Last January, France’s Institute for Radiological Protection and Nuclear Safety (IRSN) detection stations began detecting the presence of radioactive iodine-131 in the air across Europe. While the concentrations of the radioactive substance are still well within accepted safety limits, its source remains a mystery, despite numerous investigations having been conducted into the event over the past month.

Iodine-131 contamination was first detected over northern Norway in mid-January, and over the course of the month the same substance was found in the air in Finland, Poland, Czech Republic, Germany, France and Spain. The highest concentrations were found in Poland, at 5.92 micro-becquerels per cubic meter — an amount that’s still within acceptable safety margins, but its presence is troubling, nonetheless.

Iodine-131 is an artificial, unstable isotope of natural iodine, typically produced as a byproduct of nuclear reactions. Early media reports blamed the spike on secret nuclear testing being carried out by Russia, however there has been no evidence of such activity, and the lack of other radioactive substances accompanying the I-131 rules out nuclear detonation as a source.

This leaves a medical or industrial accident as a potential source, but no-one has reported an incident involving the radioactive material, and current weather patterns across Europe have hampered attempts to trace the material back to a potential source. I-131 also has a short half-life, at approximately eight days, meaning its introduction into the environment was rather recent.

Strictly speaking, no amount of I-131 exposure is safe, but with lower concentrations comes a decreased likelihood that its beta-decay process will cause cellular damage. I-131 contamination was the chief health concern following the Chernobyl and Fukushima nuclear disasters, however too much time has elapsed since those events for them to be the current source of iodine-131.