CEIC4000 Chap.5 Life Cycle Assessment
Life Cycle Assessment
Life-cycle assessment follows extraction, production, distribution, use and disposal so that an apparent improvement does not shift burden to another stage, region or impact category. This chapter explains the four LCA phases, the functional unit, process boundaries, life-cycle inventory, classification, characterisation and interpretation.
Worked examples emphasise service equivalence, unit discipline and conclusions that remain inside the stated scope. The goal states the decision and audience, while the functional unit makes alternatives comparable by equal service rather than object count. A process map then exposes included stages, elementary flows, exclusions and allocation choices before the inventory is calculated.
Inventory quantities retain their physical units until validated factors convert them into category indicators; kilograms, energy and water cannot simply be added. Interpretation identifies dominant processes, checks completeness and consistency, and varies assumptions that could reverse the ranking. Climate, toxicity, water and resource results remain visible even when optional weighting is used.
The final recommendation names the functional unit, boundary, data limitations and scenario because a comparative life-cycle result does not by itself demonstrate absolute sustainability.
What this chapter covers
- 01
Life-cycle thinking
- 02
Goal and scope definition
- 03
Functional unit and system boundary
- 04
Life-cycle inventory
- 05
Impact classification and characterisation
- 06
Interpretation and sensitivity
Worked example · free
Scale an inventory to equal service
- 1Scale single-use material to 8.0 kg.
- 1Scale washing energy to 1.2 kWh.
- 1Keep mass and energy as separate inventory flows.
- 1List omitted manufacture, transport, breakage and end-of-life flows.
Key terms
- Functional unit
- The quantified service used as a common comparison basis.
- System boundary
- The processes, geography, technology and time included in the study.
- Elementary flow
- A flow taken directly from or released directly to the environment.
- Characterisation factor
- A factor converting an inventory flow into a reference impact unit.
Life Cycle Assessment FAQ
Why is the functional unit so important?
It ensures that alternatives provide equal service before their burdens are compared. One disposable object and one reusable object are not equivalent when lifetime, capacity, washing or breakage differ, so object counts can produce a false ranking.
What belongs in the life-cycle inventory?
Include the material and energy inputs, products, emissions and wastes from every process inside the stated boundary, scaled to the functional unit. Record exclusions and prioritise data quality for flows that may dominate the result.
Can one weighted LCA score decide which option is sustainable?
A single score can support a decision only after its weighting choices are disclosed. It can hide trade-offs among climate, toxicity, water and resource categories, and LCA scope alone does not establish absolute sustainability without an appropriate capacity reference.
Assessment move
Begin with the decision, audience and functional unit. Draw the process map before building the inventory, then scale every included flow to the same service. During impact assessment, preserve category results and validated factors. In interpretation, identify dominant processes, run sensitivity on lifetime and boundary choices, and keep the recommendation inside the scope.
Write the goal as a decision statement naming the alternatives, intended user and reason for comparison. Define the functional unit as a quantified service, not an object count, and list the performance properties needed for genuine equivalence. A reusable product may require lifespan, washing frequency, loss rate and capacity assumptions; an energy system may require reliability and delivered output.
Draw unit processes within a visible boundary and mark exchanges with the environment separately from exchanges among processes. Build the inventory row by row, attaching a unit, data source, geography, year and quality note to every important flow. Scale each flow to the functional unit before summing, and never add kilograms, kilowatt-hours and cubic metres as though they were one burden.
If a process produces several products, explain the allocation method and test a plausible alternative when it could affect the ranking. During impact assessment, show classification and characterisation before optional normalisation or weighting. Retain category-specific results so climate, toxicity, water and resource trade-offs remain visible.
Verify factors belong to the selected method and version; invented arithmetic examples teach the operation but cannot supply real environmental coefficients. In interpretation, identify contribution hotspots and distinguish a system difference from a database artefact.
Vary the assumptions most likely to reverse the conclusion, such as reuse count, lifetime, electricity mix, transport distance, recycled content or end-of-life route. Conduct completeness and consistency checks across alternatives. A wider boundary is useful only when the added processes are material and supported by adequate data.
Finally, word the conclusion with the functional unit, included stages, impact categories and scenario. State exclusions beside the recommendation. Life-cycle evidence can reveal burden shifting and comparative impacts, but a relative ranking does not alone establish absolute sustainability or settle ethical trade-offs among affected groups.
