A stretch of beach near Lannion in Brittany has been closed because of concerns that large amounts of rotting seaweed could prove to be a fatal health hazard.

The seaweed Ulva lactuca more commonly known as sea lettuce is growing abnormally fast in the region due to heavy nitrate pollution caused by intensive farming practices. The nitrates leach out of the soil and wash down in to the sea through local rivers. Wave action then washes the seaweed on to the beach in large amounts up to a metre deep in so called 'green tides' where it starts to decompose, during the decomposition process highly toxic hydrogen sulphide gas is formed. Because of its the gooey nature a thick crust forms on the top of the rotting seaweed causing the gas to accumulate and become trapped inside the rotting mounds. When the crust is broken by stepping on it the trapped gas is released.

Local mayor René Ropartz took the decision to close the beach at Trédrez-Locquémeau when a rider lost consciousness and his horse was killed after riding across the rotting sea weed last week. A council worker is also being treated in a hospital at Lannion after collapsing and falling in to a coma while clearing the deadly weed.

Earlier this year several dogs were killed after walking over the rotting seaweed, dogs and small children are particularly susceptible to the gas as they are lower to the ground and more likely to inhale larger amounts. The problem is not isolated to the beach at Trédrez, other beaches along the western coast of France are from time to time affected by the seaweed, beach goers and dog walkers should take care to avoid mounds of rotting sea weed. It should also be noted that in some areas with strong tidal movement or after storms a thin layer of sand can be washed over the rotting seaweed concealing it from view.

Hydrogen sulphide gas has a distinct and unpleasant smell rather like rotten eggs, it is highly flammable and extremely toxic. Due to its nature it is heavier than air and so has a tendency to accumulate in a higher concentration close to the ground. Although the smell is highly pungent it's deadly properties can quickly eliminate the victims sense of smell until it is too late.

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