Interpreting LCA data means comparing the figures for each product, working out a fair measure, and then deciding which has the lower impact. Here is some example data for two carrier bags, given per bag. The plastic bag is made from poly(ethene), which comes from crude oil. The paper bag is made from wood pulp.
- Plastic bag: 2 MJ of energy, 4 litres of water, 0.1 kg of carbon dioxide released, used 4 times.
- Paper bag: 4 MJ of energy, 12 litres of water, 0.2 kg of carbon dioxide released, used once.
To compare fairly, divide each total by the number of uses. Taking water as the example, the plastic bag uses 4 ÷ 4 = 1 litre per use, but the paper bag uses 12 litres per use. Even if the plastic bag were used only once, it would use 4 litres, which is still less than the paper bag. On these figures the plastic bag has the lower impact for energy, water and carbon dioxide.
You must also think about what the data leaves out. The figures do not include litter caused by plastic bags, and crude oil is a non-renewable resource. An LCA paid for by a company that sells one of the products may be biased, and harm to wildlife is hard to turn into a number.