Microplastics—particles smaller than 5 millimeters—come from a number of sources. Plastic bags and bottles released into the environment break down into smaller and smaller bits. Your washing machine is another major source : When you launder synthetic clothing, tiny microfibers slough off and get flushed to a wastewater treatment plant. That facility filters out some of the microfibers, trapping them in “sludge,” the treated human waste that’s then applied to agricultural fields as fertilizer. That loads the soil with microplastic. A wastewater plant will then flush the remaining microfibers out to sea in the treated water. This has been happening for decades, and because plastics disintegrate but don’t ever really disappear, the amount in the ocean has been skyrocketing .
In fact, this new research shows there may now be more microplastic blowing out of the ocean at any given time than there is going into it. Put another way: So much has accumulated in the ocean that the land may now be a net importer of microplastic from the sea. “That's really highlighting the role of legacy pollution,” says Janice Brahney, an environmental scientist at Utah State University and co-lead author of the new PNAS paper. “The amount of plastics that are in our ocean is just overwhelming compared to anything that we produce in any given year in the terrestrial environment.”