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A mysterious radioactive cloud that swept through much of Europe this fall has officials baffled.

The cloud was harmless and has dissipated, France’s Institute for Radiological Protection and Nuclear Safety wrote in a statement Thursday, but between Sept. 27 and Oct. 13, the radioactive nuclide Ruthenium 106 was detected in “the majority of European countries.”

Experts aren’t totally sure where the substance came from, though testing suggests it was first released during the last week of September in Russia or Kazakhstan, likely somewhere between Russia’s Volga River and Ural Mountains.

“Russian authorities have said they are not aware of an accident on their territory,” IRSN Director Jean-Marc Peres told Reuters.

Based on atmospheric tests, the report says, it’s unlikely that the cloud was the result of a nuclear reactor accident. Instead, it likely stemmed from a nuclear fuel treatment or nuclear medicine treatment center.

Ruthenium 106 is produced when atoms divide in a nuclear reactor. It’s sometimes used in cancer treatment, specifically for melanoma of the eye, though it can be dangerous at high levels. The cloud drifting over Europe, however, never reached a concerning concentration, according to the IRSN report.

“The concentration levels of Ruthenium 106 in the air that have been recorded in Europe and especially in France are of no consequence for human health and for the environment,” it says.

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