Study finds ‘biodegradable’ plastic bags that can still hold shopping after being discarded for three years.
Study finds ‘biodegradable’ plastic bags that can still hold shopping after being discarded for three years.

Big plastic bag myth exposed

Shoppers who try to do the right thing for the environment and use biodegradable plastic bags might want to sit down for this.

Plastic bags that claim to be biodegradable, are anything but in some cases. A new research paper has revealed that in certain scenarios they can still carry a full load of shopping three years after they were thrown away.

Seen as a solution to the globe's increasingly urgent plastic pollution problem, disposable bags are supposed to decompose if they are buried in landfill, or wash into the sea.

The study, published in Environmental Science and Technology, examined biodegradable, oxo-biodegradable, compostable, and standard plastic bags for a period of three years.

The bags were exposed to three natural environments: left in open-air, buried in soil, and submersed in seawater, as well as in controlled laboratory conditions. The range of environmentally friendly bags fared differently in each environment.

After nine months exposure in the open-air, all bag materials had disintegrated into fragments.

In the marine environment, the compostable bag completely disappeared within three months. The same compostable bag type was still present in the soil environment after 27 months but could no longer hold weight without tearing.

However two of the bags - technically known as oxo-biodegradable - were still able to carry shopping after spending three years in the ground or covered in seawater.

"It is therefore not clear that the oxo-biodegradable or biodegradable formulations provide sufficiently advanced rates of deterioration," the researchers from the UK's University of Plymouth wrote.

After three years, some bags were perfectly robust.
After three years, some bags were perfectly robust.

There have been at least four reports of dead whales being found with huge amounts of plastic waste in their stomach, causing them to starve, in the last year.

Professor Richard Thompson, of the International Marine Litter Research Unit, who was involved in the study said it showed that certain bags might be polluting the ocean when consumers expect them to decompose.

"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," he said.

He previously gave evidence to a government inquiry in the UK that led to a small levy on plastic bags in the country. He called for new standards to be imposed on bag manufacturers.

"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," he said.

Some have suggested that the manufacturers of the bags - many of which reside in China - could be skimping on the required biodegradable additives to make them truly compostable.

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."

In Australia, Coles and Woolworths have banned single-use plastic shopping bags across their stores.