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Microplastics, tiny bits of plastic measuring 5 millimeters or less, are often the result of larger pieces of plastic breaking down on land before making it into the ocean.

Plastic starts breaking down, or degrading, when exposed to light and high temperatures from the sun. Ultraviolet B variation (UVB), the same part of the light spectrum that can cause sunburns and skin cancer, starts this process for plastics.
This process, known as photo-oxidation, is a chemical reaction that uses oxygen to break the links in the molecular chains that make up plastic. It also happens much faster on land than in the comparatively cool waters of the ocean.

For example, a hot day at the beach can heat the sandy surface - and plastic trash sitting on it - up to 40°C. The ocean, on the other hand, gets darker and colder the deeper you go, and the average temperatures at its surface in July can range from 7°C near Adak Island, Alaska, to 31°C in Cannon Bay, Florida.

Back on that sunny, warm beach, a plastic water bottle starts to show the effects of photo-oxidation. Its surface becomes brittle and tiny cracks start forming. Those larger shards of plastic break apart into smaller and smaller pieces, but they keep roughly the same molecular structure, locked into hydrogen and carbon chains. A brisk wind or child playing on the beach may cause this brittle outer layer of plastic to crumble. The tide washes these now tiny plastics into the ocean.

Once in the ocean, the process of degrading slows down for the remains of this plastic bottle. It can sink below the water surface, where less light and heat penetrate and less oxygen is available. In addition, plastics can quickly become covered in a thin film of marine life, which further blocks light from reaching the plastic and breaking it down.

Cleaning up a few plastic bottles on a beach can make a big difference when it comes to keeping microplastics from entering the ocean.