Besides the particles' danger to wildlife, on the microscopic level the floating debris can absorb organic pollutants from seawater, including PCBs, DDT, and PAHs.[40] Aside from toxic effects,[41] when ingested, some of these are mistaken by the endocrine system as estradiol, causing hormone disruption in the affected animal.[38] These toxin-containing plastic pieces are also eaten by jellyfish, which are then eaten by fish. Many of these fish are then consumed by humans, resulting in their ingestion of toxic chemicals.[42] While eating their normal sources of food, plastic ingestion can be unavoidable or the animal may mistake the plastic as a food source.[43][44][45][46][47]
Marine plastics also facilitate the spread of invasive species that attach to floating plastic in one region and drift long distances to colonize other ecosystems.[15] Research has shown that this plastic marine debris affects at least 267 species worldwide.[48]
Great Pacific garbage patch: https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch
In people, fails to degrade, in sanitary sewer, fails to degrade, back into the ocean, etc.
But actually plastic slowly degrading at the magnitute it is at, make the small particles spread all over. Probably gives people cancer.
Plastic breaks down into smaller plastic which becomes embedded in biological tissue.