in mallards and swans. Swimmer's itch is quite rare. It is recommended to shower immediately after swimming and dry thoroughly to limit the risk.

It is transmitted through rodent urine, which contaminates rivers and ponds. Most often benign in humans, this disease can in some cases lead to severe kidney failure, according to the Pasteur Institute . The most common symptoms are vomiting, fever, headaches, and diarrhea. If you practice aquatic activities in fresh water, it is advisable to disinfect and protect wounds, wash your hands and avoid contact with your eyes, mouth, and nose, and wear protective equipment.

• Cyanobacterial toxin poisoning

Skin irritations, digestive problems, even nervous system problems... These are signs of cyanobacteria poisoning, bacteria that proliferate in the heat. Several cases of death have been reported in dogs. It is best to avoid swimming if the water turns green or if a brown film accumulates. Sometimes, cyanobacteria also form clumps resembling algae. The risk is particularly high in lakes, which are less oxygenated, especially in the south of France, but they also thrive in slow-flowing rivers, such as the Tarn .

Chemical discharges, pesticides, drug molecules... Many human activities directly influence the quality of waterways, with two major unknowns: at what concentration level, and with what consequences?

In some extreme cases, waterway pollution is known to be associated with serious health consequences. This is the case in China, where more than 450 villages lived for years with drinking water contaminated by upstream factories. This polluted water led to cancer rates 50% higher than in the rest of the country.

In France, such extreme situations are rare. "In the Seine, there are some traces of pesticides, mainly herbicides, but they are very low," explains Bernard Legube. The risk is inversely proportional to the flow rate. In running water, between currents, oxygenation, and dilution in rivers, pollution does not last long, the researcher explains.

Currently, there is no precise monitoring of chemical pollution at the national level. "It's expensive to set up and the issues are not the same from one region to another," explains Marc Valmassoni. The NGO Surfrider is campaigning for their inclusion in the next revision of the European directive on bathing water quality.