UV radiation and erosion cause sea-borne plastic to dissolve

Plastics washed out to sea, such as drinking bottles and plastic bags, are broken down by UV radiation and the action of the waves and decompose into the sea as micro-plastic.

The solar UV radiation which reaches the Earth's surface affects not only human health but also the ageing of materials. Although many plastics are very durable, the sun's UV radiation is able to break them down. Other factors affecting the decomposition of plastics at sea include the surrounding temperatures and oxygen levels, the impurities that have accumulated on the surface of the plastic, and the wind and waves that produce mechanical abrasion on sandy or rocky beaches.

Ultraviolet radiation plays an important role in the breakdown of plastics at sea because it gives a starting impulse to the decomposition of plastics floating on the surface or scattered along coastlines. The decomposition of plastic starts with the surface layer, on which UV radiation and the oxygen in the air work together to form microscopic cracks. These cracks penetrate gradually into the deeper layers and cause the plastic to break up into smaller pieces.

"Our current understanding of the decomposition process is still insufficient", explains Finnish Meteorological Institute Researcher Anu Heikkilä. "We know, however, that in addition to the macroscopic breakdown process, the weakened surface of the plastic also ‘peels away' due to the effect of oscillating temperatures and humidity, and it is this mechanism which produces large amounts of plastic particles which are washed into the sea water."

With time, even large pieces of plastic that have been washed into the sea eventually decompose into small pieces which are referred to as micro-plastic. Depending on its original use, some of the micro-rubbish in the sea was in micro-form to begin with. This kind of micro-plastic finds its way into the water system from a variety of sources, such as industrial run-off, plastic films used in agriculture, cosmetic and hygiene products, and the washing of clothes made from synthetic fibres.

Photo: NIOZ Sea Research / E. Zettler

Micro-plastic normally refers to particles of plastic that are less than 5 mm in diameter, or in some cases less than 1 mm. Micro-plastic has even been found in seabed sediments and in polar ice.

"Micro-plastic has wide-ranging effects which are not yet fully understood", says Ms Heikkilä. "Plastic floating around at sea can serve as a vessel for transporting marine microzoans as foreign species to new areas. Another big question mark is the effect of micro-plastic on the food chain."

The Finnish Meteorological Institute has researched the working life of different kinds of materials and developed unique methods for studying the sensitivity of materials to UV radiation. Using this information, products can be developed to be more durable, ecological and economical. This knowledge is also being utilised now in international, interdisciplinary joint research focused on the origin and life-cycle of micro-plastic.

Further information:

Researcher Anu Heikkilä, tel. + 358 50 338 4854, anu.heikkila@fmi.fi