Researchers have found chemical pollution at all 262 swimming locations examined across the Netherlands.
Environmental organisation Natuur & Milieu and the Netherlands Institute of Ecology carried out the research.
They studied data from 205 official swimming locations and 57 other popular urban swimming spots.
The researchers looked for 122 pollutants in ponds, rivers and lakes. They found excessive levels of 35 substances.
Multiple chemical safety limits were exceeded at every location included in the 2024 data analysis.
The most common pollutants included PFOS, which belongs to the PFAS family, flame-retardant chemicals, mercury, pesticides and substances released during burning.
Some pollutants exceeded recommended safety limits at up to 90% of the locations.
Researchers said pollution at official swimming locations was more likely to come from agricultural pesticides. Urban swimming waters contained more pollution connected to industry and traffic.
The chemicals can enter the water through factories, agriculture, shipping, traffic and wastewater.
Some of the substances have been banned for years. However, they break down very slowly and remain in water and soil for a long time.
The study also found that none of the waters had good ecological quality. Excessive amounts of nitrogen and phosphorus were found at many locations.
Incidents involving blue-green algae have also increased during the past five years.
Current Dutch swimming-water inspections mainly focus on immediate health risks such as blue-green algae and bacteria including E. coli.
They do not regularly check every location for industrial chemicals and pesticides. This means a location can receive a safe or excellent rating while still exceeding chemical water-quality limits.
The researchers stressed that the findings do not mean someone will immediately become ill after swimming once.
However, some of the chemicals have been linked to damage to the nervous and hormonal systems and an increased risk of cancer. They can also harm aquatic animals and make drinking-water treatment more difficult and expensive.
Natuur & Milieu wants authorities to monitor chemicals at all official swimming locations. It is also calling for stricter rules and inspections covering wastewater discharges.
The organisation has asked the government to ban the production and use of harmful substances that do not naturally break down, including PFAS.
