Life cycle assessment — AQA GCSE Chemistry
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Life cycle assessment explained
Life cycle assessment covers 5 specification points, including Extracting and processing raw materials… and Use of water, resources, energy sources and….
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Life cycle assessments (LCAs) are carried out to assess the environmental impact of products in each of these stages: extracting and processing raw materials manufacturing and packaging use and operation during its lifetime disposal at the end of its useful life, including transport and distribution at each stage.
A life cycle assessment, or LCA, examines a product from the cradle to the grave. The four stages are extracting and processing raw materials, manufacturing and packaging, use and operation during its lifetime, and disposal at the end of its useful life. Transport and distribution must be considered at every stage, because moving materials and products uses energy and produces emissions. For example, an LCA of a plastic bottle would consider the oil used to make the plastic, the energy used in moulding and packaging, any energy used during its use, and whether it is recycled, incinerated or sent to landfill. LCAs help compare products, but they are only as reliable as the data and assumptions used, and different assessors may reach different conclusions.
Use of water, resources, energy sources and production of some wastes can be fairly easily quantified.
A life cycle assessment (LCA) tracks a product through raw material extraction, manufacture, use and disposal. Some inputs and outputs are measurable: a factory's water meter records litres used, electricity bills give kilowatt-hours, and a balance gives the mass of material consumed. Waste masses sent to landfill or recycling can be weighed. For example, making 1000 paper bags might use 2500 dm³ of water and 400 MJ of energy, while 1000 plastic bags might use 500 dm³ of water and 700 MJ of energy. These quantities are objective because instruments give numbers in defined units. However, quantification is only 'fairly easy' for measurable flows; it does not tell us which impact matters more.
Allocating numerical values to pollutant effects is less straightforward and requires value judgements, so LCA is not a purely objective process.
Once an LCA has measured emissions, someone must decide how harmful each pollutant is. Assigning a number to that harm is not a simple measurement. For example, 1 kg of carbon dioxide and 1 kg of sulfur dioxide have different effects: one contributes to climate change, the other to acid rain. Deciding that one is ten times worse than the other is a value judgement, not a reading from an instrument. Different people may weight health, habitats or cost differently, so two valid LCAs can reach different conclusions. This is why an LCA is not purely objective: the data collection can be objective, but the interpretation involves choices.
Selective or abbreviated LCAs can be devised to evaluate a product but these can be misused to reach pre-determined conclusions, eg in support of claims for advertising purposes.
A full LCA is lengthy and expensive, so companies often use a selective or abbreviated version that studies only chosen stages or impacts. This can be useful: a quick comparison of manufacturing energy for two designs may guide a decision. The risk is misuse. If a company studies only the stage where its product performs well and ignores stages where it performs badly, the result can support a pre-determined conclusion. For example, an advertisement might claim a bag is 'eco-friendly' because it is recyclable, while ignoring the energy used to make it. The LCA itself may be valid for its limited scope, but presenting it as a complete comparison is misleading.
Students should be able to carry out simple comparative LCAs for shopping bags made from plastic and paper.
A simple comparative LCA sets the same stages side by side for two products. For plastic and paper bags, compare raw materials, manufacture, use and disposal. Paper comes from trees, needs water and energy to pulp and form, and may be recycled or composted. Plastic comes from crude oil, needs energy to polymerise and mould, and may be reused many times before disposal. A fair comparison uses the same functional unit, such as carrying the same shopping for one year, and the same stages. Data might show paper uses more water and energy in manufacture, while plastic persists as litter. The conclusion depends on how many times each bag is reused and which impacts are weighted most.
Your focus
- List and describe the four stages of a life cycle assessment.
- Explain why transport and distribution are considered at each stage of an LCA.
- Evaluate the usefulness and limitations of LCAs when comparing products.
Show all 15 objectives
- List measurable inputs and outputs at each stage of an LCA.
- Interpret given data on water, energy, resources and waste for two products.
- Explain why measured quantities are more objective than opinions.
- Describe why pollutant effects need numerical weightings.
- Explain how value judgements affect LCA conclusions.
- Evaluate whether an LCA can be described as objective.
- Describe what makes an LCA selective or abbreviated.
- Explain how a selective LCA can be misused in advertising.
- Suggest how to judge whether an environmental claim is trustworthy.
- Carry out a simple comparative LCA for paper and plastic bags.
- Use a functional unit to make the comparison fair.
- Justify a conclusion using data and stated assumptions.
