Elevated iodine-129 detected in West Philippine Sea seawater, Philippine scientists trace likely pathway to the Yellow Sea
MANILA (Jan. 18, 2026) — Scientists from the University of the Philippines Marine Science Institute (UP MSI) and partners have detected elevated concentrations of iodine-129 (¹²⁹I) in seawater samples from the West Philippine Sea (WPS), a finding that researchers say is consistent with long-range transport of nuclear-signature contaminants through regional ocean currents rather than any local source. (Philstar.com)
What was found
The results come from an analysis of 119 surface seawater samples collected across multiple Philippine waters, including the WPS, Philippine Rise, Sulu Sea, and other locations. In the WPS, researchers measured iodine-129 concentrations about 1.5 to 1.7 times higher than those observed at other sampling sites in the country. (Philstar.com)
A peer-reviewed study describing the dataset reports WPS iodine-129 concentrations in the range of 6.54 to 14.8 × 10⁶ atoms per kilogram of seawater, and notes the difference is statistically significant, suggesting an increasing trend relative to prior records. (ScienceDirect)
The work was carried out by scientists from the Department of Science and Technology – Philippine Nuclear Research Institute (DOST-PNRI), UP MSI’s Geological Oceanography Laboratory, and the University of Tokyo. (Philstar.com)
Why iodine-129 matters (and what it does not mean)
Iodine-129 is a long-lived radionuclide widely used by oceanographers and radioenvironmental scientists as a tracer of human nuclear activities—including historical weapons testing, nuclear accidents, and especially spent fuel reprocessing, which has been a major global source to the environment over decades. (ScienceDirect)
Importantly, detecting iodine-129 does not automatically indicate an active, nearby nuclear facility or a current release. In many ocean settings, iodine-129 behaves like a water-mass “fingerprint” that can be transported far from its original source and mixed through large-scale circulation.
Where it may be coming from
Based on the spatial pattern of the measurements and comparison with regional literature, the team traced the likely source region to the Yellow Sea. (Philstar.com)
UP MSI also pointed to the alignment of their findings with recent Chinese studies that link iodine-129 observed in the Yellow Sea to a combination of:
decades-old atmospheric nuclear weapons testing, and
iodine-129 that entered the environment from European nuclear fuel reprocessing, later moving into soils and rivers in northeastern China. (Philstar.com)
From there, researchers say iodine-129 could plausibly reach Philippine waters via ocean circulation systems, particularly the Yellow Sea Coastal Current and the Chinese Coastal Current, though they emphasize that dedicated oceanographic modeling is still needed to confirm the transport pathways and timing. (Philstar.com)
Is it dangerous?
Despite the “nuclear” label, the researchers stressed that the measured iodine-129 levels currently pose no threat to human health or the marine environment. (Philstar.com)
Why it’s a policy and monitoring issue
Scientists involved in the work argue the finding underscores a broader reality: radioactive tracers can cross borders through air and ocean circulation, meaning environmental baselines and monitoring systems need to be strong even in countries without nuclear power plants.
They called for strengthened monitoring and regulation of radioactive materials, particularly those that can move through shared seas and international current systems. (Philstar.com)
What comes next
Researchers say the next steps should include:
high-resolution ocean circulation modeling to test the proposed current pathways and estimate transit times; (Philstar.com)
expanded sampling across seasons and depths to separate local variability from basin-scale transport;
continued development of iodine-129 as a regional tracer to improve understanding of how the WPS connects to the broader Northwest Pacific system.
The research was funded by the DOST-National Research Council of the Philippines and the DOST-Philippine Council for Agriculture and Resources Research and Development. (Philstar.com)