Some biodegradable and compostable bags hold groceries with ease even after being buried in soil or at sea for three years, posing questions about how eco-friendly these alternative bag options really are.

Because of their lightweight, strong, and durable nature, plastic bags have become a staple of everyday life since they were first introduced in the 1970s. Today, world plastic production exceeds 335 million tonnes (369 million tons) with around half of all plastics chucked out after one use. As such, the UN Environment Assembly named the accumulation of plastic debris as a major global crisis in 2015, sparking the innovation of alternative plastic-like materials. Today, the global plastic battle is one at the forefront of international environmental policy. 

But the study begs the question: are biodegradable or compostable bag options really all that much better?

To find out, researchers at the University of Plymouth took four different types of commonly used opaque bags most prevalently used by UK retailers. A total of 16 of these bags – made of biodegradable, oxo-biodegradable, compostable, and conventional high-density polyethylene materials – were either buried in soil, submerged in seawater, left in the open air, or placed in a controlled lab for up to three years in order to determine how efficiently they break down.

After spending nine months in the open air, all materials disintegrated into fragments but the overall results varied by material and environment. Biodegradable, oxo-biodegradable, and conventional plastic stayed together well enough for use after being in the soil or marine environment for more than three years. Meanwhile, compostable bags disappeared in the marine environment after three months but were present in the soil after more than two years.

“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,” said lead researcher Imogen Napper in a statement. “When you see something labeled 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.”

Altogether, the researchers say their work suggests even the products we think of as eco-friendly may be doing just as much damage as ordinary plastics.

“This research raises a number of questions about what the public might expect when they see something labeled as biodegradable. We demonstrate here that the materials tested did not present any consistent, reliable and relevant advantage in the context of marine litter,” said Richard Thompson, Head of the International Marine Litter Research Unit at Plymouth. “It concerns me that these novel materials also present challenges in recycling. Our study emphasizes the need for standards relating to degradable materials, clearly outlining the appropriate disposal pathway and rates of degradation that can be expected.”

Plastics have been found in nearly every ecosystem on the planet, from the deepest depths of the ocean to the tops of some of Earth's highest peaks. 

Collectively, the results show that “none of the bags could be relied upon to show any substantial deterioration over a 3-year period in all of the environments,” wrote the authors in Environmental Science & Technology “It is therefore not clear that the oxo-biodegradable or biodegradable formulations provide sufficiently advanced rates of deterioration to be advantageous in the context of reducing marine litter, compared to conventional bags.”