Sargassum seaweed: limit the exposure of residents and workers to hydrogen sulphide

Since August 2014, the French Caribbean and French Guiana have been experiencing successive waves of Sargassum seaweed washing up on their coastlines. Despite the efforts made to clean it up, the seaweed decomposes in situ. This leads to the production of hydrogen sulphide (H2S), which can sometimes be detected at high concentrations. Doctors' reports concerning the health effects suffered by people exposed to H2S, and complaints from the general public relating to the problem of odours, have increased sharply. Following a request from the Ministries of Health, the Environment and Labour, in March 2016 ANSES published an initial expert appraisal on the emissions from decomposing Sargassum seaweed. In 2017, this expert appraisal was supplemented by a revision of the toxicological profile of H2S and a summary of the ecology, accumulation, chemistry and decomposition of Sargassum seaweed.

Toxicity of hydrogen sulphide

Individuals are mainly exposed to H2S by the respiratory route. Absorption via the oral and dermal routes is possible, but only makes a small contribution to overall exposure.

While the effects in humans related to acute exposure (i.e. for a short period of time) are well documented (neurological and respiratory effects whose severity increases with the concentration of exposure), less is known about the effects related to H2S exposure over longer periods. The effects observed first are irritation to the upper airways and eyes. Neurobehavioural effects and neurological symptoms (headache, loss of balance and memory) are suspected. Moreover, in the current state of knowledge and in light of the few studies available, no conclusions can be drawn as to the carcinogenic potential of H2S. The strong unpleasant odour associated with this gas should also be emphasised.

In light of the data available, the Agency believes that its recommendations on prevention, made during an earlier expert appraisal concerning workers in contact with decomposing green algae on the coasts of Brittany, should be considered for workers in contact with Sargassum seaweed.

In addition, the expert appraisal published in 2017 revealed that Sargassum seaweed has a strong ability to trap and accumulate heavy metals, particularly arsenic and cadmium, which may present a risk to human health and the environment.

ANSES’s recommendations

In its "OPINION on gaseous fumes emitted by decaying Sargassum seaweed in the Caribbean and in French Guiana (PDF)" of March 2017 (in French), the Agency stated:

"[The Agency] stresses that there are currently enough observations and knowledge of the emissions/health risks associated with the decomposition of Sargassum seaweed washed up on the shore to justify implementing preventive measures right away. [It] believes that no assessment of the health risks associated with exposure of the inhabitants of the French départements in the Americas to H2S in the air resulting from the decomposition of the seaweed should be conducted before implementing these preventive measures.

Therefore, considering the :

  • measurements of H2S concentrations already carried out in the ambient air in the vicinity of the areas where the seaweed is washing up;
  • data on the chronic toxicity of H2S, in particular for the upper airways.

it appears that in the presence of washed up seaweed, part of the population is already exposed to H2S concentrations above the existing chronic TRVs proposed by the US EPA (1.43 ppb - 2 µg·m-3, 2003) and the OEHHA (7.14 ppb - 10 µg·m-3, 2000).

Moreover, an increase in the number of medical consultations related to the effects suffered by the population of Martinique exposed chronically to H2S has been reported by the network of sentinel physicians.

This therefore justifies the immediate implementation of preventive measures in the presence of washed up seaweed."

General recommendations:

  • implement the regular, systematic collection of seaweed washed up on the shore;
  • clearly mark the seaweed collection zones and restrict access to operators;
  • inform the population of the health risks associated with exposure to H2S, especially in the vicinity of the beaches where the seaweed is decomposing. The population also needs to be informed that they must not handle the seaweed.

Recommendations for workers:

During seaweed collection, transport and processing operations, ANSES recommends that:

  • each worker, including those inside machine cabins, be equipped with a portable H2S detector, sited near the respiratory tract;
  • workers wear personal protective equipment, in particular gloves, boots and gas filter half-masks, or hoods with forced ventilation when the H2S concentration exceeds 10 ppm;
  • mechanical collection be prioritised, while considering the environmental constraints;
  • worker training and information be provided regularly;
  • traceability be established for work involving exposure.

In addition, following its expert appraisal published in 2017, the Agency recommended prohibiting the possible use of this seaweed for human food or animal feed, pending the completion of more extensive studies on seaweed contamination by heavy metals. 

The Agency also recommends continuing research on:

  • exposure associated with situations where Sargassum seaweed has accumulated, and the effects on human health;
  • the toxicity of H2S and more particularly on the effects of chronic exposure to low doses of H2S;
  • the indirect environmental and health impacts associated with the washing up of Sargassum seaweed (composition of the seaweed, presence of heavy metals);
  • the proliferation and phenomenon of washed up seaweed in the French départements in the Americas.

Reminder of the recommendations made by the French High Council for Public Health (HCSP) in its Opinion of 3 September 2015:

  • for H2S concentrations between 0.2 and 1 ppm on beaches in the vicinity of washed-up seaweed: public information;
  • for values between 1 and 5 ppm on beaches: public information, access not recommended for susceptible or vulnerable individuals;
  • for values above 5 ppm on beaches: access limited to professionals equipped with individual H2S measuring equipment with alarms.