Environmentalists have long denounced plastic as a long-lasting pollutant that does not break down. A new study indicates that, in the oceans, plastic does decompose, but says that is not a good thing either.

Thousands of tons of plastic debris wind up in the oceans every year, some of it washing up on coasts, some being swirled by currents into the Great Pacific Garbage Patch between California and Hawaii, said to be larger than Texas.

“Plastics in daily use are generally assumed to be quite stable,” Katsuhiko Saido, a chemist at Nihon University, Chiba, Japan, said in a statement.

“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,” said Saido, who presented his findings at the annual meeting of the American Chemical Society on Wednesday.

Saido reported that the decomposing plastics release potentially toxic chemicals such as bisphenol A and PS oligomer, which can disrupt the functioning of hormones in animals.