Hundreds of customers were evacuated from Grants Shopping Centre in Croydon after a chemical incident on Monday at 11am.
It is believed that an excess of chemicals in a gym toilet at the centre caused the incident.
200 people from the shopping centre and 40 people from Virgin Active Gym were evacuated.
Kevin Smith, manager of Croydon Fire Station, told the Croydon Guardian: “We think the incident was caused by some chemical products being used in the toilets.”
Nearly 50 fire fighters are still at the scene following an investigation into the problem by crewmembers wearing breathing apparatus.