ECHA issues guidelines for enforcing mixture classification based on bridging principles

Background: classifying chemicals in the EU

• The EU uses the United Nations (UN) Globally Harmonized System (GHS) as the basis for classifying, labeling and packaging hazardous substances and mixtures. This framework is implemented in Regulation (EC) 1272/2008 on Classification, Labeling, and Packaging (CLP). Currently, the CLP is aligned with the seventh revised edition of the GHS.
• In the EU, suppliers of hazardous substances and mixtures – manufacturers, importers and downstream users – must first identify these chemicals, then label their properties and package them accordingly, before they can be placed on the market. The task of assessing chemicals and identifying their hazardous properties – hazards such as carcinogenicity, explosivity, and aquatic toxicity – is referred to collectively as “classification.” This is one of the main objectives of the CLP, along with ensuring appropriate packaging and labeling of products to clearly convey to actors in the supply chain the hazards associated with these chemicals. Chemical classification is predicated on intrinsic hazardous properties, rather than extrinsic factors such as the likelihood of exposure and risk considerations. This entire exercise of classifying, labeling and packaging chemicals is often associated with the term “hazard communication,” which also includes the compilation of safety data sheets (SDSs), and workplace signage used in spaces like construction sites or factory settings, among other types of hazard communication.
• Annex I of the CLP contains the formal criteria for classifying all physical, health, and environmental hazards, and their differentiations or types, referred to as categories. Specifically, they are found in Parts 2, 3, 4 and 5 of this annex.
• Concerning the classification of substances, the CLP differentiates between harmonized classification and self-classification. The former entails assigning the classifications associated with the hazardous chemicals listed in Annex VI of the CLP, the List of harmonized classification and labeling of hazardous substances. That is, for all substances listed in Annex VI, suppliers are required to use the associated classifications for purposes of classifying, labeling and packaging their substances under the CLP. On the other hand, self-classification is the task of the suppliers themselves assessing and identifying the hazards of substances not listed in Annex VI by comparing them, based on adequate and reliable information and test data, with the classification criteria laid down in the CLP.
• Suppliers are required to self-classify all hazardous mixtures of more than one substance because, unlike for single substances, the CLP does not provide any list of mixtures already pre-classified in harmonized fashion. Thus, for products composed of hazardous mixtures, suppliers must independently determine physical, health and environmental hazards. They must also properly communicate these hazards with appropriate labeling when placing their products on the market, regardless of the volume produced.
• Accurate classification under the CLP is predicated on a clear understanding of the concept of weight-of-evidence (WoE) determination via expert judgment. In accordance with Article 9(3), when the CLP Annex I criteria on classification “cannot be applied directly to available identified information,” duty holders are required to “carry out an evaluation by applying a weight of evidence determination using expert This su in accordance with section 1.1.1 of Annex I [of the CLP], weighing all available information having a bearing on the determination of the hazards of the substance or the mixture, and in accordance with section 1.2 of Annex XI to Regulation (EC) No 1907/2006.”

ECHA’s new enforcement guide seeks to clarify the tiered approach for mixture classification, focusing on when test data on the mixture to be classified is not available

• This summer, ECHA issued a guidance document for enforcing the classification of mixtures that is based on so-called bridging principles. According to the guidance, the tiered approach for mixture classification indicates that underlying test data from the mixture itself must be prioritized for classifying that mixture. If such primary data is inadequate or not available, then duty holders are required to employ bridging principles under the CLP by looking to data on mixtures similar in composition that have been tested, as well as test data on the component ingredients making up the mixture to be classified. As per section 1.1.3 of Annex I of the CLP, “Where the mixture itself has not been tested to determine its hazardous properties, but there are sufficient data on similar tested mixtures and individual hazardous ingredient substances to adequately characterise the hazards of the mixture, these data shall be used in accordance with [the bridging rules] referred to in Article 9(4) for each individual hazard class in Part 3 and Part 4 of [Annex I of the CLP], subject to any specific provisions for mixtures in each hazard class.”
• Annex 4 of the guide, on page 22, presents a helpful overview of the tiered approach for mixture classification. Essentially, once all relevant data are collected and reviewed, the duty holder determines whether sufficient data are available on the mixture itself. If not, the next step is to determine if enough relevant data exist on similar tested mixtures to apply the Part 1, Annex I bridging principles for classification. On the other hand, if sufficient data are available, the next step is to undertake classification of the mixture based on the bridging principles. They include dilution (1.1.3.1); batching (1.1.3.2); concentration of highly hazardous mixtures (1.1.3.3); interpolation within one hazard category (1.1.3.4); and substantially similar substances (1.1.3.5).
• If after this step, such data for similar tested mixtures and individual chemicals are not available or are found to be inadequate, the second sub-paragraph of Article 9(4) mandates duty holders to apply other available classification methods, as specified in each section of the CLP’s Parts 3 and 4 of Annex I. These sections are dedicated to each class and category of the health (Part 3) and environmental (Part 4) hazards. These alternative methods include, for example, the calculation method using concentrations of constituent substances as well as the additivity method.

