CLP Classification of Mixtures: Calculation Methods and Automation

The CLP Tiered Approach to Mixture Classification

The CLP Regulation defines a tiered approach for the classification of mixtures:

• Tier 1 — Experimental data on the mixture: If experimental data on the mixture itself are available (toxicity tests, flammability tests, etc.), these have priority.
• Tier 2 — Bridging principles: If there is no data on the mixture but there is data on similar mixtures, bridging principles (dilution, batches, concentration, interpolation, substantially similar mixtures, aerosols) apply.
• Step 3 — Calculation: If neither experimental data nor bridging are available, the classification is calculated based on the classification of the individual components

Summation method

The summation method is the main calculation method for classifying mixtures in many health hazard classes. It consists of adding up the concentrations of the components classified in a given hazard class/category and comparing the total with the concentration limits (generic or specific).

Example for acute oral toxicity:

• The acute toxicity estimates (ATE) of each component are calculated
• The additivity formula is applied: 100/ATE(mix) = Σ(Ci/ATEi)
• The resulting ATE determines the acute toxicity category (Cat. 1, 2, 3, 4)
• Components without ATE data are handled according to the cut-off rules in Annex I CLP (variable thresholds for hazard class and exposure route)

Generic Concentration Limits (GCL)

GCLs (Generic Concentration Limits) are the standard threshold values defined in Annex I of the CLP for each hazard class and category. If the concentration of a hazardous component exceeds the GCL, its contribution to the classification of the mixture is considered.

Specific Concentration Limits (SCL)

SCLs (Specific Concentration Limits) are customized limits assigned to specific substances when generic limits do not adequately reflect the hazard potential of the substance. SCLs may be more or less restrictive than GCLs. They are defined in Annex VI, Table 3 of the CLP (harmonized classifications) or may be self-classified by the registrant.

M factors (Multipliers)

M factors apply to substances with acute (H400) and chronic (H410) aquatic toxicity. They serve to amplify the contribution of substances that are highly toxic to the aquatic environment in the classification of the mixture. An M factor = 10 means that the concentration of the substance is multiplied by 10 in the calculation of the sum for aquatic toxicity.

Bridging Principles

When experimental data on the mixture are not available and direct calculation is not possible, the CLP provides six bridging principles:

• Dilution — If a classified mixture is diluted with a diluent of equal or lower hazard, the new mixture retains at least the same classification.
• Batching — A production batch can be considered equivalent to another batch from the same process if the composition is substantially the same.
• Concentration of highly hazardous mixtures — If a mixture is classified in the most severe category and the concentration of the components increases, the classification remains in the most severe category
• Interpolation — If three mixtures A, B, and C contain the same hazardous components, with B having concentrations between A and C, and A and C have the same classification, then B has the same classification
• Substantially similar mixtures — If a component is replaced with one of similar hazard at a similar concentration, the classification of the new mixture can be extrapolated
• Aerosols — An aerosol product retains at least the classification of the non-aerosol mixture, with the additional physical hazards specific to aerosols

Classification by Hazard Class

Physical Hazards

The classification of physical hazards of mixtures often requires experimental data: flash point for flammable liquids, explosivity tests, oxidizing tests. kemXpro handles both classification based on experimental data and calculation where applicable.

Health Hazards

For each health hazard class, kemXpro applies the appropriate calculation method:

• Acute toxicity — ATE additivity formula
• Skin corrosion/irritation — Summation with additivity rule for corrosives/irritants
• Serious eye damage/irritation — Summation with specific rules for extreme pH
• Sensitization — Comparison with concentration limits (GCL/SCL)
• Mutagenicity, Carcinogenicity, Reproductive toxicity (CMR) — Comparison with concentration limits for individual CMR components
• Single/repeated STOT — Summation with concentration limits per category
• Aspiration hazard — Specific rules based on viscosity and concentration of hazardous components

Environmental hazards

The environmental classification of mixtures uses the summation method with M factors. kemXpro automatically calculates the acute and chronic aquatic toxicity of the mixture by summing the weighted contributions (with M factors) of the individual components and applying the classification limits.

The kemXpro Classification Engine

kemXpro implements a comprehensive CLP classification engine that:

• Automatically classifies mixtures based on the composition and classification of the components
• Applies all methods — Summation, ATE additivity, bridging principles, SCL, M-factors
• Uses the harmonized classification from Annex VI Table 3 and the ECHA C&L Inventory
• Manages ATP — Adaptations to Technical Progress with automatic reclassification
• Generates the classification report — Detailed document with all calculation steps, verifiable for audits
• Automatic recalculation — When the composition or classification of a component changes, the classification of the mixture is recalculated
• Version comparison — Highlights the differences between the previous and current classifications