Lesson plan: Does a bag for life actually help? Life-cycle assessment with real data (ages 15–18)

Lesson overview

• Title: Does a bag for life actually help? Life-cycle assessment with real data
• Age band: 15–18 (England Years 10–13 / General Certificate of Secondary Education (GCSE) and A-level; US Grades 9–12; Australia Years 10–12)
• Duration: 60 minutes
• Subject: Chemistry, with Geography, Economics and Business links
• Made by: No Plastic Today. Funded by: No Plastic Today (no sponsors, no industry funding). Licence: CC BY 4.0.

Learning objectives

• I can describe the four stages of a life-cycle assessment (LCA) and explain what "functional unit" means.
• I can calculate a reuse break-even point from published LCA data and state its assumptions.
• I can evaluate a policy (a bag charge or a deposit return scheme) using evidence, and propose one refinement.
• I can explain why "reusable" is not automatically better and what makes it better.

Curriculum codes

• England: GCSE Chemistry "Earth and atmospheric science" (life-cycle assessment and recycling; using materials); GCSE Biology (impact of human activity on biodiversity, pollution); GCSE Geography (resource management). A-level Chemistry (polymers, LCA); A-level Economics (externalities, extended producer responsibility).
• Next Generation Science Standards (NGSS): HS-ESS3-4 (evaluate or refine a technological solution that reduces impacts of human activities on natural systems); HS-ETS1-3 (evaluate a solution to a complex real-world problem based on prioritised criteria and trade-offs); HS-LS2-7 (design, evaluate and refine a solution for reducing the impacts of human activities on the environment and biodiversity).
• Australian Curriculum v9: Science Year 10 (global systems and human impact) (verify code); senior Chemistry syllabus (polymers, life-cycle analysis) (verify by state); Geography "Sustainable places" (verify); Economics and Business (verify).
• International Baccalaureate (IB): Diploma Programme (DP) Environmental Systems and Societies (ESS; solid domestic waste; pollution management; topic numbering differs between the 2017 and 2024 guides); DP Chemistry (polymers); creativity, activity, service (CAS) project link.

Materials

• Data sheet 1, "Bag weights and break-even" from the UK Environment Agency, Life Cycle Assessment of Supermarket Carrier Bags (2011, report SC030148; copy at biodeg.org/wp-content/uploads/2021/04/uk-ea-publishes-lca-of-supermarket-carrier-bags-.pdf), Table 3.1 (bag weights) and executive summary (break-even uses). Single-use HDPE bag 8.12 g. Bag for life (LDPE) 34.94 g, break-even 4 uses. Non-woven polypropylene bag 115.83 g, break-even 11 uses. Cotton bag 183.11 g, break-even 131 uses. Paper bag 55.20 g, break-even 3 uses. Break-even is on global warming potential. These break-even numbers assume the thin HDPE bag is used once and not reused as a bin liner. The report also gives higher numbers for when thin bags are reused as bin liners (ask students to find them).
• Data sheet 2, "Policy outcomes": England's 10p single-use bag charge; 399 million bags sold by large retailers from April 2025 to April 2026, down 81% since 2016 and down from 7.6 billion in 2014 before the charge (UK Government: gov.uk/government/publications/carrier-bag-charge-summary-of-data-in-england/single-use-plastic-carrier-bags-charge-data-for-england-2025-to-2026). UK Deposit Return Scheme: 20p deposit on drinks containers from 1 October 2027 (House of Commons Library: commonslibrary.parliament.uk/research-briefings/cbp-10453/). EU Single-Use Plastics Directive 2019/904: bans on cutlery, plates, straws and expanded polystyrene food containers; 77% separate collection of bottles by 2025 and 90% by 2029 (eur-lex.europa.eu/eli/dir/2019/904/oj).
• Data sheet 3, "Scale and the carbon link": about 9% of plastic waste recycled globally in 2019 (Organisation for Economic Co-operation and Development (OECD): oecd.org/en/about/news/press-releases/2022/06/global-plastic-waste-set-to-almost-triple-by-2060.html). Making new (virgin) plastic, rather than recycled plastic, emitted 2.24 billion tonnes CO2-equivalent in 2019, about 5.3% of global greenhouse gas emissions (Lawrence Berkeley National Laboratory, 2024: osti.gov/biblio/2336721). Reuse, recycling and material substitution together could cut plastic pollution by 80% by 2040 with existing technology (UN Environment Programme (UNEP), Turning off the Tap, 2023: unep.org/resources/turning-off-tap-end-plastic-pollution-create-circular-economy).
• Calculators; graph paper or a spreadsheet; the class survey slip (see Part 3).
• A single-use bag, a bag for life, a cotton bag and a paper bag on the front bench.

