There is, of course, green algae, the large-scale stranding of which has dramatically increased and intensified on the Breton coast in this instance, but there are also all sorts of other algae that are just as 'intriguing'.
When these organic materials decompose (anaerobic fermentation), they release a gas that is easily recognisable (at very low concentrations) by its "rotten egg" smell, which disappears at higher concentrations. This is hydrogen sulphide (H2S).
Hydrogen sulphide is considered a toxic and dangerous gas. It is a colourless gas, heavier than air. Hydrogen sulphide is a stable compound at normal temperatures, but at very high temperatures it can dissociate and eventually burn in the air, creating toxic fumes of sulphur oxides. Prolonged exposure to and inhalation of this gas can have serious health consequences. Indeed, if the gas concentration reaches or exceeds 1000 ppm by volume, it can result in harm to the central nervous system, with loss of consciousness and symptoms of respiratory distress. These are the main risks for people's health if they are exposed to this gas. If the concentration of hydrogen sulphide is less than 1,000 ppm by volume, the main health effects will be irritation of the mucous membranes of the eyes and respiratory tract, conjunctivitis, rhinitis and dyspnoea, or degeneration of the olfactory nerve (making it impossible to detect the gas).
There are devices for detecting the presence of gas, in this case hydrogen sulphide, in the air in a given area. The safest way to prevent hydrogen sulphide is to use a portable H2S gas detector (single-gas detector like the Senko SGT disposable gas detector or multi-gas such as the portable 4-gas detector), which will indicate the presence of hydrogen sulphide in the atmosphere. The detector will also indicate the instantaneous concentration of the gas, as well as the TWA (Mean Exposure Limit) and the STEL (Threshold Limit Value). If a limit value is exceeded, the gas detector will give an audible signal (up to 95 dBA) and a light signal.
If H2S is detected, you will need to take the necessary precautions and use appropriate protection, such as a respiratory protection mask with a type B filter cartridge (ideally B2), to guarantee optimum safety. You can also use a half-mask (anti-gas), but hydrogen sulphide is still quite irritating for the eyes, and a full mask will protect them better than a half-mask...
So, even on holidays and on the beach, stay vigilant …
Have a good swim!