Plastics decompose with surprising speed in the oceans, releasing contaminants into the water, according to new research.
The huge amount of plastic waste in our seas has previously been regarded as a long-lasting pollutant that does not break down easily.
Researchers who presented their work at a meeting of the American Chemical Society (ACS) suggest otherwise.
Thousands of tonnes of plastic debris end up in the oceans every year.
Much of it washes up on coasts, but vast areas of waste - composed mainly of plastic - float in the oceans.
The so-called Great Pacific Garbage Patch between California and Hawaii is one such expanse, which is thought to be about twice the size of Texas.
Most attention has focused on dangers that visible items of plastic waste pose to seabirds and other wildlife.
"Plastics in daily use are generally assumed to be quite stable," said Katsuhiko Saido, lead author of the new study.
"We found that plastic in the ocean actually decomposes as it is exposed to the rain and sun and other environmental conditions, giving rise to yet another source of global contamination that will continue into the future."
Dr Saido, a chemist at Nihon University in Chiba, Japan, said his team found that when some plastics decompose they release the chemicals bisphenol A (BPA) and PS oligomers into the water.
Previous studies in animals suggest that, at particular doses, exposure to BPA can disrupt hormone systems.
Plastics do not usually break down in an animal's body after being eaten. However, the substances released from decomposing plastic could be absorbed, say the researchers.
But it is unclear whether marine animals are being exposed at sufficient concentrations to cause concern about the effects of these compounds.
The work was presented at the Fall Meeting of the American Chemical Society in Washington DC.