France's PM Francois Fillon warned of the health risk from the weed while visiting the beaches in August.
A horse that had been ridden across the sands had already collapsed and died, and the animal’s rider had become unconscious.
Investigators from France's National Institute for Environmental Technology and Hazards (Ineris) discovered hydrogen sulphide on the beach in such high amounts that it could be "deadly in few minutes".
Sea lettuce is harmless in the sea, and is actually an edible seaweed, but as it rots on the beaches it releases the poisonous gas.
