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Sheet 12: Health impacts


Closed Schools due to Sargassum events

In May 2018, the mayor of the seafront commune of Petit-Bourg decreed the unprecedented measure to close nursery, middle, elementary and high schools. Within the 8 closed establishments, some 2,000 students were deprived of class for 10 days.

The day before, the ARS (Regional Health Agency) published a statement warning about the high level of Hydrogen Sulfide in the air (6 ppm) due to Sargassum fermentation on surrounding beaches. A few days earlier, 3 schools had been subjected to the same decree in the commune of Goyave, in Guadeloupe still, while the mayor launched a massive plan to combat Sargassum. It included the relocation of the school of Christophe’s classes to two other establishments inland, in order to get the students out of the first line of exposure to gas emanations. The same year, in the town of Robert in Martinique, all schools remained closed again following the decision of the municipality for 4 days.

The French territories of the Caribbean are not the only ones to close schools since in Barbados, Mexico or Saint Lucia, excessive Sargassum events also forced local authorities to take similar mesures.

Following this succession of closures, the Regional Agency of Health installed sensors in all the French Islands of the Caribbean, which read the daily rates of Hydrogen Sulfide present in the air on the coasts.

Appareil de mesure du gaz H2S Martinique

What is Hydrogen Sulfide and how can it be measured?

This neurotoxic and corrosive gas released by Sargassum fermentation on beaches is indeed dangerous for humans when present in the air in large quantities. It has a very characteristic smell of rotten egg, and can be deadly when in very high doses. It is used in the chemical industry to manufacture Sulfuric Acid, as well as to produce heavy water used to eliminate impurities of certain ores in the nuclear and metallurgical industries.

Gas is measured in ppm (parts per million) which indicates the level of Hydrogen Sulfide particles in one million air particles. The higher the number of Hydrogen Sulfide particles, the more present and potentially dangerous. Released by algae, our olfactory system is able to smell it even in small quantities (from 0.02 ppm).

Pièce de monnaie oxydée

What are the human risks associated with exposure to the gas ?

The first health issues appear around 14 ppm of Hydrogen Sulfide and begin with an irritation of the mucous membranes of the eyes (conjunctivitis) and of the respiratory system (rhinitis or bronchitis). The first preventive measures must then be taken, by closing schools and recommending vulnerable people (pregnant women, infants or people over 65 years old, people with asthma or diabetes) to avoid affected areas.

In 2018, in Capesterre de Marie-Galante in Guadeloupe, 35 families heavily exposed to the fermentation gasses of Sargassum were waiting to be moved, while the island was cut off from the world due to the intensity of the Sargassum invasion. As boats could no longer reach them, an emergency helicopter was finally sent.

If humans are however exposed to higher levels of it for a long time, severe symptoms can be observed : nausea, pulmonary edema, convulsions and coma. They do not necessarily lead to death, but in the worst cases there can be sequelae, including neurological disorders.

In January 2023, a woman who worked unprotected collecting Sargassum was hospitalized at the University Hospital Center of Martinique (CHUM) after acute hydrogen sulphide poisoning. She presented several symptoms (respiratory discomfort, loss of consciousness and vomiting) and was placed on oxygen for almost 12 hours. An elevated amount of thiosulfate (hydrogen sulfide metabolism) was found in his blood and urine.

In the event of critical measurements of Hydrogen Sulfide levels on the coastline, local residents will have to evacuate in order to preserve their health.

Chronic toxicity can also impact the nervous system and provoque fatigue, insomnia, loss of libido and memory problems. Women with long-term exposure have higher risks of spontaneous abortions or premature deliveries.

In Martinique, the team of Professor Dabor Résière from the toxicology department of the CHUM carries out consultations in the communes of François and Robert to assess these cases of chronic poisoning. In these towns on the Atlantic coast, 60% of patients examined presented with chronic hydrogen sulphide poisoning. An assessment made with the use of two measuring devices (operating like a breathalyzer): the Spirometer (measurement of ventilatory flow, or lung capacity) and the Niox (measurement of exhaled nitric oxide).

Several cases of COPD (chronic obstructive pulmonary disease) are also to be noted throughout the Caribbean, like in Guadeloupe, St. Vincent the Grenadines and Cuba. The disease linked in 80% of cases with smoking, is characterized by coughing and shortness of breath. These symptoms are regularly underestimated and may require hospitalization if they worsen.


Risks for the animals

Humans are not the only ones affected by the risks associated with exposure to Sargassum decomposing gas. Animals can also be affected with much more direct and devastating effects, as the gas attacks their central nervous system directly. Many death cases of cats and dogs were reported , or animals suffering from cardiac rhythm and respiratory disorders throughout the Caribbean, especially in 2018 (a record year before 2021 in terms of quantities of stranded algae). It is assumed that these cases are related to hydrogen sulphide fumes, but no field study can corroborate this.

According to the results of laboratory studies, hydrogen Sulfide also produces cellular damage in animal cornea, their cerebral cortex, liver and lungs. The systemic effects in animals are therefore quicker and more severe. An exposed dog will first be peculiarly excited, its heart rate will accelerate from the first minutes of inhalation, before the appearance of tremors. Its heart rate will then slow down significantly, leading to respiratory and cardiac arrest.

For rabbits, a ten-minute exposure to 400 ppm causes tracheal cells and ciliary movements to stop functioning. For other rodents, the liver and kidneys will quickly be affected and cease to function. In these cases we can speak of chronic effects since they do not survive moderate exposures.


Guadeloupe la 1ère
Martinique la 1ère
Agence Régionale de Santé – Guadeloupe
Agence Régionale de Santé – Martinique

Science et vie
Université des Antilles
Haute Autorité de Santé