A takeaway box is marked ‘biodegradable’, so you throw it away with a clear conscience.
A plastic bag advertises itself as ‘biodegradable’. It sounds like an environmental victory: unlike ordinary plastic, surely it will simply disappear. Yet a growing body of research suggests that the label tells us much less than shoppers might assume.
Conventional plastics are generally made from fossil fuels. Many alternatives begin with biological resources, including corn starch, sugar cane and other plant sugars. For example, microorganisms can ferment sugars into lactic acid, which manufacturers convert into polylactic acid (PLA) and mould into packaging. Other biodegradable polymers, including some petroleum-derived materials, are made through different chemical processes. Crucially, ‘bio-based’ refers to a material’s origin, whereas ‘biodegradable’ describes its potential to be broken down biologically. The two labels are not interchangeable.
Microorganisms can convert suitable polymers into simpler substances, but the speed and completeness of the process depend on temperature, oxygen, moisture, material thickness and surrounding conditions. Industrial composting plants carefully control these factors. A product certified for industrial composting is not necessarily suitable for home composting, and neither label guarantees rapid breakdown in the ocean.
Scientists are now investigating what happens before complete biodegradation. Plastics can fragment into particles less than five millimetres across, known as microplastics. Some may persist while microorganisms slowly consume the polymer. In research published in 2025 on PHBV microplastics mixed into soil, only approximately 1.5–5% degraded after eight weeks under the conditions studied. This result cannot be applied to every bioplastic, but it demonstrates why actual conditions matter. A 2024 analysis in Nature Chemical Engineering also found trade-offs: faster degradation could reduce certain aquatic toxicity impacts while increasing greenhouse-gas emissions.
The stakes are enormous. OECD estimates show that global plastic waste rose to 353 million tonnes in 2019, of which only 9% was recycled. Under its baseline scenario, annual waste could reach 1.014 billion tonnes by 2060, approaching three times the 2019 level. Simply switching materials without tackling consumption may leave that growth largely untouched.
Does this make biodegradable plastics pointless? Not at all. Certified compostable packaging can make sense in specific systems, especially if it is collected with food scraps and processed appropriately. Yet agricultural land, irrigation, fertilisers, factory energy and emissions all affect sustainability. Some materials can also contaminate conventional recycling streams if sorted incorrectly.
The crucial question is not merely ‘Will this plastic biodegrade?’ It is ‘Under which conditions, how completely, and what happens to it in the real world?’ When possible, reducing disposable packaging and reusing durable products remain powerful solutions.
Even certified materials may degrade gradually, releasing microplastics before mineralisation. Compost from appropriate treatment can be valuable, but any sustainable claim needs evidence.
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