Nine people were taken to hospital after a cloud of chlorine gas spread through a gym in west London.
Seventy people fled Gold Gym in Hanwell, west London at 8.30pm on Monday before firefighters arrived to cordon off the entire high street.
People were seen wrapped in silver foil outside the gym as firefighters in hazmat suits worked to clean up the chemical spillage inside a plant room.
Nine people were treated at the scene by paramedics before being taken to hospital for further treatment.
London Fire Brigade said unsafe chemical fumes were most likely “caused by a mixture of chemicals which produced a chlorine gas”.
Firefighters moved into help ventilate the gym after high readings of chlorine gas were detected in a sweep of the building. A Gold Gym spokesman said: “Everybody’s fine.”
A London Fire Brigade spokesperson said: “Most cleaning chemicals are either acid or alkali based – and if they were to be mixed, there is a very real risk that there will be a chemical reaction.
“This reaction can often release a harmful, irritable or even toxic gas which can have an incredibly strong smell and have a harmful effect on anyone breathing in these gases, especially the young and the elderly.
“Inhaling this gas can cause breathing difficulties as well as choking and irritation of the airway. The gas can also cause eyes to water and, in stronger concentrations, can cause loss of consciousness.”