Detection and activity of iodine-131 in brown algae collected in the Japanese coastal areas

Iodine-131 (physical half-life: 8.04 days) was detected in brown algae collected off the Japanese coast. Brown algae have been extensively used as bioindicators for radioiodine because of their ability to accumulate radionuclides in high concentration factors. The maximum measured specific activity of 131I in brown algae was 0.37 ± 0.010 Bq/kg-wet. Cesium-137 was also detected in all brown algal samples used in this study. There was no correlation between specific activities of 131I and 137Cs in these seaweeds. The specific activity of 137Cs ranged from 0.0034 ± 0.00075 to 0.090 ± 0.014 Bq/kg-wet. Low specific activity and minimal variability of 137Cs in brown algae indicated that past nuclear weapon tests were the source of 137Cs. Although nuclear power stations and nuclear fuel reprocessing plants are known to be pollution sources of 131I, there was no relationship between the sites where 131I was detected and the locations of nuclear power facilities. Most of the sites where 131I was detected were near big cities with large populations. Iodine-131 is frequently used in diagnostic and therapeutic nuclear medicine. On the basis of the results, we suggest that the likely pollution source of 131I, detected in brown seaweeds, is not nuclear power facilities, but nuclear medicine procedures.