Biodegradable and compostable plastic bags can still carry full loads of shopping after three years of being exposed to a natural environment, a study has found.
Researchers from the University of Plymouth examined five plastic bag materials widely available from high street shops in the UK.
The bags were exposed to air, soil and sea – environments they could encounter if left as litter – and monitored at regular intervals.
After nine months in open air, all the materials had completely disintegrated into fragments, the study published in Environmental Science and Technology found.
But biodegradable, oxo-biodegradable and conventional plastic materials remained functional as carrier bags after being in soil or the sea for more than three years.
The compostable bag disappeared completely from the test rig in the marine environment within three months but was still present in soil after 27 months, though it showed signs of deterioration.
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 bags 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.”
In the study, the scientists quoted a 2013 European Commission report that suggested about 100 billion plastic bags were being issued each year, although various governments including the UK have since introduced levies.
Many bags are known to have entered the marine environment, where they can be broken down into microplastics by marine creatures.
Professor Richard Thompson, head of the International Marine Litter Research Unit, said: “This research raises a number of questions about what the public might expect when they see something labelled as biodegradable.
“We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter.
“It concerns me that these novel materials also present challenges in recycling.
“Our study emphasises the need for standards relating to degradable materials, clearly outlining the appropriate disposal pathway and rates of degradation that can be expected.”