Nine children and two adults have been taken to hospital following a reported chlorine leak at a swimming pool in west London, the London Ambulance Service (LAS) said.
Crews were called to the Vale Farm Sports Centre on Watford Road, Sudbury, at 13:34 BST.
The 11 people were treated at the scene before all being taken to hospital, a LAS spokesperson said.
BBC News understands that while most of the patients are not seriously injured, three of the children may require more investigation.
A London Fire Brigade (LFB) spokesperson said its crews “carried out a sweep of the building where a chlorine leak was found in a learner pool on the ground floor of the three-storey building”.
“Around 150 people left the building before the arrival of the brigade,” they added.
The LAS also sent its hazardous area response team to the swimming pool.
A Metropolitan Police spokesperson said the leisure centre had been “evacuated as a precaution” and roads had been closed.
Chris Williams, the area manager for Everyone Active, which operates the centre, said: “Safety is our top priority and we are taking this incident very seriously.”
“We are conducting a full investigation into the cause and will implement necessary measures to prevent similar occurrences in future.”
He added that the centre had reopened at 16:00 BST but the pool is still closed.
Councillor Neil Nerva, the cabinet member for community health and wellbeing at Brent Council, said she is “seeking reassurance” from Everyone Active and the pool will remain closed “until further notice”.
What is chlorine and is it dangerous?
According to Public Health England (PHE) guidance, chlorine is part of the group of chemicals called halogens, which include fluorine, bromine and iodine.
In its liquid form it is used to treat water supplies and swimming pools to prevent the transmission of waterborne diseases.
Exposure to liquid or gaseous chlorine may cause irritation or burns to the skin and eyes, says PHE.
Children are expected to be affected by chlorine in the same way as adults.