In November 2019, materials researcher Mark Miodownik, project leader Danielle Purkiss and their colleagues began asking people across the United Kingdom to put a familiar promise to the test. Packaging marked “home compostable” was appearing in kitchens, but making it disappear required more than the words on the pack. A person had to recognise the right mark, have access to a compost heap and create conditions in which the material could break down.
The University College London team called its project the Big Compost Experiment. Over the next two years, 9,701 people completed a survey and 1,648 tried composting packages at home. They photographed the items, placed them in systems ranging from wormeries to outdoor heaps, then looked for what remained. Eventually, participants submitted 1,307 test results.1
The first difficulty appeared before the compost had done anything. In the researchers’ sample of uploaded packaging, 14 per cent carried a mark for industrial composting rather than home composting, while 46 per cent had no identifiable compost certification. “Compostable”, “biodegradable” and “home compostable” looked close enough to lead different promises into the same experiment.1
The heaps exposed the consequence. Of the 1,307 reported results, 55 per cent of items were still clearly visible and another 11 per cent had left small visible pieces. Thirty-four per cent could no longer be seen or found. The study did not measure whether every vanished item had completely changed into water, carbon dioxide and biological material, and the participants were unusually interested in composting. Even so, their experience made one thing tangible: compostability is a process with a place, temperature, moisture and time. It is not a promise that an item vanishes wherever it lands.
The word describes a route, not a raw material
Plastic is not defined by whether its carbon began in oil or a plant. It can be shaped, and it keeps that shape while in use. A “biobased” plastic contains matter made from living sources. A “biodegradable” plastic is made so living things can break it down. That breakdown only happens under certain conditions, while “compostable” has a narrower meaning. It describes breakdown in a composting process, often at an industrial plant after collection.2
A compostable plastic can therefore be made from plants or fossil fuels. It can also do useful work: a food-waste caddy liner can hold wet scraps. A small sticker can stay attached to fruit peel. A film or coating can seal a pack or keep moisture out. In each case, the claim does not mean the object has no plastic. It means the whole item has a planned route after use.
Industrial plants can control heat, moisture, oxygen, microbes and time. Garden heaps change with the weather, the material added and how often they are tended. A home-composting mark thus answers a different question from an industrial mark. Neither mark sends the item to normal plastic recycling. Neither says it will break down safely as litter.
The Big Compost Experiment showed how easily the two routes can be confused. Among items with the TUV OK Compost Home mark, 60 per cent stayed clearly visible. A further 12 per cent left small visible pieces in the varied home systems. In that marked group, 28 per cent were no longer visible or not found. The researchers also said the small pieces might break down with more time. A mark defines the conditions of a test. A real heap may be cooler, drier, wetter or in use for less time.1
An unfinished fragment may affect soil life
Visible pieces raise another question. If compostable plastic does not finish breaking down, what can those bits do in soil?
David Gutiérrez-Rial and his team explored that question with Eisenia andrei, an earthworm used in composting work. Their 2025 lab study compared particles of polylactic acid, called PLA, with two other plastics. One was a biodegradable plastic called PHB. The other was standard polyethylene, or PE. The particles had no added chemicals. In tests lasting four, 49 and 112 days, the team checked survival, growth, cell health and breeding.3
The result was not a simple finding that biodegradable plastic poisoned the worms. The short and medium tests found no broad effect on survival or growth. They also found no sign of the type of cell damage they checked. Breeding was different. In the 49-day test with artificial soil, 67.7 per cent of control cocoons hatched. The rates were 35.1 per cent with PLA, 33 per cent with PE and 44.2 per cent with PHB. The team used three levels of particles. These were controlled lab tests, not a garden heap.3
The team also ran a 112-day test. It added particles equal to 0.125 per cent of the compost by weight. Worms in the control made far more cocoons than those exposed to PLA or PE. Under these test conditions, PLA particles could cut breeding in this worm species.3
That result does not prove that a pack in the Big Compost Experiment harmed worms in a home heap. The citizen project did not identify the plastic in every piece. Nor did it check the amount around worms. The lab team used pure particles with no added chemicals, not old packaging. A field test could make the link clearer. It would need to track a known pack, its pieces, the amount in soil and the same effect on breeding. For now, the sound lesson is narrow but useful. “Biodegradable” does not mean a fragment has no effect on living things. It is wise to avoid making those pieces.
This investigation continues below.
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Labels need a working destination
The route can fail even when the material works as planned. A compostable item in normal plastic recycling can act as a lookalike impurity. Normal plastic in food waste creates the reverse problem. A European Commission review said this confusion raised impurity levels. It also cut the amount and quality of material recovered4.
The European Commission’s Joint Research Centre then tested a label design it called Prototype 2. Its study involved 11,096 people in eleven European Union countries. With Prototype 2, people sorted 42.6 per cent of packs correctly. Without it, they sorted 32.7 per cent correctly. Yet the label did not help them sort packs made of two or three parts. The team said home and industrial composting needed clearer words and signs5.
The separate European Union Packaging and Packaging Waste Regulation began applying in stages on 12 August 2026. For household sorting, its relevant change is a common packaging label planned from 2028.6 It may make the promise easier to spot. But it cannot create a collection service. It cannot split a mixed pack or make a local plant accept an item.
England’s current household recycling guide shows why the local route still counts. Councils do not have to collect every item marked compostable or biodegradable with food waste. They may accept caddy liners. The guide recommends liners made from bio-based compostable plastic. It says it does not recommend liners made from fossil-based compostable plastic.7 The rule does not say that every council takes every liner. It also does not say that every accepted item gets the same treatment. The exact item and the current local guide must match.
Choose a route that exists
The surest way to avoid the wrong route is to skip an unneeded throwaway item. Where a durable reusable product performs the same job and can genuinely be used many times, there is no compostability promise to interpret after one use. When an item must be thrown away, ask two questions. What is the whole item made to do? Does its planned route exist here?
An item meant to return to a home heap needs proof about the whole item, not just one layer. Look for an all-natural material that can return safely in the conditions you have. For a dry, separable package with a reliable local recycling route, a simple material that can be collected and recycled may be the clearer choice. Compostable plastic can still be useful. It fits jobs such as holding wet food waste when its mark, collection and treatment all match.
Each choice has limits. Reuse can require washing and enough repeated use to justify a heavier object. Paper or natural-fibre formats may not provide the same moisture barrier or strength. A compostable liner can keep food-waste collection cleaner, but only where the collector takes it. Price and local access can also narrow the choice.
At the bin, read past the large word on the front. Check whether the mark says home or industrial composting. Check that every part can follow that route. Then check whether the council’s current guide names the item. Keep compostable plastic out of normal plastic recycling unless the local service asks for it. Never treat the label as a licence to litter.
The pack at the start of the Big Compost Experiment did not make a false claim just by joining “plastic” and “compostable”. It gave only part of the needed instruction. The promise works when the material, the whole item, the mark, the collection system and the treatment all meet. When they do not, choose a route that already works—or use less material in the first place.