Holidaymakers 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 say 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.

The health scare is a new blow to the French tourism industry, already suffering from a big fall in the number of British visitors.

The dangers were highlighted after Vincent Petit, 27, a veterinary surgeon from Paris, said that rotting seaweed a metre deep had killed his horse last week as he rode across St-Michel-en-Grève beach. Mr Petit lost consciousness and was pulled off the beach. A post-mortem on the horse showed that it had died of pulmonary oedema caused by inhaling hydrogen sulphide given off by the rotting seaweed.

Jean-François Piquot, a spokesman for the environmental group Eau et Rivières, said that toxic seaweed had been present on beaches in Brittany for decades and was spreading. “There are about five beaches that are unusable. The problem is getting worse.” Up to 70,000 cubic metres of seaweed is cleared off about 70 beaches every summer in Brittany, according to Eau et Rivières.

“There is no doubt that farming is to blame,” said Mr Piquot. “Britanny has 5 per cent of French agricultural land but 60 per cent of the pigs, 45 per cent of the poultry and 30 per cent of the dairy farms. As our rivers are not long, the pollution does not have time to clear before the water reaches the sea. If it enters a closed bay and there is sunlight, that produces the seaweed.”

There was further dismal news for French tourist industry leaders when a beach in Villefranche-sur-Mer on the Mediterranean coast was closed last week because of toxic seaweed — this time blamed on global warming.

In Marseilles, 13 beaches were shut at the weekend after heavy rainfall washed the contents of the city’s sewer into the sea.

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