Life cycle assessment: Students should be able to carry out simple comparative LCAs for shopping bags made from plastic and paper.

The idea

A life cycle assessment (LCA) tracks a product through four stages: extracting raw materials, manufacturing, using it, and disposing of it. For shopping bags, compare a plastic bag (made from crude oil, a limited resource) with a paper bag (made from wood, a renewable resource if replanted). Judge each stage: extraction uses energy and can damage habitats; manufacture uses energy and water and may release pollutants; use depends on reuse — a plastic bag reused many times spreads its impact, while paper bags tear and may be replaced; disposal differs because plastic is durable and non-biodegradable but can be recycled, whereas paper is biodegradable and recyclable but may release methane in landfill. Weigh the stages to decide which bag has the smaller overall impact.

What earns the marks

• Identifies the four LCA stages: raw material extraction, manufacture, use and disposal, and applies each stage to both bag types.
• Compares raw materials: plastic bags use crude oil, a finite resource, while paper bags use wood, which can be renewable if forests are managed sustainably.
• Compares manufacture and use: both need energy and water, but reuse of a plastic bag many times can lower its impact per use, while paper bags may need replacing more often.
• Compares disposal: plastic is durable and non-biodegradable but recyclable, whereas paper is biodegradable and recyclable but can produce methane if buried in landfill.
• Reaches a justified comparative conclusion that weighs the stages rather than judging on one stage alone, and recognises that the answer depends on assumptions such as number of reuses.

Common mistakes and how to put them right

• Judging paper as always better because it is biodegradable.

  Put it right: Biodegradability is only one disposal factor, and growing and processing wood also uses energy, water and land.

• Ignoring the use stage and comparing only manufacture.

  Put it right: How many times a bag is reused strongly changes its impact per shopping trip.

• Treating an LCA as a single score with no uncertainty.

  Put it right: LCAs involve estimates and value judgements, so state the assumptions behind your comparison.

Exam tips

• Structure your answer stage by stage (extraction, manufacture, use, disposal) so every clause of the comparison is covered.
• Use comparative connectives such as whereas, however and therefore to make the weighing of stages explicit.
• Finish with a clear conclusion that names the assumptions, for example how many times each bag is reused, rather than declaring one bag universally better.