TRUE OR FALSE. Is the sulfur cloud linked to the eruption of a volcano in Iceland dangerous for health in France?

While eruptions can cause air pollution, emissions from the one that began Thursday are not significant enough and are circulating at too high an altitude to be dangerous, physicists say.

A slight smell of sulfur in the air. A gas cloud has been crossing France since Sunday, August 25, and should disperse on Tuesday, according to a model from the European Center for Medium-Range Weather Forecasts. This outgassing coming from the Icelandic volcano Sundhnuksgigarod, erupting since Thursday, is particularly loaded with sulfur dioxide.

Should we be worried about this cloud? In a widely shared post on X, business leader Silvano Trotta, known for being a disinformation agent, draws a parallel with the Chernobyl cloud scandal, whose danger was downplayed by French authorities in 1986. Franceinfo takes stock with physics researchers.

When a volcano erupts, it can spew lava, but also plumes containing ash, or gases such as carbon dioxide (CO2), water vapor or sulfur dioxide (SO2). It is the latter that appears on the map showing a red cloud crossing the hexagon. “SO2 is harmful to health”confirms Hervé Herbin, teacher-researcher in physics at the atmospheric optics laboratory of the CNRS and the University of Lille. “It is irritating to the skin, eyes and respiratory tract. It can be problematic, even affecting the entire respiratory system, right down to the lungs.”explains the man who studies in particular the effects of volcanic eruptions on the atmosphere.

However, there is no need to worry about the effects of the latest eruption of the Sundhnuksgigarod volcano. “These SO2 levels remain limited and do not represent a danger to health, especially since the cloud tends to dilute”assures meteorologist Guillaume Séchet on X.

This is confirmed by Marie Boichu, also a researcher at the atmospheric optics laboratory of the CNRS and the University of Lille. “For there to be pollution, there must be particles on the ground”that is to say in the air which circulates at human height, she explains. “However, we measure the air quality on the ground and we do not see any anomaly on the surface. The cloud is rather located at high altitude, about 5 km from the ground”.

Moreover, “This gas has a very short lifespan, of the order of a day”adds his colleague Hervé Herbin. “According to satellite data, when it arrives in France, it falls back in a diluted form. This can contribute to a deterioration of the air, but it is not the main factor of pollution.”he decides.

“On a global scale, volcanic eruptions occur all the time. They represent 10 to 15% of SO2 emissions, while 90% come from industry.”

Herve Herbin

teacher-researcher in physics at the CNRS and at the University of Lille

Since the eruption in Iceland, the levels of sulfur dioxide observed in the air are far from the reference threshold established by the World Health Organization, which considers its concentration to be alarming above 40 μg/m3 over 24 hours. “There have to be hundreds of micrograms per cubic meter for several hours for it to be dangerous, especially for babies and people with asthma.insists Marie Boichu. Here, we are at a few micrograms per cubic meter, and it is not certain that this concentration is of volcanic origin.”

“It’s a false alarm.”

Marie Boichu

teacher-researcher in physics at the CNRS and at the University of Lille

Generally speaking, France does not have much to fear from SO2 emissions linked to volcanic activity. The volcanic regions closest to France are Italy, where Etna erupted in early August, and Iceland. “The winds rarely bring plumes back towards France and when this happens, the concentrations are not high enough” to be harmful, reassures physicist Hervé Herbin.

According to the researcher, the most worrying danger linked to sulfur dioxide is that for the environment, due to emissions of human origin. “SO2 dissolves very easily in water and in air humidity, which causes acid rain. This can cause problems locally, in the context of agriculture”he notes.

“We remain attentive in volcanology, because we have already had very significant Icelandic eruptionsMarie Boichu nevertheless recognizes. There was the Laki disaster in 1783, just before the French Revolution, causing excess mortality linked to sulfur dioxide emissions..” More recently, “We were also able to observe an episode of air pollution in Reunion, linked to the Piton de la Fournaise volcano”adds the physicist. In July 2023, the alert thresholds for sulfur dioxide pollution had been exceeded at the Bourg-Murat station.

Especially since the SO2 level is not the only indicator of pollution to take into account. When in contact with air, gaseous sulfur dioxide can transform into sulfate aerosols, ““fine and persistent particles”as recalled by a CNRS study led by Marie Boichu, on the subject of the eruption of the Icelandic volcano Bardarbunga between September 2014 and February 2015. This had led to “vast air pollution” to France, in particular linked to these particles capable, due to their small size, of “penetrate deep into the lungs and have adverse health consequences.” But the volcano had then released a quantity of sulfur significantly greater than that linked to Thursday’s eruption, and had done so “at low altitude in the atmosphere”while the current cloud is circulating much higher.

The most publicized Icelandic eruption, that of the Icelandic volcano Eyjafjallajökull in 2010 which paralyzed European air traffic, did not cause significant sulfur dioxide pollution. But around 70 to 80 million cubic meters of tephra, the rock fragments ejected during a volcanic eruption, were released into the atmosphere, according to an estimate by the French Bureau of Geological and Mining Research. However, “the level of concentrations [de particules] on the ground did not exceed a few dozen μg/m3, which is within a range of usual values ​​characteristic of an episode of urban pollution”according to a note from the Institute for Radiological Protection and Nuclear Safety.