Bridging principles in the context of enforcement

• The guidance provides details on mixture classification that national enforcement authorities (NEAs) need to consider when assessing how suppliers (e.g., manufacturers, importers, downstream users) have classified their mixtures in a tiered approach pursuant to Articles 6(5) and 9(4). These provisions apply when no test data on the complete mixture itself to be classified are available, or the data available are inadequate to classify the mixture’s possible hazard or hazards.
• In such cases, Article 6(5) stipulates using “other available information on individual substances and similar tested mixtures which may also be considered relevant for the purposes of determining whether the mixture is hazardous.” These data may only be used if the duty holder finds them to be adequate, reliable and scientifically valid.
• Article 9(4) then specifies that bridging principles (as outlined above) are required to be used where only test data on these “reference mixtures,” that is, similar tested mixtures and the individual component substances, are available.

The enforcement guide’s “areas of interest”

• The enforcement guide focuses on four “areas of interest” for NEAs to scrutinize when duty holders classify untested mixtures by applying bridging principles to use similar tested mixtures as the basis of the classification, while also employing a weight-of-evidence (WoE) determination using expert judgment.

• These areas of interest are (1) the bridging method applied; (2) application of WoE and expert judgment; (3) the “substantially similar mixtures” bridging principle; and (4) the classification method for tested reference mixtures.
• Generally, for each of these areas, the guide underscores the importance for NEAs to confirm that duty holders have adhered to the relevant CLP requirements. For example, when examining and identifying information to be used to support the classification of substances and mixtures, WoE determination using expert judgment can only be employed pursuant to Articles 5(1) and 6(1). Likewise, only information determined to be adequate, reliable, and scientifically valid may be used as the basis of classification. This condition applies both to data on the mixture itself as well as to data on reference mixtures. Below are highlights from each area of interest.

Bridging method applied

• The authors of the guide emphasize that “only one bridging principle can be applied per hazard class evaluated for the untested mixture.” They note that the aerosols hazard class represents the only exception to this rule, “where a mixture classified based on another bridging principle is used in an aerosol container,” according to the ECHA Guidance on the application of the CLP. The ECHA Guidance continues, “However, different bridging principles may apply to different hazard classes.”
• The guide focuses on the substantially similar mixtures bridging principle in practice, stating that this is the main principle applied by duty holders.

Application of weight of evidence using expert judgment

• The authors stress, in the context of using a bridging principle (i.e., mixture classification based on test data from a reference mixture), a weight-of-evidence determination using expert judgment can only be applied to evaluate multiple similar tested mixtures to choose the most suitable one for purposes of classifying the untested mixture.
• NEAs should ensure that duty holders are not incorrectly applying “weight of evidence using ‘bridging’ from multiple tested mixtures.” The authors indicate that WoE can only be used to select one tested, fully classified mixture, and caution against duty holders using “multiple tested mixtures, each with insufficient information to adequately characterise the hazard of the tested mixtures.” Here, the authors again underscore the importance of selecting only underlying data that is adequate and reliable.
• WoE determination using expert judgment is required when any of the bridging principles are used to compare the mixture to be classified with the reference mixture and several reference mixtures are available to bridge from – but each with a different hazard classification.

