How does sunlight and seawater turn larger plastic objects into microplastics?

Larger plastic objects are transformed into microplastics—defined as plastic pieces less than 5 mm (0.2 inch) in length—through a process of physical weathering and fragmentation.12

While conventional plastics are not biodegradable and resist natural decay, they are susceptible to environmental forces that break them down into smaller bits. Exposure to ultraviolet radiation from sunlight, known as photodegradation, works in tandem with seawater to embrittle the plastic material. This chemical and physical weakening is further accelerated by mechanical forces, including wave action and wind abrasion.3142

As these larger items fragment, they become secondary microplastics, which persist in the environment for hundreds of years. This process is a significant concern in plastic pollution, as it renders plastic debris small enough to be ingested by a wide range of marine life, from zooplankton to large predators, potentially introducing toxic chemicals into food chains.132