Biocidal products regulation: All that glitters is silver?

Silver is widely used in paints and coatings on account of its anti-microbial effect and is classified under the Biocidal Products Regulation (Regulation (EU) No. 528/2012; BPR) in product-type 2 (Disinfectants and algaecides not intended for direct application to humans and animals).

Biocidal active substances and their harmonised classification

As far back as 2020, the assessing authority, in this case Sweden, submitted a draft CLH report (Proposal for Harmonised Classification and Labelling) to the Risk Assessment Committee (RAC). As part of its work, the RAC then assessed numerous toxicological studies of both elemental silver (CAS: 7440-22-4) and its various forms (nano-silver, silver in powder form, massive silver) with regard to its intrinsic properties. The goal of the process was to make a proposal for the harmonised classification of the substance in accordance with CLP that would then also apply to the biocidal active substance. The fact that the assessment alone took more than two years shows how complex and lengthy an assessment of the intrinsic properties of substances can be.

Toxicological approach to the assessment of the available studies

In its report, the Swedish authority proposed that silver (particle diameter ≥ 1 mm), silver powder (100 nm < particle diameter < 1 mm) and nano-silver (1 nm < particle diameter ≤ 100 nm) be classified as category 1B reproductive toxicants (Repr. 1B: Presumed human reproductive toxicant).

To this end, its experts employed a read-across approach by which they assessed data obtained from studies of various silver salts (e. g. silver nitrate and silver acetate), other silver compounds having their own CAS numbers (e. g. silver-zinc zeolite, CAS: 130328-20-0), and nano-silver in order to use such data for their assessment.

The central premise of the read-across approach was that the silver ions (Ag+) released in biological systems was the cause of the toxicity of all tested substances (Figure 1). This approach is commonly used for assessing the toxic properties of metals, in part because pure metals are difficult to test in biological systems. By contrast, metal salts are generally more water-soluble and more biologically available to the organism than the pure metal.

In that regard, it was not surprising that a great many studies of various silver salts were available for the assessment. In general, a read-across approach uses results from studies conducted on other, similar substances to predict the toxicological effects of a given substance.

The RAC‘s Classification

The RAC, after having assessed all studies of relevance to reproductive toxicity, concluded that silver did not meet the criteria for classification as a category 1B reproductive toxicant, in part because of a lack of results from standard tests and uncertainties in the test results. It proposed classification in category 2 (Repr. 2: Suspected human reproductive toxicant).

The RAC deemed that the effects exerted by the various silver compounds in the nanometre to the micrometre range on the fertility of the animals in the assessed animal studies were proven and it therefore proposed a classification with the hazard statement H361f (Suspected of damaging fertility).

In general, the reproductive toxicity of a substance is assessed on the basis of various endpoints, such as fertility, development of the offspring during pregnancy and lactation, and the effects on subsequent generations.

Impact on use as a biocidal active substance

Had it been confirmed by the RAC, the proposed classification as a category 1B reproductive toxicant could have had a major impact on the approval of silver as a biocidal active substance. Under Article 5 BPR, silver would have been classified as an active substance that meets the exclusion criteria. Article 5 covers, inter alia, substances that meet the criteria for classification as category 1A and 1B carcinogens, mutagens, reproductive toxicants, that have endocrine-disrupting properties or that are PBT (persistent, bioaccumulative, toxic) or vPvB (very persistent, very bioaccumulative) under Annex XIII of Regulation (EC) No. 1907/2006 (REACH). Approval of such active substances is only possible under certain conditions and, as the case may be, only for certain uses.

Responsibility for the preparation of the decisions relating to approval or non-approval and, as the case may be, restrictions on the use of biocidal active substances rests with the ECHA’s Biocidal Products Committee (BPC). This is an expert panel that prepares assessments for the ECHA in relation to various BPR processes – such as the approval of silver as a biocidal active substance – for a decision by the European Commission. The BPC must factor the RAC’s assessment of the classification into its assessment and it also examines the efficacy as well as the risks arising from the use of silver as a biocidal active substance. The process for assessing silver (CAS: 7440-22-4) as a biocidal active substance is still ongoing. However, the authority is scheduled to submit its assessment report to the BPC in the last quarter of 2026.

For the moment, then, there is no need for manufacturers of biocidal products to take any action regarding the authorisation of their products.

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