The Nantes appeals court found France liable for the death of Jean-Rene Auffray, citing its "negligence" in enforcing environmental regulations to protect its waters and prevent toxic green algae blooms.

For more than five decades, tons of green algae have washed up annually on Brittany's beaches in western France, releasing hydrogen sulphide gas as it rots - a toxin that can prove deadly in high concentrations.

Auffray, a keen trail runner, died of sudden respiratory failure while jogging in the algae-choked Gouessant estuary near the city of Saint-Brieuc.

His family sued over his death, but in 2022 a court rejected the claim, ruling there was insufficient evidence to link the toxic algae to Auffray's death.

Tuesday's decision reversed that ruling, marking the first time France has been held liable for the health risks posed by deadly green algae blooms along its coast.

The court "holds the State liable for negligence due to its failure to implement European and national regulations designed to protect waters from agricultural pollution", which is "the main cause of the proliferation of green algae in Brittany", it explained in a statement.

The court found Auffray's death was caused by a rapid pulmonary oedema -- a condition in which there is too much fluid in the lungs -- which could only be explained by fatal poisoning from inhaling hydrogen sulphide at very high concentrations.

"For the first time, a French court has recognised the link between a person's death and the state's negligence in these green algae cases," said the family's lawyer, Francois Lafforgue.

The deceased's family will receive partial compensation, as the court found the state 60 percent liable, noting that he had taken a personal risk by jogging in the estuary.

The state has been ordered to pay 277,343 euros ($321,750) to the jogger's wife, 15,000 euros to each of his three children, and 9,000 euros to his brother.

According to a 2021 report from France's top audit court, around 90 percent of Brittany's algae blooms are caused by agriculture, where the use of nitrogen fertilisers has increased significantly since the 1960s.

Intensive farming - especially pig farming - and the resulting nitrate pollution have been linked to the spread of green algae in the western Cotes-d'Armor region.