GHS mixture classification, explained

If your product is 98% copper sulphate and 2% water, is it classified the same as pure copper sulphate? Not necessarily — and the UN GHS "Purple Book" (the Globally Harmonized System of Classification and Labelling of Chemicals) defines specific mathematical rules for answering that question, depending on the hazard class in play.

Acute toxicity uses an additivity formula. Each hazard category has a reference "ATE" (Acute Toxicity Estimate) value — Category 1 is 0.5 mg/kg, Category 2 is 5, Category 3 is 50, Category 4 is 300. For a mixture, you compute 100 divided by the sum of each component's concentration divided by its ATE. The result tells you which category band the mixture falls into. A useful (if slightly counterintuitive) consequence: at 100% concentration, the math can push a pure Category 4 substance into Category 3, because the reference ATE values sit exactly on category boundaries. That's not a bug — it's the standard being deliberately conservative.

Skin and eye hazards use concentration-cutoff summation instead. If components classified as Category 1 skin corrosives sum to 5% or more of the mixture, the whole mixture is Category 1. Below that, a second check (Category 1 concentration weighted ×10, plus Category 2 concentration) against a 10% cutoff determines whether it drops to Category 2 or clears entirely.

Aquatic hazard uses a summation method with "M-factors" — multipliers assigned to particularly potent aquatic toxicants. Acute Category 1 components, weighted by their M-factor, need to sum to 25% or more of the mixture to classify the whole product as acutely hazardous to aquatic life. A similar but separate check handles chronic aquatic hazard, with Category 1 and Category 2 contributions weighted differently.

None of this is guesswork, and none of it should be. ChemDocs implements each of these rules as real calculation logic — not an approximation — and stores the full trace of which rule fired, on which component, with what inputs, alongside every classification result. If you or an auditor ever need to know why a product came out "Danger" instead of "Warning," the answer is in the trace, not a black box.

The rules above cover acute toxicity, skin/eye hazards, and aquatic hazard — the classes most relevant to the industrial chemicals we started with (copper compounds, silver nitrate). Other hazard classes, like oxidizing solids, aren't modeled yet; we'd rather ship an honest subset than a complete-looking one that's partly guessed.