Scientists have identified a new source of chemical pollution released by the huge amounts of plastic rubbish found floating in the oceans of the world. A study has found that as plastics break down in the sea they release potentially toxic substances not found in nature and which could affect the growth and development of marine organisms.
Until now it was thought that plastic rubbish is relatively stable chemically and, apart from being unsightly, its principle threat to living creatures came from its ability to choke or strangle any animals that either got caught in it or ingested it thinking it was food.
But the latest research suggests that plastic is also a source of dissolved substances that can easily become widely dispersed in the marine environment. Many of these chemicals are believed to toxic to humans and animals, the scientists said.
The scale of plastic pollution in the sea has only been widely recognised in recent years when sailing yachts reported vast areas of ocean, such as an area estimated to be twice the size of Texas in the North Pacific, that seem to be permanently covered in a layer of floating marine litter caught up in swirling ocean currents or gyres.
Some of the items were found to be many decades old, suggesting that the plastic took a long time to degrade. However, a study by Katsuhiko Saido at Nihon University in Chiba, Japan, has found that plastics degrade relatively quickly in the conditions and temperatures that were designed to simulate the environment of the open ocean.
"Plastics in daily use are generally assumed to be quite stable. 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 said.
"To date, no studies have been conducted on plastic decomposition at low temperature in the environment owing to the mistaken conception that plastic does not decompose. The present study was conducted to clarify that drift plastic does indeed decompose to give rise to hazardous chemicals in the ocean," he said.
The scientists found that when plastics decompose in the ocean they release a range of chemicals, such as bisphenol A and substances known as polystyrene-based (PS) oligomers, which are not found naturally. Bisphenol A has been implicated in disrupting the hormonal system of animals.
A common form of plastic rubbish is styrofoam, which soon gets crushed into small pieces in the sea. However, it also releases substantial quantities of a toxic substances called styrene monomer, which is known to cause cancer, as well as styrene dimers and trimer, which are suspected of being carcinogenic. The trimer also breaks down into the toxic monomer form.
Findings from the study were released yesterday at the American Chemical Society meeting in Washington. Dr Saido said that samples of seawater collected from the Pacific Ocean were found to be contaminated with up to 150 parts per million of some of these components of plastic decomposition.
"This study clearly shows new micro-pollution by compounds generated by plastic decomposition to be taking place out of sight in the ocean. Thus, marine debris plastics in the ocean will certainly give rise to new sources of global contamination that will persist long into the future," he said.
It is estimated that there could be hundreds of millions of tons of plastic rubbish floating in the world's oceans. In Japan alone, it is calculated that 150,000 tons of plastic is washed up on its shores each year.