Biodegradable plastics don’t break down in the ocean, which is just one more worry to add to the blanket of plastic problems choking the world’s seas. A new report from the UN Environment Program (UNEP) shows that not only do these plastics not rapidly break down in the ocean, but they won’t biodegrade well anywhere in nature.

The report addresses all kinds of plastic and micro plastic debris in the world’s oceans. But biodegradable plastics are particularly misleading, it notes.

To break down biodegradable plastics, certain conditions are required. Importantly, temperatures often have to reach 50 degrees Celsius, a temperature not found in the ocean, UNEP’s chief scientist Jacqueline McGlade told the Guardian. “They are also not buoyant, so they’re going to sink, so they’re not going to be exposed to UV and break down.”

These conditions are seldom met anywhere in the natural world. Plastic trash is rarely laid out and exposed to the sun in order to help it break down. And worse, these biodegradable plastics contain metals to help them disintegrate and these also end up in the ocean. “Some common non-biodegradable polymers, such as polyethylene, are sometimes manufactured with a metal- based additive that results in more rapid fragmentation,” says the report. And speaking of fragmentation, plastics, even biodegradable ones, never truly “break down.” Instead, they just get broken into ever-smaller particles, presumably turning the oceans—eventually—into a plastic soup.

The best case would be to stop all plastics from entering waterways, where they are washed into the seas. But as we’re not doing such a great job of that, other options become more attractive. For instance, plastics might be designed specifically to disintegrate in seawater, and alternative materials could be found to do the jobs usually done by petroleum-based plastics. For example, this edible six-pack ring is made from wheat and barley, and can be safely nibbled on by marine life. Even if nothing takes a bite, the plastic is designed to break down rapidly, before it can become a hazard.

With global plastic production at around 300 million tons per year and biomass-based polymers still a more expensive alternative, the plastic situation is unlikely to get better any time soon. But the improvement has to start somewhere, and avoiding plastic wherever possible is probably a good start.

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