Shopping in a sustainable manner can be a confusing task, even for those who consider themselves educated, eco-friendly consumers. We’re told to use canvas tote bags at the grocery store, but those come with an environmental catch. Even materials that seem like they’d be easy to recycle—like yogurt containers and plastic bottle caps—can be a minefield of “do's” and “don’ts.”

Further complicating matters, a new study has revealed that “biodegradable” plastic bags didn’t actually degrade after three years in the natural environment, according to The Guardian. Researchers from the University of Plymouth in the UK tested four types of plastic bags marked as biodegradable and one conventional plastic bag as a control. Some of the biodegradable bag samples were buried in soil, some were left in open air, and others were submerged in seawater to represent conditions after the bags are thrown away by consumers. After nine months, the four biodegradable bags left in open air had disintegrated into plastic fragments; in seawater, only the bag type labeled compostable disintegrated completely. At the end of the study, all but the compostable bag were still able to hold 5 pounds of groceries after being submerged in soil or seawater. The findings were published in the journal Environmental Science and Technology.

Imogen Napper, the study's lead author, said she was shocked by the findings. “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,” Napper said 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.”

If you’re looking for a better alternative, compostable bags might be the way to go. Still, researchers concluded that none of the bags, including compostable ones, had deteriorated sufficiently enough to offset the negative effects of littering.

Co-author Richard Thompson, who serves as head of the university’s International Marine Litter Research Unit, called for more consistent guidelines regarding what can be labeled as biodegradable. “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,” Thompson said.

[h/t The Guardian]