Green algae alerts in Brittany: what health risks?

If you are currently spending your holidays in the bay of Saint-Brieuc, be careful. On Wednesday July 20, the Côtes d’Armor prefecture announced that the alert threshold for exposure to hydrogen sulphide had been exceeded in the town of Hillion, on the Saint-Guimond site. This potentially deadly gas emanates from the putrefaction of green algae, present in very large quantities on the Breton coasts.

A new alert

If these algae are naturally present at sea, they began to spread on the coasts of Brittany forty years ago. Their proliferation is explained in particular by the addition of nitrates spread in the fields, and whose surpluses are transported by rivers, to the beaches. And the phenomenon is far from diminishing: since the installation, at the beginning of July, of twelve hydrogen sulphide sensors in seven Breton bays (from Finistère to Côtes-d’Armor), this is the first time that a alert of this type is triggered.

“The surveys carried out by the association Air Breizh reached 1.565 ppm (particles per million) on 20/07/2022 at 03:45″, indicates the prefecture, which recalls that the High Council for Public Health has set the alert threshold at 1 ppm. Saint-Guimond beach is therefore closed to the public until further notice and new readings are taken twice a day to follow the evolution of the situation.

A risk of fatal poisoning

When they are at sea, green algae do not pose a danger to humans. But when they accumulate in large quantities on the beaches, they ferment in 24 to 48 hours and emit a toxic gas, hydrogen sulfide, recognizable by its smell of sulfur or rotten eggs. Walking on rotting piles can suddenly release hydrogen sulfide, which is a real health risk.

In humans, as in animals, this gas can cause intoxication ranging from simple embarrassmente (eye and nose irritation) to serious discomfort, or even cardiac arrest, in the most severe cases. The degree of severity actually depends on the amount of gas released and the proximity of the source, especially for animals and children, closer to the ground.

Note that the further away you are from the source of the fumes, the lower the risk, but the smell of rotten eggs is also a nuisance for residents.

Reminder of safety instructions:

  • Do not approach collection sites within 50 meters.
  • Do not drive in areas where there are reported algae deposits.
  • Do not come into contact with algal clumps in case of heavy accumulation on beaches, rocks or in muddy areas.
  • Do not walk your animals near green algae grounding areas.

What to do in case of suspicious symptoms?

If you have ventured near infested coasts and you have suspicious symptoms (irritated eyes or throat, watery eyes, headaches, breathing difficulties, cough, itching, etc.), quickly move away from the said area, and contact the poison control and toxicovigilance center specifying the location of the exhibition.

If you have severe symptoms, such as malaise, significant difficulty breathing, chest pain, etc., call the emergency services immediately (15, 18 or 112).

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