Dr Richard Thompson of the University of Plymouth has investigated how plastic degrades in the water and how tiny marine organisms, such as barnacles and sand-hoppers, respond.
He told the BBC: "We know that plastics in the marine environment will accumulate and concentrate toxic chemicals from the surrounding seawater and you can get concentrations several thousand times greater than in the surrounding water on the surface of the plastic.
"Now there's the potential for those chemicals to be released to those marine organisms if they then eat the plastic."
Once inside an organism, the risk is that the toxins may then be transferred into the creature itself.
At the microscopic level, plastic pollution is far worse than feared
"There are different conditions in the gut environment compared to surrounding sea water and so the conditions that cause those chemicals to accumulate on the surface of the plastic may well be reversed - leading to a release of those chemicals when the plastic is eaten."
It is as if the plastic particles act as magnets for poisons in the ocean.
In an experiment involving plastic carrier bags immersed off a jetty in Plymouth harbour, he is assessing the time taken for them to fragment.
In related projects, he and colleagues have also added plastic powder to aquarium sediment to establish how much is ingested by marine life. Research on stretches of shoreline has shown that, at the microscopic level, plastic pollution is far worse than feared.
Dr Thompson worries toxins may be released to marine organisms
In a typical sample of the sandy material gathered at the high tide mark on shorelines, one-quarter of the total weight may be composed of plastic particles.
Studies have found that plastic traces have been identified on all seven continents.
Here on Midway, Matt Brown of the US Fish and Wildlife Service echoes the warnings of a long-term threat from plastic waste.
"The thing that's most worrisome about the plastic is its tenaciousness, its durability. It's not going to go away in my lifetime or my children's lifetimes.
"The plastic washing up on the beach today - if people don't take it away it'll still be here when my grandchildren walk these beaches."
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