When comparing different types of antifouling paints against fouling on leisure boats, the results were the opposite of what many would expect. Of the paints tested, the biocide-free silicone paint worked best, and the paint marketed as environmentally friendly turned out to be extremely toxic. The study, led by Chalmers University of Technology in Sweden, was conducted in Swedish, Danish and French coastal waters, and also showed that coatings with a low copper content can be just as effective as those containing more copper.
An untreated boat hull can be covered in a thick layer of algae, barnacles and other marine organisms in just a few weeks. This makes the boat both heavier and slower in the water – which leads to higher fuel consumption, among other things. Therefore, boat owners regularly paint the hull with so-called ‘antifouling paints’ that slowly release toxic substances – biocides – to prevent the organisms from growing on the hull. The paints that dominate the market contain the biocide cuprous oxide and are effective as long as they release copper. However, it has two major disadvantages – it affects other organisms in the environment, and it contaminates the water and bottom sediments.
"The goal of our study was to see how the environmental impact of antifouling paints can be reduced, while boat owners get the effect they want," says Maria Lagerström, researcher at the Division of Maritime Environmental Science at Chalmers and lead author of the new study.
Seven different paints in different marine environments
Together with colleagues from Danish and French universities, Maria Lagerström investigated seven different antifouling paints:
The last paint is not sold in Sweden today, but in several other EU countries, including Denmark and France, where it is marketed as safe for the environment.
To test the effectiveness, the different paints were applied to panels that were immersed in the water for up to six months in three different coastal waters: Tjärnö in Sweden, Hundested in Denmark and Arcachon Bay in France. The researchers measured both the surface coverage of the fouling on the different paints, and how much biocides were released into the surrounding water.
Although the coastal waters in the different countries differ, all the results pointed in the same direction: the silicone-based biocide-free paint was the most effective, closely followed by the biocide-containing tralopyril paint. For the copper coatings, the copper content in the paint was not a deciding factor, meaning that each of the five copper paints had more or less the same effect against fouling, despite a wide range in their copper content.
"Eco-friendly" paint was the most toxic
The researchers also performed two different tests to see how much the paints impact the marine environment. The first was an environmental risk assessment where models were used to assess the levels of biocides that the paints can give rise to in the marine environment, based on how much was released into the water during the test period. The silicone paint was not included here, as it does not contain biocides.
According to the test, none of the biocidal paints would meet today’s environmental risk criteria*, even if those with a low copper content fell below limit values in French waters. But one paint clearly stood out – the tralopyril paint, which is marketed as environmentally friendly.
"I've done this kind of environmental risk model before, but I've never seen such high values! The amount of biocide released was several thousand times higher than the acceptable levels, so I'm surprised that this paint is promoted as environmentally safe," says Maria Lagerström.
The second test included all paints, where four different marine species were exposed to leachate solutions from the painted panels in the lab to see how they were affected. In this case, too, it was the paint with tralopyril that fared the worst, followed by the paint with the highest copper content.
"If we sum up the study, we can conclude that the copper-free alternatives were actually more effective than those containing copper – but with very different effects on the environment. The test winner is the silicone paint that has the best sustainability profile because it performed well against fouling while having low toxicity. It requires more prep work and a special underlying coating to get the paint to adhere to the hull, but in our comparison it looks extremely promising and is worth trying.
" Using paints with the highest copper content would have an unnecessary environmental impact now that we have shown that it works with lower levels. If we phase them out, it would make a huge difference to the environment in our European coastal waters. And I really can’t see any reason why highly toxic products like the tralopyril paint should be allowed on the market in any European country at all.
More about the three different marine environments
To investigate how the paints work under different conditions, they were applied on panels that were submerged in the sea for up to six months at three locations in Europe:
The researchers chose several environments because both the amount of fouling and the organisms that dominate vary between different sea areas.
*Although the Biocidal Products Regulation, BPR, (the EU regulatory framework for the authorisation and use of biocidal products) was introduced in 2013, not all products that have applied for authorisation have been fully evaluated. Therefore, during a transitional period, products that were already on the market can continue to be sold, even if they have not passed an environmental risk assessment. In practice, this means that products with potentially high environmental hazards can continue to be used for a very long time.
More about the study:
The scientific article Sustainability ranking of antifouling coatings for leisure boats – Balancing efficacy and environmental impact has been published in the Journal of Hazardous Materials and is written by Maria Lagerström, Florane Le Bihanic, Laura Veensalu, Rémy Boisserie-Gimenez, Lola Racofier, Ann I. Larsson, Marcel Butschle, Gwendolina Limon, Xavier Cousin, Kai Bester and Jérôme Cachot. The researchers are active at Chalmers University of Technology, University of Gothenburg, Technical University of Denmark, Aarhus University, University of Bordeaux, the Marine Natural Park of the Bay of Arcachon (PNMBA), under the auspices of the French Biodiversity Agency (OFB), University of Montpellier, Labocéa.
For more information, please contact:
Maria Lagerström, Researcher at the Division of Maritime Environmental Science, Department of Environmental and Energy Sciences, Chalmers University of Technology, Sweden: maria.lagerstrom@chalmers.se, +46 31 772 46 23
Maria Lagerström speaks Swedish and English. At Chalmers, we have podcast studios and film equipment on site and can assist with requests for TV, radio or podcast interviews.
Image caption: Different antifouling paints were applied to panels and then immersed in water for up to six months. The pictures show the panels covered with various degrees of algae, barnacles, and other marine organisms that had grown on them over the six month-period. Control panels were also used to see normal levels of growth when no paint was applied - these show the most growth. Credit: Ann Larsson / University of Gothenburg
Journal of Hazardous Materials
Observational study
Not applicable
Sustainability ranking of antifouling coatings for leisure boats – Balancing efficacy and environmental impact
15-Mar-2026
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.