A life cycle assessment (LCA) measures the effect a product has on the environment at every stage of its life. There are four stages: obtaining the raw materials, manufacturing the product, using the product, and disposing of the product when it is no longer useful.
At each stage the assessment records figures such as the energy used, the water used, the carbon dioxide released and the waste produced. Adding up the figures for the four stages gives totals that can be used to compare products.
To evaluate LCA data, compare the totals and also look at which stage causes most of the impact. A reusable cotton bag may need more water and energy to make than a single-use plastic bag, but it is used many times. If its manufacture releases 4 kg of carbon dioxide and it is used 40 times, that is 0.1 kg per use, which can be lower than a plastic bag that releases 0.2 kg per use. A product that is recycled at the end of its life usually has a smaller disposal impact than one sent to landfill.
Be careful with the data. Some figures are estimates, and some stages, such as how a customer uses the product, are hard to measure. The company that paid for the assessment may choose which factors to include, so the results can be biased. A fair evaluation also asks who carried out the assessment.