What is a life cycle assessment?
A life cycle assessment (LCA) is a way to measure how much a product harms the environment over its whole life. We do not only look at the factory. We look at four stages:
- Raw materials: mining ore, drilling for crude oil, cutting trees, growing cotton.
- Making and packaging: turning raw materials into the product. This uses energy, water and chemicals.
- Use: fuel for a car, electricity for a phone, water and soap for washing clothes.
- Disposal: landfill, burning (incineration), or reuse and recycling.
At each stage we ask: how much energy, water and raw material is used? How much pollution and waste (for example CO₂) is made? Transport between stages counts too.
Why LCAs are useful, and their limits
LCAs help companies and governments choose the product with the smallest total impact. They also show which stage is worst, so we know where to improve.
Limits of an LCA
- Energy, water and CO₂ can be measured fairly well.
- Some effects are hard to give a number to, for example how bad a smell is or how ugly a landfill site looks. People must make value judgements, so two LCAs can disagree.
- A company may write a selective or shortened LCA that only shows the good parts, for advertising. Always ask who paid for the LCA.
Comparing products: plastic vs paper shopping bags
| Stage | Plastic (poly(ethene)) bag | Paper bag |
|---|---|---|
| Raw materials | Crude oil, a finite resource | Trees, a renewable resource, but needs land |
| Making | Fractional distillation and cracking, then polymerisation; quite little energy per bag | Pulping wood uses a lot of energy and water |
| Use | Strong, can be reused several times | Weaker, usually used once |
| Disposal | Not biodegradable; lasts hundreds of years; can be recycled or burned | Biodegradable and easy to recycle |
So there is no simple winner. The answer depends on how many times each bag is used. This is why we look at the impact per use: total impact ÷ number of uses.
Ways of reducing the use of resources
Many raw materials are finite (they will run out): metal ores, crude oil, sand for glass. Getting them out of the ground also uses energy and damages land. We can cut this with the 3 Rs: reduce, reuse, recycle.
- Reuse: glass milk or drink bottles are washed and filled again. Only cleaning is needed.
- Recycle glass: crush, melt and reshape. Melting old glass needs less energy than making new glass from sand.
- Recycle metals: sort, melt and recast. Aluminium recycling uses only about 5% of the energy of extracting new aluminium by electrolysis. Steel scrap is added to the blast-furnace steel.
- Recycle plastics: sort by type, then melt and remould.
Recycling still uses some energy (collecting, sorting, melting), so reuse is often even better. In India, the kabadiwala system collects paper, metal and plastic from homes for recycling.
Key formulas and definitions
- Total impact (LCA): total = raw materials + making + use + disposal (+ transport)
- Impact per use: impact per use = total impact ÷ number of uses
- Saving from recycling: % saved = (energy for new − energy for recycled) ÷ energy for new × 100
Worked examples
1. A product uses 30 MJ for raw materials, 50 MJ to make, 10 MJ in use and 5 MJ for disposal. Find the total energy.
Total = 30 + 50 + 10 + 5 = 95 MJ.
2. A cotton bag needs 130 MJ over its life. A plastic bag needs 2 MJ. How many times must the cotton bag be used to beat single-use plastic bags?
Cotton per use = 130 ÷ n. It beats plastic when 130 ÷ n < 2, so n > 65. It must be used at least 66 times.
3. Making 1 kg of new aluminium needs 200 MJ. Recycling 1 kg needs 10 MJ. What percentage of energy is saved?
Saved = 200 − 10 = 190 MJ. % saved = 190 ÷ 200 × 100 = 95%.
4. Car A: making 60 GJ, use 180 GJ, disposal 5 GJ. Car B: making 90 GJ, use 120 GJ, disposal 8 GJ. Which has the lower total, and which stage matters most?
A = 245 GJ, B = 218 GJ, so B is lower. The use stage (fuel) is the biggest part for both, so better fuel economy saves the most.
5. Explain why reusing a glass bottle is better than recycling it.
Reusing only needs washing and transport. Recycling needs crushing and melting at high temperature, which uses much more energy.
6. A company LCA for its drink can leaves out the mining stage. Why is this a problem?
Mining bauxite uses a lot of energy and damages land. Leaving it out makes the can look greener than it is. This is a selective (shortened) LCA.
Common mistakes
- Looking only at the factory. An LCA covers all four stages, from raw material to disposal.
- Thinking a paper bag is always greener than a plastic one. It needs more energy to make; it depends on reuse.
- Thinking recycling is free. Collecting, sorting and melting still use energy, just less than making new.
- Thinking every part of an LCA is exact. Pollution effects often need value judgements, so LCAs can be biased.