A gaseous chlorine (Cl2) release occurred in the swimming pool at a person’s home subsequent to the mistaken pouring of 5 litres of a chlorinated substance into the installation’s maintenance circuit, normally reserved for a bromine by-product. The mobile chemical emergency squad evacuated the home’s occupants and ventilated the premises. The Cl2 concentration measured in the installation had reached 1 ppm, and a strong chlorine smell was noticeable inside the house; a safety perimeter was set up. Fire-fighters disassembled the filtration system and recovered a “solid aggregate” that they set aside in a bucket. The chemical waste was treated by a private firm. The emergency response team then poured sodium thiosulphate into the pipeline in order to neutralise the oxidising substances present and halt the chemical reaction.