Every year, a huge quantity of marine debris is washed up onto Japanese coastal areas, and more than half of the debris is plastic. It has been thought that plastics such as polystyrene or polycarbonate do not thermally decompose at a natural environment temperature. However, a possibility that polystyrene is decomposed at a low temperature of 30 to 50°C, such as a temperate or a subtropical beach condition, was pointed out. When polystyrene decomposes styrene oligomers are produced. The authors have carried out field investigations on Japanese beaches from 2007 to 2012 in order to understand the pollution situation of Japanese coastal areas. In this paper, we report the field investigations and laboratory analyses. High abundances of marine debris were found on Japanese beaches with limited access roads. The styrene oligomers were detected from all beach sand and shore water samples. Averages of styrene oligomers of the sand and the water are 470 µg/kg and 3.1 µg/L, respectively.

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