What is life cycle impact assessment (LCIA)?

When the inventory phase ends you hold a table hundreds of rows long: 3.2 kg of methane, 840 g of nitrogen oxides, 0.4 g of nitrous oxide, 12 kg of carbon dioxide. Technically correct, and it answers nothing. Which is worse? How much harm in total? Impact assessment fills that gap.

Classification: sorting flows into categories

The first step assigns each flow to the environmental problem it affects. Methane contributes to climate change; sulphur dioxide to acidification; phosphate to eutrophication. A flow can enter more than one category — nitrogen oxides contribute to acidification, eutrophication and photochemical ozone formation at once.

Characterisation: converting to a common unit

The second step converts each flow into the category’s reference substance. For climate change the reference is carbon dioxide; one kilogram of methane equals roughly 28 kilograms of CO₂ equivalent over a hundred-year horizon. That conversion coefficient is the characterisation factor, and it comes from scientific models.

Multiply and sum, and an inventory of hundreds of rows collapses into a handful of indicators: climate change in kg CO₂e, acidification in mol H⁺ equivalent, water use in scarcity-weighted m³. This is the moment an LCA becomes intelligible.

Midpoint and endpoint

Indicators can stop at two different points along the causal chain. Midpoint indicators stop in the middle of the mechanism: ‘this product emits 12 kg CO₂e’. Endpoint indicators follow the chain to its end: ‘that emission causes this much loss of human health years’.

Midpoint carries less uncertainty but takes expertise to interpret. Endpoint is more intelligible to a decision-maker but carries far higher modelling uncertainty. EN 15804 and most EPD programmes use midpoint.

Normalisation and weighting

Two optional steps follow. Normalisation divides results by a reference — the annual impact of one European citizen, say — so indicators in different units become comparable magnitudes. Weighting then assigns importance coefficients and collapses them into a single score.

Weighting is not scientific but a value judgement: there is no scientific answer to how many times more important climate change is than water scarcity. For that reason ISO 14044 prohibits weighting in comparative assertions disclosed to the public.

Which method to choose

• EF v3.1 — the European Commission’s reference method; mandatory in PEF work
• The EN 15804+A2 indicator set — used in construction product EPDs, based on EF
• CML — the classic midpoint method, long in use, dominant in the EN 15804 A1 era
• ReCiPe — offers both midpoint and endpoint, common in academic work
• IMPACT World+ — a current method with strong regional differentiation