Do ‘Biodegradable’ Plastic Bags Actually Degrade?

A new study has found that the bags could still hold weight after being buried in water and soil for three years

A plastic bag submerged in soil for three years could still hold a full load of shopping (Picture Lloyd Russell, University of Plymouth).jpg
A plastic bag submerged in soil for three years could still hold a full load of shopping. Lloyd Russell, University of Plymouth

It’s no secret that the world has a plastic bag pollution problem. In the United States alone, 100 billion are used each year—the European Union goes through another 100 billion bags—and these single-use plastics often end up in the environment, where they pose a threat to animals. Rather than decomposing, the standard plastic bags break down into tiny pieces that get consumed by a variety of organisms and make their way up the food chain.

In light of these sobering realities, biodegradable plastic bags have been touted as a better way to get your groceries and other purchases home from the store. But as Laura Parker reports for National Geographic, a new study has found that biodegradable bags may not actually degrade all that quickly in the environment. Some, in fact, were still able to carry nearly five pounds of groceries after being exposed to the elements for three years.

The report, published in Environmental Science & Technology, describes an experiment by researchers at the University of Plymouth, who sought to find out how five different types of plastic behaved in various environments over time. Research fellow Imogen Napper and Richard Thompson, a marine biologist and head of the university’s International Marine Litter Research Unit, tested conventional plastic bags, compostable bags, biodegradable bags and two types of oxo-biodegradable bags—or bags that do not do not need microorganisms to decompose and therefore “should biodegrade predictably in any environment,” writes Fast Company’s Mark Wilson.

Each of the bag types was attached to a wall under the sun, buried in a university garden, submerged in Plymouth Harbor and, for control purposes, placed in a black box in a lab. The researchers tested both whole bags and bags that had been cut into strips and enclosed in mesh pouches. The experiment began in July 2015, and the researchers checked on the bags regularly.

Within three months, the compostable bag in the marine environment completely disintegrated—but it was the only bag that did. By nine months, the open-air bags had all broken down into fragments. The compostable bag in soil still held its shape after 27 months, though it was too weakened to hold any weight. After spending three years in water and soil, the biodegradable, oxo-biodegradable and conventional plastic bags largely kept their original forms. And, much to the researchers' surprise, the bags were still functional, meaning that they could hold groceries without breaking.

“I was really amazed that any of the bags could still hold a load of shopping,” says Napper. “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 results of the experiment have the researchers questioning whether “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,” as they write in their study.

Some critics, however, have pointed out that biodegradable, oxo-biodegradable and compostable bags are not intended to break down in any and every environment. Ramani Narayan, a chemical engineer at Michigan State University who was not involved in the study, tells Parker that compostable bags are designed to be thrown out in industrial composters. And Symphony Environmental Technologies, which made one of the oxo-biodegradable bags used in the study, says that its products are meant to degrade on open landscapes or ocean surfaces—not in deep landfills or deep seas.

Still, the researchers say it remains unclear whether purportedly biodegrabdeable bags deteriorate any faster than the standard plastic ones. And because biodegradable and compostable bags are often not compatible with recycling infrastructure, the scientists stress the importance of providing consumers with clear information on how to properly discard these products. “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,” says Thompson.

One way to counteract the negative effects of plastic bags' persistence in the environment might be to capitalize on their longevity by using them multiple times. “Perhaps durability in the form of a bag that can and is reused many times,” the study authors write, “presents a better alternative to degradability.”

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