Think twice before you feel good about using biodegradable plastics, says the United Nations Environmental Programme (UNEP), which argues in a new report that they don't properly break down in places like the ocean.
"The real bottom line is that biodegradable plastics aren't going to be a solution for reducing the impact of marine litter," Peter Kershaw, the report's author, said. "When you see 'biodegradable' on a plastic bag, for example, does that mean if you drop it in the streets, it's just going to disappear? No, it doesn't."
The report, which references the ubiquity of plastic in the seas, says that for some kinds of biodegradable plastic to actually degrade, they need to be in a composter that reaches over 50 degrees Celsius (122 degrees Fahrenheit) for a "prolonged" amount of time.
"Such conditions are rarely if ever met in the marine environment," the report states. The degradation will also be impeded if the plastic is buried in sand or sinks into cooler water.
Kershaw is also concerned that if plastic is marked as "biodegradable," people are less likely to throw it away properly.
Ironically, he said that biodegradable plastics can in some cases complicate recycling. "If you're trying to encourage people to recycle plastics, the last thing a recycler wants, is so-called biodegradable plastics mixed in with mainstream plastics," Kershaw said. One problematic situation is when a common type of plastic called polyethylene has an added compound, which helps it fragment under ultraviolet light.
Conventional, non-biodegradable, plastic, like a soda bottle made out of a material called PET, is highly recyclable, especially when the plastic is clear and there's a monetary value on it, Kershaw said.
Ramani Narayan, a professor in the department of chemical engineering and materials science at Michigan State University, said he generally agrees with the UN report. He's a coauthor on a February study published in the journal Science that found that between 4.8 and 12.7 million metric tons of plastic made its way into the ocean in 2010 from over 190 different coastal countries.
The worst kind of plastic to end up in the ocean, he said, are items like milk jugs, made out of polyethylene, or plastic utensils, made out of polypropylene. That kind of lightweight plastic floats, breaks up, attracts microorganisms, and can enter the food chain.
He pointed out that unlike a composter, the oceans shouldn't be seen as a "disposal environment," and that even if biodegradable plastic enters the ocean, where it's cold, it will degrade very slowly and will remain in the ocean "for long periods of time."
"Therefore, biodegradability as a solution is not the right approach," Narayan said.
However, he added that the right kind of biodegradability could be valuable if a piece of plastic accidentally does end up in the ocean. "Because it will be eventually removed, unlike a regular non-biodegradable plastic which is going to be floating around for ever and ever after," he said. "There is some value in looking at it, but it's not a solution."
He gave the example of a fishing net or a lobster pot, which could become lost in the ocean. Items like that could incorporate the right kind of biodegradable polymer, even if such a kind of plastic isn't in widespread use, he said.
Ultimately, the solution, he said, is good waste management to make sure plastic doesn't get into the ocean.
George Leonard, the chief scientist at the Ocean Conservancy, agrees. He said that biodegradable plastics could have some use as a "targeted applications," specifically with fishing gear, which, when it is lost, has a major impact on the oceans.
But in terms of the big picture, he said, the Ocean Conservancy concurs with the UNEP report.
"If people are looking for biodegradability to be a silver bullet to the problem of ocean plastic, it clearly is not," he said.
"We really think that the solution to plastics in the ocean is not biodegradability," he added, "it's to keep plastics out of the ocean in the first place."
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