Vacationers have been told to keep away from beaches in northern France covered in seaweed after doctors gave warning that it could give off lethal fumes when it rots.

A stretch of beach had to be closed after a horse rider lost consciousness as a result of the putrefying algae. His horse was killed. Local residents have also been treated in hospital.

The incident was in Brittany, where green seaweed is spreading across the region’s beaches as nitrates pollute the water supply as a result of intensive agriculture.

Scientists said that as the seaweed — known locally as sea lettuce — decomposes, it forms an impermeable white crust under which hydrogen sulphide accumulates. When the crust is broken, the gas is released.

Alain Menesguen, director of research at the French Institute for Sea Research and Exploitation, said: “This is a very toxic gas, which smells like rotten eggs. It attacks the respiratory system and can kill a man or an animal in minutes.” Some scientists believe that a build-up of hydrogen sulphide in the atmosphere wiped out the dinosaurs 300 million years ago.

Pierre Philippe, of the Lannion hospital in Brittany, said that hydrogen sulphide was as dangerous as cyanide. He said that he had treated several cases of poisoning caused by the seaweed among local residents, including a council worker paid to clear beaches of the algae who was taken to hospital in a coma.

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