Assessment of efficacy and excess toxicity of antifouling paints for leisure boats - A guide for copper-based antifouling paints intended for use in the Baltic Sea region

Report, 2021

The regulation of antifouling paints in the European Union falls under the Biocide Products regulation (BPR, Regulation (EU) 528/2012) and consists of two assessments: an environmental risk assessment (ERA) and an efficacy assessment. The efficacy assessment is key for the placement of an antifouling paint on the market as a biocidal product must be shown to be effective to gain approval. At the same time, the BPR states clearly that biocidal products should not be excessively toxic, i.e. release active substances to the environment in excess of the minimum necessary to achieve the desired effect. According to the BPR guidance document, an acceptable efficacy for antifouling paints is obtained if a static panel test is able to demonstrate a surface coverage of macrofouling below 25% on the treated surface when the control has at least 75% coverage. Guidance on how to determine whether a paint is excessively toxic is however lacking from the document.

Objective and method
The overall objective of this report was to compile the current knowledge on the efficacy of antifouling products and the minimum dose of copper. Regional pleasure craft marina scenarios for emission estimation were recently introduced for the harmonised environmental risk assessment of antifouling paint, whereby marine EU waters have been divided into four regions (Baltic, Baltic Transition, Atlantic and Mediterranean). This report focuses solely on the Baltic, Baltic Transition and Atlantic regions and all results were related to these three regions. A review was conducted where both peer-reviewed scientific articles and previously unpublished data relating to fouling pressure and efficacy assessments of copper and biocide-free antifouling paints in these regions were compiled. Even though the Atlantic region is included in the assessment it must be emphasised that the data for this region is exclusively collected from the northern Swedish west coast (Skagerrak). Most of the data were obtained from the EU BONUS-project CHANGE (Changing antifouling practices for leisure boats in the Baltic Sea) which contained both fouling and efficacy assessment of copper coatings in marinas during up to four consecutive years (2013–2016). Additional studies from 2018 and 2020 were also included in the compilation. The efficacy of a total of 10 copper coatings (cuprous oxide or copper powder) available on the Swedish market could thus be assessed at as many as 18 locations across 6 different years.

Fouling pressure
The minimum necessary dose will depend on the fouling pressure (i.e. the intensity and type of fouling organisms) of the region where the paint is intended for use. The fouling pressure, measured as the surface coverage of macrofouling on static control panels in the Baltic Sea region, was found to exceed 25% at all studied marinas (17 locations) and years (4 years). However, in 20% of the cases, the macrofouling coverage was below 75%, indicating that the brackish Baltic Sea does not confine well to the current requirements for efficacy testing. The fouling pressure was highest at the Atlantic site but varied considerably between sites in the Baltic and Baltic Transition regions. Also, high interannual variation in macrofouling cover was observed for several of the marinas in these two regions. The two most northern sites of the Baltic region were dominated by mainly soft fouling indicating a lower need for biocidal coatings. No general patterns of fouling pressure could however be concluded for the three regions.

Efficacy of copper paints
The efficacy assessments of 10 commercial copper-based coatings showed acceptable results at 82 – 100% of evaluated locations. Products currently on the Swedish market are thus highly efficient, with macrofouling coverages on static panels well below the 25% macrofouling criteria. A combination of high fouling pressure and low surface seawater temperatures were often found to coincide at the few instances where some copper paints failed to meet the set efficacy criteria. The lower temperatures may have acted to slow the release of copper, resulting in some paints failing to withstand the presiding high fouling pressure. Hence, both biotic (fouling pressure) and abiotic factors (temperature) may influence the results of an efficacy assessment in the Baltic Sea region, in particular for low-leaching copper paints.

Efficacy of biocide-free paints
The efficacy of a fouling release coating (i.e. silicone coating) was assessed during a 5-months long field experiment in 2020 at one test site in the Baltic and two test sites in the Atlantic region. The result showed the control panels deployed in the Atlantic region to be heavily fouled with macrofouling (100% coverage) but the fouling release coating had no macrofouling and was as effective as two copper-based coatings for professional use. Thus, silicone paints present an effective biocide-free antifouling strategy already available to boat owners.

Excessive toxicity
Efficacy test results offer limited support for the evaluation of excessive toxicity, especially if the test is carried out in a region other than that of intended use. An evaluation of excessive toxicity is therefore proposed based on field release rates. Copper release rates of 2 and 7 μg/cm2/day were found to be sufficient to prevent all macrofouling settlement in the Baltic and Baltic Transition regions, respectively. Copper paints with field release rates in excess of these values can thus be considered excessively toxic. In absence of field release rates, a model is proposed for their estimation. The use of the model is however limited to hard and polishing paint only, due to lack of data for self-polishing paints. Gradient panels with paint stripes of increasingly diluted paint (i.e. decreasing amounts of biocide(s)) could also be used for the assessment of excess toxicity. Ideally, this assessment should be coupled with environmental release rate data to justify the need for the dose delivered by a given product.

Considerations specific to the Baltic Sea region
The combined findings of this report show that the conditions of the Baltic Sea region require specific consideration. The release rate of copper needed to deter macrofouling is lower in the Baltic than the Baltic Transition and Atlantic regions. The evaluation of efficacy and excessive toxicity should therefore be carried out in the marine region of intended use. Variability in fouling pressure and environmental parameters both between locations and years, even within the same marine region suggest however that care should be taken when choosing the test location. Additionally, the duration of the efficacy test should reflect product use. A period of 5 – 7 months of exposure is therefore suggested.