Impact assessment

An inventory can list hundreds of elementary flows. Life cycle impact assessment (LCIA) turns them into a handful of indicators people can act on.

The mandatory steps

ISO 14044 requires three elements:

1. Selection of impact categories, category indicators and characterisation models, justified by the goal.
2. Classification: assigning each inventory flow to the categories it contributes to. CO₂, CH₄ and N₂O go to climate change. SO₂ goes to acidification (and to particulate matter formation).
3. Characterisation: multiplying each flow by a characterisation factor and summing within each category.

For climate change, the characterisation factors are Global Warming Potentials (GWPs):

climate change score = Σ (mass of gas i × GWP₁₀₀ of gas i)

Using IPCC AR6 (2021) GWP₁₀₀ values:

Gas	GWP₁₀₀ (AR6)
CO₂	1
CH₄, fossil	29.8
CH₄, non-fossil	27.0
N₂O	273

So 1 kg of fossil methane counts as 29.8 kg CO₂-eq. Older methods use the IPCC Fifth Assessment Report (AR5, 2013) or AR4 values. AR5 Working Group I, Chapter 8, Table 8.7 gives GWP₁₀₀ values for CH₄ and N₂O in two versions. Without climate-carbon feedbacks they are 28 for methane (30 for fossil methane) and 265 for N₂O; with them, 34 (36) and 298. So “AR5” can mean more than one value set, and so can a method built on it: the EF 3.0 method, which is based on AR5, uses 36.8 for fossil methane, because its factors include indirect effects that the plain IPCC table values leave out. Which set you use is set by your method or PCR, not by preference, and mixing them is a common review finding.

Optional steps

• Normalisation: dividing results by a reference (for example the annual impact of an average EU citizen) to see which categories are relatively large.
• Grouping: sorting or ranking categories.
• Weighting: multiplying categories by value-based weights to get a single score.

ISO 14044 does not allow weighting to be used in comparative assertions disclosed to the public. Weighting reflects values, not natural science, so always show unweighted results alongside it.

Midpoint and endpoint

• Midpoint indicators sit early in the cause-effect chain: kg CO₂-eq, mol H⁺-eq, kg P-eq. They are more certain and more widely used.
• Endpoint indicators model further, to damage to areas of protection: human health (e.g. DALYs), ecosystem quality (species loss) and resources. Easier to communicate, much more uncertain.

Choosing a method

Method	Typical use
EF 3.1 (Environmental Footprint)	EU PEF studies; new EN 15804+A2 EPDs where the programme requires it
EF 3.0	EN 15804+A2 EPDs calculated before the switch to EF 3.1
ReCiPe 2016	General studies needing both midpoint and endpoint results
TRACI 2.1	North American studies (developed by the US EPA)
CML-IA baseline	Older EN 15804+A1 EPDs and legacy studies
IPCC GWP₁₀₀ alone	Carbon footprints (ISO 14067, GHG Protocol)

EF 3.0 and EF 3.1

The EN 15804+A2 indicators are based on the European Commission’s Environmental Footprint (EF) method, maintained by the Joint Research Centre (JRC). Two versions are in use:

• EF 3.0: its climate change factors are based on IPCC AR5. EN 15804+A2 EPDs were first calculated with EF 3.0.
• EF 3.1: published in July 2022, with climate change factors updated to IPCC AR6 (Working Group I, Table 7.15). For fossil methane the factor drops from 36.8 to 29.8 (JRC 2023, Table 3).

Programmes move from one to the other on their own dates. EPD International’s construction PCR (PCR 2019:14) requires characterisation factors based on EF 3.1 or later from 1 September 2024. So two EN 15804+A2 EPDs can rest on different EF versions: check which one before you compare them.

Tip

If an EPD programme, PCR or client specification applies, it names the method and version. Start there. Otherwise, pick a current, widely used method, name the exact version in the report, and don’t cherry-pick categories after seeing results.

What you should now be able to do

• Calculate a GWP result from an inventory of CO₂, CH₄ and N₂O by hand.
• Explain the difference between midpoint and endpoint indicators.
• Choose a method for a given study goal and justify it.

Sources

• IPCC (2021), Climate Change 2021: The Physical Science Basis, Working Group I contribution to AR6, Chapter 7, Table 7.15 (GWP₁₀₀ for CH₄ and N₂O): https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
• IPCC (2013), Climate Change 2013: The Physical Science Basis, Working Group I contribution to AR5, Chapter 8 (Myhre et al.), Table 8.7 (GWP₁₀₀ for CH₄ and N₂O, with and without climate-carbon feedbacks; note b gives the fossil methane values): https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter08_FINAL.pdf
• Andreasi Bassi, S. et al. (2023), Updated characterisation and normalisation factors for the Environmental Footprint 3.1 method, JRC, EUR 31414 EN (section 3.1: EF 3.0 based on AR5, EF 3.1 published July 2022 and based on AR6; Table 3: methane factors): https://publications.jrc.ec.europa.eu/repository/handle/JRC130796
• EPD International, PCR 2019:14 Construction products, v2.0.1 (2025), section 4.9, p. 43 (EF 3.1-based factors required from 2024-09-01; EF 3.0 factors include indirect radiative forcing): https://www.environdec.com/pcr-library
• ISO 14044:2006 (mandatory and optional LCIA elements; no weighting in comparative assertions disclosed to the public), described in our own words: https://www.iso.org/standard/38498.html

Checked: 2026-10-07 (IPCC AR5 Table 8.7, read in the chapter PDF); 2026-10-04 (JRC and PCR 2019:14 entries, per the programme sources ledger).