Green algae

Assessment of the risks associated with green algae

Every summer for more than 30 years, massive quantities of green algae have been washing up on parts of the French coastline. Once washed up on beaches, these algae deposits decompose and produce large quantities of gases, especially hydrogen sulfide (H2S), potentially exposing walkers, local residents and workers required to collect these algae, to risks. The issue of the risks associated with these algae when bathing or consuming shellfish, as well as with regard to animal health, is also raised. 

Episodes of massive proliferation of macro-algae on the coasts and beaches of Europe have been reported in the scientific literature as early as 1905 on the Irish coast. This phenomenon seems to have increased over the past forty years. The proliferation of these algae is mainly linked to the presence of nitrates in the water generated by human activities (particularly agriculture), and the shape of the coastline (many bays).

Once washed up on the beaches, these massive deposits of algae decompose. During the decomposition process, large quantities of gases are released, including hydrogen sulfide (H2S) which may cause disagreeable smells and health problems for walkers and local residents. To remedy this situation, the algae are removed from some beaches, and are then processed on the land. These various steps expose workers to these gases.

To try to curb this phenomenon, in February 2010, the government developed a five-year plan to control the green algae. It has three components: 

  • Actions to limit the flow of nitrogen towards the coast; 
  • Improved collection, and development of treatment capacity for beached washed up algae;
  • Improved knowledge and risk management. 


In this context, the Agency received several formal requests and conducted expert appraisals focusing on three aspects of this problem:

  • assessment of the risks associated with gas emissions by green algae to the health of local residents, walkers and workers; 
  • identification of hazards and potential exposures related to swimming and shellfish consumption in areas affected by green algae blooms:
  • assessment of the toxicity of hydrogen sulfide (H2S) in animals and identification of available toxicity values in the literature on this subject.