Loftus Leisure Centre in Loftus, East Cleveland, was evacuated with fire crews, paramedics and police officers at the scene. Cleveland Fire Brigade advised the public to avoid the area which was cordoned off.
Pictures show firefighters in chemical suits entering the leisure centre which is run by EveryoneActive.
An eyewitness, who has not been named, told TeessideLive: “I came down to have a look. There was loads of fire and ambulance here.
“Firemen were in special suits. We then got told to move back. Someone said it was a chemical leak but I couldn’t smell anything.”
The North East Ambulance Service confirmed that paramedics were called to the centre at 3.10pm on Tuesday.
In a statement, it said 10 patients with breathing difficulties were treated with six of them taken to hospital.
Three of the patients were taken to James Cook University Hospital in Middlesbrough.
The rest were transferred to the University Hospital of North Tees in Stockton.
South Tees Hospitals NHS Foundation Trust, which runs James Cook University Hospital in Middlesbrough, confirmed that they treated patients from the incident.
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Firefighter Craig Strike told TeessideLive he believed it was a harmful substance which can be released by some chemicals used for cleaning swimming pools.
Centre operator Everyone Active declined to comment when asked to confirm it by TeessideLive.
Peter Smith, contract manager, said: “Loftus Leisure Centre was evacuated this afternoon due to a gas leak in the spa. The emergency services were called and we can confirm individuals were taken to hospital.
“The building remains closed and we are working closely with the emergency services while investigations are underway. We are unable to provide any further detail at this stage, but we will keep our customers informed accordingly.”
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Cleveland Police was called in by the ambulance and fire services with officers cordoning off the area.
A spokesperson from the force confirmed: “At 3.17pm police were contacted by ambulance and fire and attended to provide a cordon.”
Three fire engines and two ambulance incident response units were at the scene.
A statement on the leisure centre’s Facebook page said: “Customer notice! Due to unforeseen circumstances, the centre will be closed tonight which means there will be no swimming lessons.
“Sorry for any inconvenience.”