The number of designated swimming locations with health warnings over poor water quality has more than doubled since the end of June, an ANP analysis of Zwemwater.nl data shows. While 46 locations were flagged at the previous measurement, there are now warnings in place at 104 swimming spots.

There are about 950 official swimming sites across the Netherlands, where authorities routinely monitor the water for health risks including blue-green algae and bacterial contamination.

The number of swimming locations where authorities advise against entering the water has also risen. Swimming is currently discouraged at 56 sites, up from 35 when the previous assessment was conducted on June 23.

Authorities say 781 official swimming sites currently have water quality considered safe for swimming. However, prolonged warm weather can cause conditions to deteriorate. Monitoring of official swimming locations will continue through October 1, the end of the swimming season.

Blue-green algae thrive in stagnant, warm water. It releases toxins, causing skin irritation, severe stomach cramps, diarrhea, vomiting, and occasionally mild neurological issues.

High concentrations of bacteria like E. coli or Intestinal Enterococci primarily cause gastrointestinal, ear, or eye infections, along with flu-like symptoms.

Sunlight and heat accelerate toxic algae growth and speed up bacterial reproduction. In addition, heavy, localized summer storms overwhelm municipal drainage networks. Water boards like Waterschap Amstel, Gooi en Vecht regularly issue broad swim advisories because flooded sewers spill untreated waste directly into local canals, ditches, and rivers, requiring roughly three days to clear.

Water managers and provinces test official bathing zones at least once a month. If a location fails standard parameters, testing frequency increases to every two weeks.

Anyone wishing to swim can download the free, official Zwemwater App or use the interactive map on Zwemwater.nl to track real-time changes before entering the water.