The question a life cycle assessment tries to answer
Is a glass bottle better for the environment than a plastic one? A paper bag than a cloth one? An electric car than a petrol one? These questions are hard because every product causes impacts at many stages, and a choice that looks good at one stage can look worse at another.
A life cycle assessment (LCA) is a structured way of adding those stages up. Instead of judging a product only by how it is used or disposed of, it looks at everything from getting the raw materials to the end of the product’s life, and measures a set of environmental effects along the way.
The main stages
Most LCAs follow a similar sequence.
- Raw materials. Mining, drilling, farming or forestry to obtain what the product is made from.
- Manufacturing. Processing the materials, making parts and assembling the product, including the energy used in factories.
- Transport and distribution. Moving materials and finished goods.
- Use. Energy, water, consumables and maintenance while the product is in service. For some products, such as a bottle, this stage is small. For others, such as a fridge or a car, it is often the largest.
- End of life. Reuse, recycling, incineration or landfill, plus any credits for recovered materials.
A study covering all five is often called “cradle to grave”. One that stops at the factory door is “cradle to gate”, which leaves out use and disposal and should not be read as the full picture.
What gets measured
Carbon is the impact most people think of, usually expressed as carbon dioxide equivalent. However, a full LCA often covers several categories, such as:
- energy and resource use;
- water use;
- acidification and nutrient pollution of land and water;
- land use and effects on habitats;
- waste.
This is important because a product can do well on carbon and poorly on water, or the reverse. Studies that report only a single measure may be telling a partial story. The ideas behind embodied carbon, the emissions built into a product before you use it, are one slice of this picture.
The choices that shape the result
LCAs follow recognised international standards, which set out the general method, but there is still a lot of room for judgement. These are the main places where results can diverge.
The functional unit. To compare fairly, a study must define what is being compared, such as “carrying 1 kg of shopping” or “heating a room for a year”, not just “one bag” or “one heater”. If one bag is used ten times as often as another, comparing a single item of each would be misleading.
The system boundary. Which stages and processes are included? A study might leave out transport, packaging or the manufacture of equipment because the data are hard to find.
Lifespan and use pattern. A product that causes more impact to make can come out ahead if it lasts much longer. As a simple illustration, if two products have the same manufacturing impact and one lasts twice as long, its manufacturing impact per year of use is half as large. For the same reason, the number of times a reusable item is actually reused is often the deciding factor. This also underlies the idea of cost per use, which applies the same reasoning to money.
Energy sources. The emissions from making or using a product depend heavily on the local electricity mix. A study based on one country’s mix may not apply in another, or in the same place ten years later.
Recycling and allocation. If a material is recycled, who gets the credit: the first product or the next one? When a process creates more than one output, how are impacts divided between them? Reasonable methods give different numbers.
Why different studies disagree
When two studies reach opposite conclusions, it is often not because one is wrong. They may have used different functional units, different assumptions about how long a product lasts, a different country, or a different list of impacts. Industry-funded studies are not automatically biased, but it is fair to check who commissioned a study and whether it was independently reviewed.
Results for products like electric vehicles show this clearly. Studies typically find that the battery adds to manufacturing impacts, while running on electricity can reduce use-phase emissions, with the balance depending on the cleanliness of the electricity, how far the car is driven and how long it lasts. That is why you will see a range of conclusions rather than one answer.
Using LCA results sensibly
- Look for the question being asked. A finding that applies to “a typical user in one country” may not apply to you.
- Be cautious of large claims from small studies. Treat a single study as one input, not a verdict.
- Check whether use or manufacture dominates. If use dominates, efficiency matters most. If manufacturing dominates, making things last and reusing them matters more. Related topics include electronic waste and reusable bottles, where the number of uses matters.
- Remember what is left out. Many LCAs do not fully capture effects such as litter, microplastics or biodiversity loss, which are hard to quantify.
Companies increasingly cite LCAs in marketing, and a claim like “30% lower impact” is only as meaningful as its method. If a figure appears without a comparison point, boundary or date, it deserves caution, as discussed in the piece on greenwashing claims.
What it means for everyday choices
LCAs rarely give a simple rule for shoppers, but a few general patterns come up repeatedly: products that are used for a long time tend to do better per use, avoiding unnecessary purchases avoids the whole chain of impacts, and for energy-using appliances the use phase often matters a lot. These patterns support the broader approach to sustainable shopping, while the specifics for any one product depend on the assumptions discussed above.