The average person uses a typical plastic bag for as short a time as 12 minutes before throwing it away, never thinking of where it may end up.

Yet once consigned to a landfill, that standard grocery store tote takes hundreds or thousands of years to break down — much more than a human lifetime. Bags make up an alarming amount of the plastic found in whale stomachs or bird nests, and it’s no wonder — globally, we use between 1 and 5 trillion plastic bags each year.

Biodegradable plastic bags are marketed as more eco-friendly solutions, able to break down into harmless material more quickly than traditional plastics. One company claims their shopping bag “will degrade and biodegrade in a continuous, irreversible and unstoppable process” if it ends up as litter in the environment.

Biodegradable bags that had been left underwater in a marina could still hold a full load of groceries.

In a study published this week in Environmental Science and Technology, researchers put supposedly eco-friendly bags made from various organic and plastic materials and sourced from U.K. stores to the test. After three years buried in garden soil, submerged in ocean water, exposed to open light and air or stashed in a laboratory, none of the bags broke down completely in all the environments.

In fact, the biodegradable bags that had been left underwater in a marina could still hold a full load of groceries.

“What is the role of some of these really innovative and novel polymers?” asked Richard Thompson, a marine biologist from the University of Plymouth and the study’s senior author. A polymer is a repeating chain of chemicals that makes up a plastic’s structure, whether biodegradable or synthetic.

“They’re challenging to recycle and are very slow to degrade if they become litter in the environment,” Thompson said, suggesting these biodegradable plastics may be causing more problems than they solve.

What the researchers did

The researchers collected samples of five types of plastic bags.

The first type was made of high-density polyethylene — the standard plastic found in grocery store bags. It was used as a comparison for four other bags labeled as eco-friendly:

A biodegradable plastic bag made in part from oyster shells

Two kinds of bags made from oxo-biodegradable plastic, which contain additives that companies say helps plastic break down faster

A compostable bag made from plant products

Each bag type was placed in four environments. Whole bags and bags cut into strips were buried in garden soil outdoors, submerged in salt water in a marina, left exposed to daylight and open air, or sealed in a dark container in a temperature-controlled lab.

Oxygen, temperature and light all change the structure of plastic polymers, said Julia Kalow, a polymer chemist from Northwestern University, who was not involved in this study. So too can reactions with water and interactions with bacteria or other forms of life.

What the scientists found

Even in a tough marine environment, where algae and animals quickly covered the plastic, three years wasn’t long enough to break down any of the plastics except for the plant-based compostable option, which did disappear underwater within three months. The plant-derived bags, however, remained intact but weakened when buried under garden soil for 27 months.

The only treatment that consistently broke down all of the bags was exposure to open air for more than nine months, and in that case even the standard, traditional polyethylene bag disintegrated into pieces before 18 months had passed.

“I would take that timescale to be too long for these products to be regarded as providing an environmental advantage,” Thompson said.

Even if these bags take less time to break down than traditional plastic bags, as litter they would still have enough time to become potentially deadly food for ocean animals like seabirds, whales, turtles or fish. Moreover, they would still be an eyesore and take up space at waste facilities for months or years.

And when some of the plastic bags did seem to break down, such as the bags left to the open air, it was unclear if the disintegration was complete.

“Did the plastic that was lost just become smaller pieces of plastic?” Kalow asked, “Or did it become molecules that could dissolve in water and be consumed?”

Future studies, she said, should dig into the fate of those disintegrated plastic particles, to ascertain whether they truly break down and disappear — or become microplastics and harmful chemicals.

Why it matters

Even standard plastic bags can’t be recycled from your home recycling bin, so most end up in landfill or are swept away by water or wind, becoming litter.

Labels like “biodegradable,” “compostable” or even “recyclable” are theoretical — they don’t reflect the reality of what happens to the materials.

Biodegradable and compostable bags are meant to solve these problems, but the study indicates that’s not the case so far.

These alternative bags aren’t meant to end up as litter in the street or in the natural environment — ideally, they’d all be treated just as manufacturers expect. Biodegradable bags would be landfilled or, in some cases, recycled into new plastics — at least in theory.

But “even if we can make something that’s recyclable, that doesn’t mean any commercial recycling plant would be interested in dealing with it,” Kalow said. Biodegradable plastics can’t generally be recycled with other plastics — in fact, they can ruin other batches of recyclable plastic, degrading the product until it becomes unusable.

Meanwhile, the eco-conscious should hope their compostable bags end up in industrial composting facilities where high temperatures and favorable conditions for bacteria and other living things would break them down. (Compostable bags in chilly, oxygen-starved landfills can actually be preserved rather than destroyed.

These are the real problems, Thompson said. Labels like “biodegradable,” “compostable” or even “recyclable” are theoretical — they don’t reflect the reality of what happens to the materials we throw away or litter into the oceans, and they don’t help people accurately understand how to get rid of them.

Three years after the researchers’ plastic bags were buried or submerged, they were nearly as useful — and as harmful — as the day they were made.

Some simple solutions might help. Thompson suggested that standardized products, made of the same sets of materials, could streamline our waste management systems and allow much more of our waste to be profitably recycled. Kalow, the polymer chemist, said there may yet be hope for new, improved biodegradable plastics if only we could discover that technology.

In the meantime, it doesn’t hurt to remember your reusable bag on your way to the store.