A recent article published in the Geochemical Journal by Miyake et al. has found that there was 31.6 times as much iodine-129 than iodine-131 released in the early days of the Fukushima catastrophe. Iodine-129 is a long-lived radionuclide with a half-life of 15.7 million years. So it doesn’t go away.
The EPA document “Health Risks from Low-Level Environmental Exposure to Radionuclides”
indicates the mortality risk for I-129 is about 3 times that of I-131. This is mainly from thyroid cancer. European nuclear reprocessing plants (mainly La Hague) release a huge amount of I-129 – they released around 1,800 times as much of it as Chernobyl did (up to the year 2000). Gee, I wonder why there is a worldwide epidemic of thyroid cancer.