Cut-off rules: what you are allowed to leave out

No life cycle assessment models every flow entering a product's production. Try to follow the oil in the machine that makes a screw, the tyre on the truck that carried that oil and the production of that tyre, and the chain never ends. Cut-off rules are the mechanism that stops this infinite regress and documents where you stopped.

Three criteria, one decision

ISO 14044 asks the cut-off decision to be made against three criteria and expects them to be applied together. Mass: a flow below a defined percentage of total input mass may be excluded. Energy: likewise as a share of total energy input. Environmental relevance: the flow's contribution to environmental impact.

The third criterion is the one that can override the first two, and it is the most often skipped. An additive at one part per thousand by mass can account for half the total in a toxicity or resource depletion indicator. Cutting off on a mass threshold alone renders such lines systematically invisible.

What EN 15804 says

Cut-off rules are not open when preparing an EPD. EN 15804 defines quantitative thresholds for flows that may be excluded from the product stage and requires the total excluded share to stay under a defined limit. It also requires a flow known to be environmentally significant to be included even below the mass threshold.

• Capital goods (plant buildings, machinery) are generally out of scope — but the PCR may say otherwise
• Staff commuting and administrative activities are typically excluded
• Packaging must be included where it is delivered with the product
• Catalysts and process aids are small by mass but can be environmentally significant
• Every excluded item must be listed in the report with its justification

The most common mistake: silent cut-off

The cut-off rule itself is not an error; failing to document it is. If a flow never entered the model and never appears in the report, a reader cannot tell whether it was deliberately excluded or simply forgotten. It is one of the first questions a verifier asks.

A good report's cut-off section contains the thresholds applied, the list of excluded flows, a justification for each and an estimate of the total excluded share. That section is also the basis of the study's reproducibility.