The big idea: A life-cycle analysis counts the materials and energy going INTO a product at every stage, and the emissions and waste coming OUT.
It is an inventory, not an opinion — and because it counts, it regularly contradicts what everybody assumed.
Five stages, each with what goes in and what comes out counted separately.
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| Impact category | What it counts | Where it usually comes from |
|---|---|---|
| Global warming potential | Greenhouse gases, converted into kilograms of CO₂ equivalent so different gases can be added together | Energy in extraction and manufacture, and for a powered product the energy used while it is in service |
| Air pollution | Particulates, sulphur and nitrogen oxides, and volatile organic compounds | Smelting, firing, solvent-based finishing, and every transport stage |
| Water pollution | Chemicals and heat discharged into rivers and groundwater | Plating baths, dyeing, paper and textile processing, and mine drainage |
| Soil pollution | Contamination from spills, landfill leachate and mine tailings | Disposal, and the extraction stage that most consumers never see |
| Ecotoxicity | Harm to living things — not just to people | Heavy metals, persistent chemicals, and microplastics released in use and at disposal |
| Resource depletion | How much of a finite material is used up for good | Rare metals in electronics, and any material that is downcycled rather than recycled |
CO₂ equivalent is why totals can be added: Methane traps far more heat than carbon dioxide, so a life-cycle analysis converts every gas into the mass of CO₂ that would do the same damage.
That is what lets a single number be quoted for a whole product, and it is why the unit is kg CO₂e rather than kg CO₂.
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| The assumption | What counting shows | Why |
|---|---|---|
| A cotton bag is better than a plastic one | It must be reused dozens of times before it breaks even | Growing and processing cotton uses enormous water and energy; a thin plastic bag uses very little of either |
| Recycling solves the problem | It helps, and extraction and use usually dominate | Recycling reduces one stage of five, and for a powered product the use stage is larger than all the others together |
| Transport is the big impact | For a dense product it is usually small | A container ship moves a tonne a very long way for very little fuel; a lorry moving air in bulky packaging is a different matter |
| A biodegradable material is better | Only if it reaches the conditions it needs | Most need an industrial composter. In landfill they behave like any other plastic, and some release methane doing it |
| A longer-lasting product costs more impact | It nearly always costs less | Doubling a product's life halves the impact of everything that went into making it, which almost no material change can match |
An LCA is only as honest as its boundary: Where the analysis starts and stops decides the answer. Leave out the extraction stage and a recycled product looks free; leave out the use stage and an inefficient appliance looks harmless.
So the first question about any LCA is what was counted, and the second is who paid for it.
How this is tested — explaining and discussing life-cycle analysis considerations. It comes up two ways:
Paper 1 — multiple choice
- Identify the impact category a described effect belongs to.
- Choose the stage that dominates for a named product.
Paper 2 — analysing a product
- Explain the considerations an LCA measures for a named product.
- Evaluate an LCA result that contradicts a common assumption.
The trap: Treating an LCA as a general argument that something is bad for the environment. It is an inventory, and the marks are for naming the category, the stage and the number.
IB-style questionEvaluate[6 marks]
A supermarket claims its cotton tote bag is better for the environment than a plastic carrier. Evaluate this claim using life-cycle analysis considerations.
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