Taken From Sky News British holidaymakers have been warned to avoid beaches in northern France because of toxic fumes from rotting seaweed. Rotting seaweed is cleared from Saint Efflam beach in Brittany One stretch of beach at Saint-Michel-en-Greve in Brittany has already been shut after a horse rider lost consciousness and his horse died. In another case, a council worker paid to clear beaches of the noxious plants was taken to hospital in a coma. Gilles Youenou, of the French Institute for Sea Research and Exploitation, said: "We would advise people to avoid the beaches in this area as the seaweed can give off hydrogen sulphide fumes. "This is all a result of a problem with agriculture. Nitrates have got into the water and polluted it. "A crust forms over stacks of rotting seaweed and when this crust is broken, fumes get out. It can be particularly lethal to dogs and other animals." There are about five beaches that are unusable. The problem is getting worse. French environmental group Eau et Rivieres Jean-Francois Piquot, a spokesman for the French environmental group Eau et Rivieres, told The Times that toxic seaweed has been present on beaches in Brittany for decades and is spreading. He said: "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. Pierre Philippe, of the Lannion hospital in Brittany, said hydrogen sulphide was as dangerous as cyanide. He said he had treated several cases of poisoning among local residents.