Life-cycle assessments measure impact at every stage

1. Obtaining raw materials covers mining ores, drilling for crude oil or felling trees, and the energy and land these use.
2. Manufacture covers turning the raw materials into the product and its packaging.
3. Use covers the energy and resources the product needs during its working life, such as the fuel a car burns.
4. Disposal covers what happens at the end of the product's life, whether it goes to landfill, is burned or is recycled.
5. Transport between the stages is also counted, because moving materials and products burns fuel.

A life-cycle assessment values each impact to compare products

1. Some impacts can be measured as numbers, such as the energy used in megajoules or the mass of carbon dioxide released.
2. Other impacts, such as the effect of a quarry on the landscape, are hard to measure and depend on value judgements.
   1. Different people weigh these effects differently, so two assessments of the same product can reach different conclusions.
3. Products should be compared when they do the same job, such as one drinks container against another.
4. An assessment may be selective, because a company could leave out stages that make its own product look worse.
   1. A shortened assessment used to advertise a product can therefore give a misleading picture.

Interpreting life-cycle data by comparing overall values

1. Read every row of the table, including raw materials, energy, percentage recycled and cost, before deciding.
2. Convert values given per kilogram into values per product, because containers made of different materials have different masses.
   1. Multiply the value per kilogram by the mass of one product in kilograms.
3. A material with a lower energy use per kilogram can still need more energy per product if each product is heavier.
4. One material is rarely best on every row, so weigh up the factors and state which one decides the choice.

Recycling for a different use needs suitable properties

1. Used PET bottles are collected, sorted by type of plastic, washed and shredded into flakes.
   1. The flakes are melted and pushed through tiny holes to make fibres, which are spun into yarn for fleece clothing.
2. This works because PET keeps the properties a fibre needs, being strong, flexible and able to be melted and reshaped.
3. A fleece does not need the food-grade purity of a drinks bottle, so small traces of contamination do not matter.
4. Making fibres from recycled PET uses less energy and crude oil than making new PET, so the product can be cheaper.
5. Crushed waste glass is reused in a similar way as a substitute for sand in road surfaces and concrete.

Recycling decisions weigh energy, cost, quality and demand

1. Recycling conserves finite resources, such as metal ores and crude oil, and reduces the waste sent to landfill.
2. Recycling aluminium uses only about 5% of the energy needed to extract new aluminium by electrolysis, so it is very worthwhile.
3. Collecting, sorting and transporting waste also use energy and cost money, which can cancel out the savings for low-value materials.
4. Mixed or dirty waste, such as plastic packets with food on them, is hard to separate, so contamination can make recycling uneconomic.
5. Some materials lose quality each time they are recycled, as the fibres in paper get shorter, so they can be recycled only a few times.
6. Recycling needs a market for the recycled material, since collected waste that nobody buys still ends up as waste.