Similarity of mixtures in the framework of “substantially similar mixtures”

• The authors cite section 1.1.0 of Annex I of the CLP, stating that every duty holder (no matter whether acting alone or in cooperation with other suppliers) is required to “document fully the basis on which classification decisions are made and shall make available to the competent authorities and, on request, to the relevant enforcement authorities the documentation, together with the data and information on which classifications are based.”
• The guide states that NEAs “can require” certain information “as part of the documentation referred in Article 9(4) [of the CLP].” This includes “composition of the tested and untested mixtures, including the identifiers of each substance in the mixtures (and in each mixture contained in the concerned mixture). Percent concentrations and hazard classifications can be also required”; “Bridging method applied and the demonstration of its applicability for each affected hazard category”; “Potential effects on mixture classification imposed by ingredients that are not similar to both, the tested and the untested mixture or that do not have the same concentration (e.g. information on potency, synergistic/antagonistic effects, buffering capacity, acid alkaline reserve, SCLs, pH, etc.)”; and “If more than one tested mixture is included, the documentation for any weight of evidence applied for the selection of the most relevant reference mixture(s) for the classification of the untested mixture.”
• The authors cite a CARACAL document from 2017 (at the bottom of page 8 of the guide), stating that “the general view was that similarity only in classification does not suffice to allow application of this bridging principle ‘Substantially Similar Mixtures’.” Similarity can only be demonstrated, the document states, “at the level of ingredient substance identity.”
• The guide emphasizes that NEAs “should consider the legal requirements for ingredients A, B and C as defined in section 1.1.3.5 of Annex I of the [CLP].” This is the relevant section addressing the rules for the “substantially similar mixtures” bridging principle.
• The authors provide a non-exhaustive list of substance groups which, when present in an evaluation using the “substantially similar mixtures” bridging principle, “might be important” to subject to “a more detailed assessment” given their “specific chemical characteristics.” Examples the authors provide include surfactants, fragrances and preservatives.

Classification method for tested reference mixtures

• This area concerns the methods and underlying information used to classify the reference mixtures that, in turn, are employed in the classification of untested mixtures. The authors underscore that the underlying information used for this purpose is commonly based on “non-standard test methods,” and that NEAs are often not equipped with the requisite expertise to assess “such non-standard approaches for the classification of the tested reference mixture.” As such, “contact with experts from the competent authorities might be required” to address such cases. Further, they recommend that NEAs begin investigations about compliance of mixture classifications based on a bridging principle by looking first at the bridging method applied or the similarity of mixtures in the framework of “substantially similar mixtures.”
• Most of the section is dedicated to the testing conditions and requirements that must be met for the tested reference mixtures to be used. For example, the guide states that “any of in vitro test methods used to classify the reference mixture must be suitable to conclude on classification in accordance with Article 6 of the CLP Regulation and the concerned ECHA guidance, Chapter R.7a ‘Endpoint specific guidance’.”

Concluding thoughts

• Entities selling products containing hazardous mixtures into the EU will want to familiarize themselves with this guide, particularly the “areas of interest” dedicated to a discussion of WoE and expert judgment, the similarity of mixtures in the context of the ‘substantially similar mixtures’ bridging principle, and the classification method for tested reference mixtures. The guide does provide critical insight into the elements of mixture classification that ECHA considers important for compliance purposes; it is also a document that’s not very clearly written and sometimes difficult to parse.
• However, to better illustrate the concepts presented in the guide, the authors have provided two useful case studies in annexes 1 and 2. These go a long way in helping to clarify step by step how a mixture classification based on bridging principles is conducted, including which concepts have been applied – and why.

To contact the author of this piece, email Scott Stephens ( sstephens@agencyiq.com).
To contact the editor of this item, please email Kari Oakes ( koakes@agencyiq.com)).

Key Documents and Dates

• Guide for enforcement of mixture classification based on bridging principles (Article 9(4) of the CLP Regulation Weight of evidence / Expert judgements), ECHA, July 2024
• Guidance on the Application of the CLP Criteria, ECHA, Version 6.0, January 2024
• Regulation (EC) No 1272/2008 on classification, labelling and packaging of substances and mixtures