Life cycle assessment, or LCA, is a method for measuring the environmental impacts of a product, service, or process from raw material extraction to disposal. This cheat sheet helps students compare choices using evidence instead of assumptions. It is useful for environmental science, sustainability projects, engineering design, and consumer product analysis.
A clear LCA reference also helps students organize data and explain tradeoffs.
Key Facts
- The four main LCA phases are goal and scope definition, inventory analysis, impact assessment, and interpretation.
- A functional unit defines the exact service being compared, such as 1,000 liters of bottled water delivered to consumers.
- System boundaries state which life cycle stages are included, such as raw materials, manufacturing, transport, use, and end of life.
- Total impact can be estimated with Total impact = sum of activity data x emission factor for all included processes.
- Carbon footprint is often reported as CO2e, where CO2e = mass of gas x global warming potential.
- Energy intensity can be calculated as Energy intensity = total energy used / functional unit.
- Allocation divides shared environmental burdens among co-products, often by mass, economic value, or energy content.
- Interpretation checks whether conclusions are supported by the data, assumptions, uncertainty, and sensitivity analysis.
Vocabulary
- Life Cycle Assessment
- A method for evaluating environmental impacts across the full life of a product, service, or process.
- Functional Unit
- The measured amount of service used as the basis for comparison in an LCA.
- System Boundary
- The set of life cycle stages and processes included or excluded from the assessment.
- Life Cycle Inventory
- A list of all material inputs, energy inputs, emissions, and waste flows for the system being studied.
- Impact Category
- A type of environmental effect measured in LCA, such as climate change, water use, or eutrophication.
- Sensitivity Analysis
- A test of how much results change when key assumptions, data values, or methods are adjusted.
Common Mistakes to Avoid
- Comparing products without the same functional unit is wrong because the products may not provide the same amount of service.
- Ignoring system boundaries is wrong because excluding stages such as transport or disposal can change the final conclusion.
- Treating carbon footprint as the only impact is wrong because products may also differ in water use, toxicity, land use, and waste.
- Using average data without checking assumptions is wrong because local electricity sources, transport distances, and use patterns can strongly affect results.
- Assuming recycling always lowers impact is wrong because collection, processing, contamination, and material quality also affect the net benefit.
Practice Questions
- 1 A product uses 4.0 kWh of electricity per functional unit. If the emission factor is 0.45 kg CO2e per kWh, what is the climate impact from electricity use?
- 2 A reusable bottle causes 2.4 kg CO2e to manufacture and saves 0.08 kg CO2e each time it replaces a single-use bottle. After how many uses does it break even?
- 3 A shipment travels 300 km by truck with an emission factor of 0.12 kg CO2e per ton-km. If the shipment mass is 2.0 tons, what is the transport impact?
- 4 Two products have the same carbon footprint, but one uses much more water in a drought-prone region. Explain why an LCA conclusion should not be based only on carbon.