Starter (8 minutes)

• Hold up the cotton bag and the thin plastic bag. Ask for a show of hands: "Which is better for the environment?" Then ask: "Better at what, and compared over what?" Introduce the four LCA stages (raw materials, manufacture, use, disposal) and the functional unit: "carrying one week's shopping home". Everything is compared per functional unit, not per bag.

Main activity (40 minutes)

• Part 1, calculate break-even (12 minutes): using data sheet 1, students verify the ratio of masses (for example 183.11 g / 8.12 g = 22.5) and compare it with the published break-even of 131 uses for cotton. Ask why the break-even is far higher than the mass ratio (cotton growing, water, fertiliser and processing carry a large impact per gram; global warming potential is not proportional to mass). Students record: the number, the assumption behind it, and one thing that would change it.
• Part 2, what people actually do (10 minutes): run a two-question survey in class or with data gathered beforehand: "How many times has your household's bag for life been used?" and "How many cotton bags does your household own?" Plot the distribution. Compare the median reuse count with the break-even numbers. Draw the conclusion: reusables only win when they are actually reused, so the design question is "how do we make reuse the default?"
• Part 3, evaluate a policy (12 minutes): groups take data sheet 2 and evaluate either the England bag charge or the deposit return scheme against three criteria they set (effectiveness, fairness, cost to households). They must use at least one number with its source, name one unintended consequence (for example bag-for-life purchases rising, or "bin liner" purchases replacing free bags) and propose one refinement (HS-ESS3-4).
• Part 4, the scale question (6 minutes): whole class. Data sheet 3. If only about 9% of plastic waste is recycled and making plastic is about 5.3% of global emissions, which of the UNEP levers (reuse, recycling, substitution) should a school prioritise, and why? Link to the school's own audit numbers if available.

Plenary (12 minutes)

• Each group states its policy refinement in one sentence with one number.
• Return to the starter question. Ask students to rewrite "Which is better?" as an answerable scientific question.
• Exit ticket: (1) Define functional unit. (2) State the cotton-bag break-even and its source. (3) Give one reason a higher recycling percentage could still mean more plastic waste in total.

Assessment

• Calculation and assumption statement (Part 1).
• Policy evaluation: criteria set, evidence used with source, unintended consequence and refinement (rubric aligned to HS-ETS1-3).
• Exit ticket.

Differentiation

• Support: pre-calculated ratios; give the three criteria for Part 3; provide sentence frames for the evaluation.
• Challenge: ask students to find the EA's figures for when thin bags are reused as bin liners, and explain why the break-even rises; compare the 2011 UK figures with a more recent LCA from another country and explain differences in electricity mix; model the effect of a 20p deposit on return rates using a simple elasticity assumption.
• English as an additional language (EAL): glossary with life-cycle assessment, functional unit, break-even, externality, extended producer responsibility, deposit return.

Extension and coursework links

• IB ESS internal assessment: "Does the reuse rate of bags for life in our community exceed the LCA break-even?" A-level Economics: a short essay on the bag charge as a Pigouvian tax (a charge that makes the buyer pay for harm the product causes to others). CAS or enterprise: run a bag-for-life count at the school gate and publish the result on the School Pledge Board.

Safety

• No practical hazards. Be mindful that household spending is a sensitive topic; keep survey answers anonymous.