Biodegradable plastic bags can still hold full load of shopping after three years in ocean, study shows

Call for clearer standards on biodegradable plastics as customers likely to be surprised so few eco-alternatives will degrade in any meaningful way

Supposedly biodegradable plastic bags marketed as eco-friendly alternatives to cut pollution are still able to carry a full load of shopping after three years in the soil or ocean, a study has found.

Increasing public awareness of the levels of plastic pollution has led to ministers introducing a levy on plastic bags, as well as calls for single-use plastics in coffee cups and other materials to be phased out.

University of Plymouth researchers trialled eco-alternative plastics from major supermarkets and found that biodegradable and so-called oxo-biodegradable bags left in the ocean could still carry a load of groceries.

While bags labelled “compostable” broke down into larger chunks in the sea, they were still able to carry shopping after three years buried in soil.

Dr Imogen Napper, lead author of the study published in Environmental Science & Technology, said she had been motivated to see if these new bags lived up to their promises and “did what they said on the tin”.

“The most surprising result of this research was knowing that not one of the bags could completely degrade in all environments,” she said.

While all the bags left in the open air for three years broke down into smaller chunks within nine months, Dr Napper said she hopes people will consider what their bags will do when they’re put in the bin.

Experts said plastic bag pollution is a sign of humans “abusing the convenience of plastics”, with billions produced a year for a single use.

Many retailers have switched to compostable plastics for an array of products in recognition of customer opposition, but most do not realise they are not as easily recycled, and will only break down in a specific industrial facility.

“We need clearer international standards on what we mean when we say something is biodegradable,” said Professor Richard Thompson, head of the International Marine Research Unit and a senior author of the study.

“Do we mean biodegradable in an industrial composting facility at 50-60C, specific pH, humidity and oxygen? Or do we mean something in the sea, a river or soil?”

The study defined biodegradable as degrading within “any meaningful timescale in the natural environment”.

Peter Andrews, head of sustainability policy for the British Retail Consortium said compostable bags should go to industrial facilities.

“Compostable carrier bags are designed to be used twice,” he said. “First, to carry shopping home and again as food waste caddy liners to be collected by local councils for composting.

“Such systems are an important step in the industry’s goal of making 100 per cent of plastic packaging recyclable, reusable or compostable.”