The Greenest Paint in the Boatyard Was the Most Toxic

A new study tested seven antifouling coatings across three European coasts. The one marketed as eco-friendly tested as the most toxic, and the one that ranked most sustainable contains no biocide at all.

Every spring in the boatyards along the Ligurian coast there is a smell, part low tide and part solvent, that means the hulls are coming out of the water. A boat that has spent a season afloat surfaces wearing a coat it never asked for: barnacles in hard white clusters, weed trailing like wet hair, a fur of algae over everything below the waterline. Someone in overalls scrapes it back, sands it down, and rolls on a fresh layer of antifouling paint, and the slow siege starts over.

That paint is the reason a boat moves cleanly through water rather than dragging a small reef behind it. An untreated hull can grow a thick layer of fouling in a few weeks, which makes the boat heavier, slower, and far thirstier for fuel. The paints that dominate the market solve the problem by leaking poison on a schedule. Most contain cuprous oxide, a copper compound that keeps anything from settling for as long as it keeps shedding copper into the water around it. It works. It also does not stay put: the copper drifts, settles into sediments, and goes to work on organisms that were never anywhere near a boat.

So a market has grown for gentler alternatives, tins that promise the same clean hull with a lighter touch on the water. Whether those promises hold up is exactly the question a team led by Maria Lagerström at Chalmers University of Technology set out to answer. Working with colleagues in Sweden, Denmark, and France, they tested seven coatings: five copper paints ranging from six to thirty-two per cent cuprous oxide, one paint built around a newer biocide called tralopyril, and one biocide-free silicone coating that contains no poison at all. They painted the lot onto panels and left them in the sea for up to six months at three sites, one on the Swedish west coast, one in Denmark, one in the Atlantic shallows of Arcachon Bay, then measured both how much fouling each paint stopped and how much chemistry each one released.

The biocide-free silicone won. Not by poisoning anything, because it has nothing to poison with, but by being too slippery to grip: a foul-release surface, the marine equivalent of a non-stick pan, that leaves barnacle larvae and algae nowhere to take hold. Close behind it came the tralopyril paint. And the copper paints clustered together regardless of how much copper they carried, the six-per-cent tin holding off fouling about as well as the thirty-two-per-cent one. Which is its own quiet indictment: the heavily loaded copper paints were buying no extra protection for all that extra metal in the water.

Then the researchers ran the chemistry through an environmental risk model, and the rankings turned over. None of the biocide paints cleared the environmental bar cleanly, though the low-copper ones slipped under the limit at the French site. But one result sat far outside the others. The tralopyril paint, the one sold in France and Denmark as the environmentally friendly choice, came back at levels Lagerström said she had never seen in this kind of modelling, thousands of times above what counts as acceptable. A second test, exposing four marine species to the runoff from the painted panels in the lab, named the same loser. The greenest-sounding tin on the shelf was, by a wide margin, the most toxic thing in the trial.

It is worth being precise about what went wrong here, because it was not the chemistry alone. The word on the tin was doing work the formula could not support. “Environmentally friendly” is a marketing claim before it is a measured one, and in this case there was a gap wide enough to drive a regulatory regime through. The EU’s Biocidal Products Regulation has been in force since 2013, but it came with a transitional clause: products already on the market may keep selling while they wait to be evaluated, even if they have never passed an environmental risk assessment. In practice a paint can carry a reassuring label and a worrying profile for years, and nothing about the shelf will tell a boat owner which is which.

Two decades of watching conservation language get bought and sold has left me warmly suspicious of any product that describes its own virtue. The label is the cheapest part to change. What this study does, and what makes it worth more than its abstract, is put numbers under the suspicion: it shows that the most sustainable option on the test rack was the one making no claim about poison at all, and that the worst was the one leaning hardest on the word green. The silicone needs more preparation and a special primer to stick, which is presumably why it has not swept the market. But Lagerström’s conclusion is hard to argue with: the high-copper paints are doing unnecessary harm for no added benefit, and a coating that markets itself as safe while testing as the most toxic in the study has no obvious business being on sale anywhere in Europe.

None of which the person in overalls in the Ligurian boatyard can see, tin in hand, choosing between cans that all promise roughly the same thing. That is the part I keep turning over. Every spring, in thousands of small decisions made on the strength of a label, the water quality of a coastline gets quietly set by people who think they are only choosing a paint.

About Chalmers University of Technology

Chalmers University of Technology is a research university in Gothenburg, Sweden, working across technology, natural sciences, architecture, and maritime studies. The antifouling study was led by researcher Maria Lagerström, with colleagues in Sweden, Denmark, and France.

Lead image: Ann Larsson / University of Gothenburg. SEVENSEAS Media thanks Chalmers University of Technology for sharing this research. The findings are reported in the study “Sustainability ranking of antifouling coatings for leisure boats: balancing efficacy and environmental impact.”