You may have noticed it in the sky or on the snow in recent weeks: a cloud of sands from the Sahara crossed Europe and in particular France on two occasions, at the beginning of February and at the end of February.

These clouds caused an increase in fine particle pollution in several regions: Grand-Est, Corsica and the Alps at the beginning of February, then more recently Auvergne-Rhône-Alpes and Nouvelle-Aquitaine.

But it also caused a slight rise in a radioactive element, Cesium-137, according to measurements unveiled on March 4 by theInstitute for Radiation Protection and Nuclear Safety (IRSN).

???? Small comparison between the episode of February 6 and the current one. France should be a little more covered, with plumes going further north (as far as Scandinavia!).

???? #MOCAGE (Large Scale Atmospheric Chemistry Model) | CNRM (@Meteo France /@CNRS) pic.twitter.com/gN2M3ctp4R

– Météo-France (@meteofrance) February 21, 2021

A rate up to 11 times higher

In its information note, the public institute unveils the Cesium-137 (Cs-137) readings taken in early February by some of its air measurement stations scattered across France. “It is observed that the volume activities of Cs-137 in the air are, for several stations, higher by a factor of 1.2 to 11 compared to the average of the activities measured in February 2019 and 2020 “ details the institute.

The most significant increase was noted at the Pic du Midi station in Bigorre, in the Hautes-Pyrénées.

A “negligible” radiation

But do these rates pose risks to our health? IRSN would like to reassure: these levels are much lower than those measured during a similar episode that occurred in February 2004 and even more after the Fukushima (2011) and especially Chernobyl (1986) accidents. The impact of this episode on dosimetry, ie the amount of radiation received by an organism, “is negligible“, insists IRSN.

Cesium comes from… French bombs

But where does this radioactive element come from? “Like all the soils of the northern hemisphere, those of the Sahara are marked by the fallout from all the atmospheric nuclear tests carried out in the 1960s.“, recalls IRSN.

The great powers then routinely detonated atomic bombs in the atmosphere for experimental purposes. France in particular carried out four shots in the Sahara.

Following “conditions conducive to the formation of a desert dust plume“in the Sahara at the beginning of February, soil particles containing Cs-137 were found in suspension in the air, describes the IRSN. They then crossed the Mediterranean and were deposited in Europe.