On July 28, 2009 Vincent Petit was riding his horse on a beach at Saint-Michel-en-Greve in western France. The beach was covered with an odd and smelly slime. The horse lost its footing and fell to the ground. The horse died immediately while the unconscious rider was rescued by nearby onlookers. The fall did not kill the horse; the fumes from the slime did. Other residents have been treated at hospitals after breathing the contaminated air.

French officials ordered tests on the slime and discovered extremely high levels of hydrogen sulfide (H2S), commonly referred to as sewer gas. The gas is being emitted by large amounts of rotting algae that has covered beaches in northwestern France.

Some experts blame the phenomenon on local farmers that use nitrate fertilizers. The rain run-off from heavily fertilized farmland is polluting the local waters and causing excessive algae deposits to appear. Others place the blame on local pig farmers and the nitrate pollution caused by swine waste draining into waterways.

The French environmental agency, INERIS, took measurements of the H2S gas and found levels to be dangerous and have closed several affected beaches. H2S gas concentrations measured 1,000 parts per million (PPM), a level that can be deadly in a matter of minutes.

Workers have been dispatched to clear away the rotting algae and are equipped with H2S detectors to monitor gas levels in real time. H2S gas is released when organic matter decomposes; a totally natural and harmless event under normal conditions. The problem on France’s beaches is the huge amount of decomposing algae and its proximity to human activity. More than 80 communities have been affected by the rotting organic matter and this has had a negative impact on tourism as well as the safety of local residents.

Experts report that when seaweed is deposited on beaches in large quantities and decomposition begins, a hard white crust forms on the surface. When the crust is broken, hydrogen sulfide gas (H2S) is released into the air. Some scientists believe that high levels of H2S gas destroyed the dinosaurs 300 million years ago.

Nitrate pollution causes algae to grow at an incredible rate. When nitrates are released into water and the sun shines brightly on the water, algae growth is nearly uncontrollable. The beach where Vincent Petit and his horse met disaster was reported to be covered by as much as 1 meter (3 feet) of rotting algae.