LCIA: Midpoint, Endpoint, Characterization Factors and Method Choice
Midpoint versus endpoint
Midpoint indicators sit close to the emission - kg CO2e, kg SO2e, kg N eq, CTUh - and carry lower model uncertainty. Endpoint indicators aggregate midpoints into areas of protection: human health (DALY), ecosystem quality (species·yr), resources (USD). Endpoints are easier to communicate but add damage modelling and value choices on top of the characterization step.
Characterization factors
A CF converts a flow into an indicator unit; multiply each flow and sum. GWP100 factors come from IPCC assessment reports, and mixing report versions is a frequent error: AR6 gives fossil CH4 ≈ 29.8 kg CO2e/kg and N2O = 273, where AR5 gave 28 and 265.
Method selection
Decide on impact-category coverage for the goal, geographic validity of the CFs (regionalized water and land-use factors), scientific maturity, and any mandated scheme. ReCiPe 2016 provides 18 midpoint and 3 endpoint indicators across hierarchist, egalitarian and individualist perspectives; EF 3.1 with 16 categories is mandatory for PEF studies; CML-IA, TRACI 2.1 and IMPACT World+ are also widely used. Normalization and weighting are optional LCIA elements carrying extra value judgements, and ISO 14044 restricts their use in comparative public claims.
Worked example
An inventory holds 1,000 kg fossil CO2 and 1 kg fossil CH4. GWP100 (AR6) = 1,000 + 29.8 = 1,029.8 kg CO2e, reported as 1.03 t CO2e. Keep unrounded values internally so aggregation does not drift.
Traps
Applying AR5 factors to an inventory characterized with AR6; claiming toxicity or eutrophication coverage when the method lacks it; treating normalization as a substitute for weighting; using regionalized CFs without matching location codes; assuming endpoint results are simply "worse" rather than differently aggregated.