WATCH: Biodegradable bag intact after 3 years in the sea - study

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Research Fellow Imogen Napper with one of the plastic bags tested as part of the study (Photo Lloyd Russell, University of Plymouth)
Research Fellow Imogen Napper with one of the plastic bags tested as part of the study (Photo Lloyd Russell, University of Plymouth)

If you buy a biodegradable shopping bag, you could be forgiven for thinking that you're doing the environment a favour. But a new study has found that this might not be the case. 

Researchers from the University of Plymouth in the UK looked at the degradation of five plastic bag materials widely available from retailers in that country.

The results surprised the team of scientists, who found "biodegradable" shopping bags still intact after three years in the environment. 

The bags were left exposed to air, soil and the sea – environments that carrier bags often end up in when they are discarded.

After nine months in the open air, all the materials had completely disintegrated into fragments.

But the biodegradable, oxo-biodegradable and conventional plastic formulations remained functional as carrier bags after being in the soil or the marine environment for more than three years. They were even capable of holding a full load of shopping.

Research Fellow Imogen Napper, who led the study as part of her PhD, said: "After three years, I was really amazed that any of the bags could still hold a load of shopping. For a biodegradable bag to be able to do that was the most surprising. When you see something labelled in that way, I think you automatically assume it will degrade more quickly than conventional bags. But, after three years at least, our research shows that might not be the case."

The compostable bag completely disappeared from the experimental test rig in the marine environment within three months but, while showing some signs of deterioration, was still present in soil after 27 months.

'Require very specific conditions to degrade'

Professor Richard Thompson OBE, Head of the International Marine Litter Research Unit said the study demonstrated the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter.  

Bags and other single use plastic items are responsible for contaminating the oceans and breaking down into microplastics. There is mounting evidence that these tiny plastic particles are impacting on marine ecosystems all the way through the food chain.

Professor Peter Ryan, Director of the University of Cape Town's FitzPatrick Institute of African Ornithology, said the study underscored the findings from a 2018 United Nations Environment Programme (UNEP) report that "degradable" alternatives to plastics were not the solution to the plastics problem that people think them to be.

"Many so-called compostable alternatives require very specific conditions to degrade, which are seldom found in nature and, as this paper shows, the rate at which even the truly degradable biopolymers decompose varies greatly, depending on where they end up in the environment."

"The danger is that people will litter more because they think that these items will rapidly degrade, and thus pose little risk to the environment. But they might remain for years, reducing the aesthetic appeal of natural environments and threatening a host of animals with entanglement and ingestion," he said.

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