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, is also raised. Below are details of the Agency’s work on this topic.
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Updated on 03/08/2016
Green algae, bathing and shellfish consumption
Identification of the hazards and potential exposures related to bathing and shellfish consumption in areas affected by green algae blooms
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 (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.
Alongside the issue of risks associated with these masses of washed up green algae for the health of exposed populations, the issue of risks associated with their presence when bathing or consuming shellfish has also been raised. In order to provide answers to the health authorities, ANSES produced a review of the available data on thehazards and potential exposures related to bathing and shellfish consumption in areas affected by green algae blooms.
The work of the Agency
An internal expert appraisal was conducted on the basis of the scientific literature and available monitoring data (in May 2011) in order to assess the relevance and feasibility of conducting such a risk assessment.
The main objective of this prospective study was to collect information on the micro-organisms and chemicals associated with the massive presence of macro-algae, in order to identify the potential hazards faced by bathers and consumers of shellfish harvested nearby.
The species involved in these so-called green tides in Brittany are overwhelmingly Ulva armoricana and Ulva rotundata. The algae in this genus, Ulva spp., belong to genera of macro-algae used in human food and referenced at European level as cosmetic ingredients.
With regard to the microbiological hazards, the literature indicates the presence of bacteria on macro-algae or in clusters of decaying algae, but without establishing a profile of the bacterial populations that develop there. The available data are scarce and are insufficient for estimating the level of concentration of pathogens in these various cases. In addition, there are few specific data for the French coast.
With regard to chemical hazards, data were collected on the algae’s composition regarding various metal trace elements, on contamination by persistent organic pollutants and pesticides, and on potentially toxic substances produced by certain algae (e.g. dimethyl sulfide, acrylic acid, dopamine). Again, there are few specific data for the French coast and none refer to any episodes of massive quantities of green algae washing up on beaches. It is therefore difficult to identify and characterise the chemical hazards associated with the occurrence of green tides in France.
From an epidemiological point of view, none of the publications analysed as part of this work that dealt with microbiological or chemical hazards related to an episode of proliferation of macro-algae, mentioned any symptoms in humansafter bathing or consumption of shellfish harvested in these areas (e.g.: foodborne infections, irritations, etc.).
Estimating the population’s exposure to the hazards identified proved impossible due to a lack of suitable data, especially on the concentrations of green algae when they are are washed up on the French coast. The limited data available for the French coast come from the monitoring of bathing water, monitoring of shellfish contamination, and reports of green tides.
Before any quantitative health risk assessment can take place, gathering epidemiological data could help identify some possible health signals. Where appropriate, acquiring the knowledge needed for qualifying the risks could be included in the scope of research work.