A life-cycle assessment looks at the impact of a material or product on the environment over its whole life, from the start to the end. This means different products can be compared fairly, for example a plastic bag and a paper bag.
The assessment covers four stages. First is obtaining the raw materials and the manufacture of the product, including the process used to make it. Second is the use of the product. Third is the transport of the product, for example by lorry, which uses fuel and releases carbon dioxide. Fourth is the disposal of the product at the end of its life, such as landfill, incineration or recycling.
At every stage two things are recorded. The first is the use of resources, such as raw materials, energy and water. The second is the impact on the environment, such as pollution, waste and greenhouse gases. Some of these are easy to measure and some need judgement, so the results should be read with care.