11. November 2025, Topic: Aquatic Ecotoxicology Sediment Ecotoxicology Risk Assessment

Sunscreen and bathing lakes: what happens to UV filters in Swiss lakes and experimental ponds?

UV filters from sunscreens have been detected in Swiss lakes in concentrations that in some cases exceed the provisional environmental quality criteria. In both lakes and experimental ponds, lipophilic UV filters such as octocrylene tend to accumulate at the water surface. Additional data on behaviour and toxicity in sediments is currently being collected in order to refine the quality criteria.

Every summer, Swiss lakes fill with bathers who cool off and protect their skin with sunscreens. But what happens to these products when they enter the water? The focus is on UV filters – the active ingredients that absorb or block ultraviolet radiation. "Some of these substances are regularly detected in waters around the world," says project manager Alexandra Kroll. "And for many compounds, there is still a lack of reliable data on their toxicity and environmental quality criteria." There is growing concern that UV filters could pose risks to aquatic organisms, especially in popular bathing areas.

To investigate this issue, the Ecotox Centre, in collaboration with the University of Applied Sciences and Arts Northwestern Switzerland, is conducting a two-year study of the concentrations of selected UV filters in five popular Swiss bathing lakes, including three mountain lakes. At the same time, the researchers are conducting controlled experiments in small test ponds: after applying sun cream, bathers enter the water and remain there for 20 minutes. These ponds were also sampled regularly after the bathing event. "For sampling at the water surface, we use a remote-controlled boat that uses a roller to separate surface water and collect it separately from the water column," explains Armin Zenker from the University of Applied Sciences and Arts Northwestern Switzerland.

Noticeable accumulation on the surface

The pond experiments provided a clear picture: a visible oily film of sunscreen residues remained on the surface for days. Lipophilic UV filters such as octocrylene were up to 10,000 times more concentrated in this surface layer than in the water below. In contrast, the more water-soluble UV filter PMDSA showed only a fourfold accumulation on the surface. The same patterns were found in natural lakes. In Lake Greifensee, for example, lipophilic filters such as 2-ethylhexyl salicylate, homosalate and octocrylene dominated the surface layer, while only small amounts were found in deeper water layers. The more hydrophilic UV filters, on the other hand, were less common on the surface but more prevalent in the water column. "It remains to be clarified what happens to the UV filters on the surface in the long term and how many of the substances ultimately end up in the lake sediment," says Armin Zenker.

Exceedances of provisional quality criteria

The Ecotox Centre has derived provisional environmental quality criteria for those UV filters for which sufficient data is available: this was possible for around half of the substances investigated. Some of the UV filters analysed were found in lakes in concentrations that exceeded these proposed environmental quality criteria. In both Lake Greifen and Lake Cresta, the levels of 2-ethylhexyl salicylate and octocrylene in the surface water – and in some cases also in the water column – were significantly higher than the ad hoc environmental quality criterion for surface waters. "This shows that risks to aquatic organisms cannot be ruled out," says Alexandra Kroll. "We urgently need more data on the toxicity of these substances in order to be able to determine the ecotoxicological risks more accurately."

Next steps: focus on sediments

The researchers are trying to remedy this situation: measurements in sediment and studies on toxicity to sediment organisms should provide the missing data. They also want to determine robust quality criteria for problematic substances. "We hope this will give us a better understanding of the path taken by UV filters and help to reconcile the protection of human health with the protection of the environment," says Alexandra Kroll.

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