Life cycle assessment exam tips
Marking Points
- An LCA assesses the environmental impact of a product across its whole life cycle.
- Stage one is extracting and processing the raw materials needed to make the product.
- Stage two is manufacturing and packaging the product.
- Stage three is use and operation of the product during its lifetime.
- Stage four is disposal at the end of its useful life, such as recycling, incineration or landfill.
- Transport and distribution must be included at each stage because they contribute energy use and emissions.
- LCAs are comparative tools, but their conclusions depend on the data and assumptions used.
- Identify the four stages of an LCA: extracting raw materials, manufacture, use and disposal.
- State that water use, resource use, energy use and waste production can be measured in defined units such as dm³, kg, MJ and kWh.
- Explain that measured quantities are objective because they come from instruments, meters or mass balances rather than opinion.
- Use a comparative example, such as water and energy totals for paper and plastic bags, to show how data are collected.
- Recognise that quantification is limited to measurable flows and does not by itself rank environmental harm.
- Explain that pollutant effects must be weighted or scored before they can be compared.
- State that weighting involves value judgements about which impacts matter most.
- Give an example of two pollutants with different effects, such as carbon dioxide and sulfur dioxide.
- Conclude that different weightings can lead to different LCA conclusions, so the process is not purely objective.
- Distinguish objective measured data from subjective interpretation.
- Define a selective or abbreviated LCA as one that covers only chosen stages, materials or impacts.
- Explain a legitimate use, such as a quick screening comparison to guide product design.
- Describe misuse: choosing boundaries or weightings that produce a desired result.
- Link misuse to advertising or other claims that present partial evidence as complete.
- Suggest a safeguard, such as stating the scope, boundaries and assumptions clearly.
- Set out the four LCA stages for both paper and plastic bags.
- Use a common functional unit, such as carrying a fixed amount of shopping for one year.
- Compare at least two quantified inputs or outputs, such as water, energy, raw material or waste.
- Consider reuse: a plastic bag used many times has a lower impact per shopping trip.
- Reach a justified conclusion that depends on the assumptions and weightings used.
Examiner Tips
- 💡Name all four stages in order before discussing any one of them in detail.
- 💡Mention transport and distribution explicitly, since it applies across every stage.
- 💡When comparing two products, comment on the reliability of the LCA data as well as the environmental impacts.
- 💡Name the unit whenever you quote a quantity, for example 400 MJ or 2500 dm³.
- 💡Link each measurable item to its LCA stage so the answer is structured.
- 💡Use a short comparison of two products to show the data are quantitative.
- 💡Use the phrase 'value judgement' and explain what choice is being made.
- 💡Contrast a measured quantity with a weighted score to show the difference.
- 💡Refer to a named pollutant effect to make the judgement concrete.
- 💡Use the phrase 'pre-determined conclusions' and explain how selective boundaries produce them.
- 💡Give a specific advertising example to show misuse.
- 💡State what information a fair report should include, such as scope and assumptions.
- 💡Present the comparison in a table with the same stages as rows.
- 💡State the functional unit before comparing numbers.
- 💡Finish with a conclusion that names the deciding factor, such as reuse or water use.
Common Mistakes
- Listing only the disposal stage; correction: an LCA must cover all four stages, from raw material extraction to disposal.
- Forgetting transport and distribution; correction: these apply at each stage and must be included in the assessment.
- Treating an LCA result as an absolute fact; correction: LCAs depend on the data and assumptions chosen, so different assessments can give different conclusions.
- Treating an LCA as only the disposal stage; correct by including extraction, manufacture, use and disposal.
- Confusing energy with power; correct by using joules or kilowatt-hours for energy and watts for power.
- Assuming all impacts can be measured equally easily; correct by noting that some pollutant effects need value judgements.
- Claiming an LCA gives one definite correct answer; correct by stating that weightings are choices.
- Saying all pollution is equally harmful; correct by explaining that effects differ in type and severity.
- Confusing the measurement stage with the weighting stage; correct by separating data collection from value judgements.
- Assuming any abbreviated LCA is automatically invalid; correct by saying it is valid only within its stated scope.
- Believing a company's environmental claim without checking which stages were included; correct by asking what was left out.
- Confusing a limited LCA with a deliberately false one; correct by distinguishing narrow scope from dishonest reporting.
- Comparing one paper bag with one plastic bag without considering reuse; correct by using a functional unit over time.
- Ignoring the extraction stage for one material; correct by covering all four stages for both.
- Giving a conclusion without stating the assumptions; correct by linking the conclusion to the data and weightings.