Stages of a Life-Cycle Assessment

Introduction

Life-cycle thinking becomes a formal study when its question, data and interpretation are organised. A life-cycle assessment (LCA) has four connected phases: goal and scope definition, inventory analysis, impact assessment and interpretation. They are not a rigid one-way checklist. If inventory data reveal an omitted stage or interpretation exposes a sensitive assumption, the study may return to its scope and improve it.

Core explanation

Goal and scope states why the study is being done, who will use the results, what alternatives are compared, the functional unit and the system boundary. It also defines data quality and allocation rules. A comparison of drinking-water containers might use “deliver 1,000 litres of safe water to users” rather than “one container,” because container capacities and reuse vary. If one option includes transport and washing while the other does not, the scope is inconsistent. The US EPA's LCA principles guide presents these four phases and emphasises that scope sets the context for all later numbers.

Inventory analysis counts inputs and outputs for each unit operation: raw materials, electricity, fuels, water, product, co-products, emissions and waste. A mass and energy balance helps catch omissions. Data may be measured at a plant, obtained from suppliers or estimated from databases. The distinction matters because proxy data for another region or technology may not represent the actual supply chain. Inventory flows are physical quantities, not yet a single environmental score. One kilogram emitted CO₂, one kilogram NOₓ and one cubic metre water withdrawal are different flows.

Impact assessment connects inventory flows to potential impact categories such as climate change, acidification, eutrophication, particulate effects, toxicity or resource use. Characterisation factors convert relevant flows into category indicators; for climate, different greenhouse gases can be expressed as CO₂-equivalents under a chosen time horizon and method. This is a model of potential impacts, not a precise prediction of one person's actual exposure. Local conditions can be especially important for water and toxicity. A low climate result can coexist with high water stress, so separate categories should be displayed before any optional weighting.

Interpretation checks completeness, consistency, uncertainty and sensitivity of the results. It identifies which stages dominate, whether alternatives are truly comparable and which assumptions could reverse a conclusion. A good interpretation reports limitations rather than simply choosing the smallest plotted bar. The EPA's LCA report describes the staged process and decision-support role. Critical review may be appropriate for public comparative claims.

Step-by-step reasoning

1. Write a goal, functional unit and equal boundaries for alternatives. 2. Gather and check physical input/output inventories by stage. 3. Assign flows to relevant impact categories with documented factors. 4. Locate hotspots and test sensitivity to uncertain data or allocation. 5. State a conclusion limited to the studied system and assumptions.

Visual explanation

Draw four boxes in a loop: goal and scope → inventory → impact assessment → interpretation. Return arrows from interpretation to every earlier box show iteration. Under inventory place kg material, kWh energy and kg emissions; under impact place category indicators, not one unexplained “green score.”

Real-world analogy

Planning a journey starts by defining destination and travellers, then listing routes and fuel, estimating time and cost, and finally checking whether traffic assumptions change the decision. LCA likewise defines the comparison, compiles flows, evaluates impacts and revisits uncertainty. It adds environmental models rather than only travel time.

Real-world example

A team compares two packaging systems. Its scope fixes equal protection of 1,000 products and includes package production, transport and disposal. Inventory shows one option uses less plastic but causes more product spoilage. Impact assessment can reveal that spoiled product dominates climate impacts, shifting attention from package mass to protective performance. Interpretation checks whether spoilage rates are well measured.

Why?

Why must interpretation loop back to scope? A result may be driven by an assumption the original question did not examine, such as reuse count or electricity source. If that assumption is uncertain or unfair across options, the team must refine data or boundary before making a robust claim.

Common misconception

“Inventory is the same as impact” confuses physical flows with modelled environmental consequences. “An LCA gives one exact universal answer” ignores functional unit, region, technology and method. “Interpretation only summarises results” misses uncertainty and sensitivity analysis.

Worked example

For one functional unit, an inventory records 2 kg polymer, 5 kWh electricity and 0.1 kg transport fuel. A second option records 3 kg polymer, 2 kWh electricity and 0.1 kg fuel. The inventory alone says option 1 uses less polymer while option 2 uses less electricity. No climate or water ranking follows until impacts of polymer and electricity are modelled for the relevant supply chains. If electricity is low-carbon, the polymer difference may dominate; if high-carbon, the ranking might change.

Quick check

1. Which LCA phase sets the functional unit and system boundary? Answer: Goal and scope definition.

Exam focus

Name all four phases in order and state each output. Distinguish physical inventory from impact categories. Give a functional unit and at least one sensitivity variable for a proposed comparison. Limit conclusions to the chosen boundary and data quality.

Advanced insight

Consequential and attributional LCAs answer different questions: one may describe burdens assigned to a current product, while another estimates changes caused by a decision. Market responses and co-product substitution can make consequential modelling difficult. The chosen approach should match the study goal and be stated transparently.

Summary

An LCA defines a comparison, inventories physical flows, models potential impacts and interprets results with uncertainty. The phases inform one another, so a credible study can revise its scope or data when interpretation reveals a weakness.

Practice questions

1. Is “5 kWh electricity used” an inventory flow or an impact category result? Answer: It is an inventory flow; its impacts depend on generation and assessment method. 2. What phase evaluates whether a result changes with reuse count? Answer: Interpretation uses sensitivity analysis, with scope and inventory revised if needed. 3. Why compare packaging for equal product protection rather than equal package mass? Answer: The functional service includes protecting goods; unequal protection can shift spoilage impacts. 4. Name one possible impact category besides climate change. Answer: Water stress, acidification, eutrophication or toxicity are examples, depending on method.