A major incident took place at the Moorways Sports Village on Moor Lane, Derby.
Fire engines, police and ambulance vehicles were at the scene.
The building, a Derby City Council-owned facility run by private operator Everyone Active, was evacuated with a cordon in place and swimmers were wrapped in foil outside.
Derbyshire Fire and Rescue Service was called to the site at 1.38pm following reports of a chemical leak. A huge emergency operation is in place. Moor Lane was closed.
One person was seen taken to an ambulance on a stretcher.
It is not known when the leisure centre will reopen.
A woman wrapped in foil said dozens were evacuated from the building while swimming at around 1.30pm.
Firefighters from the city and Stapleford in Nottinghamshire remain on site alongside a specialist environmental protection unit from Nottingham, alongside East Midlands Ambulance Service and police.
The brigade said 15 people had been taken to hospital “for precautionary checks”.
The brigade initially said “around 12 people” were being assessed by paramedics after experiencing breathing difficulties.
A Derby woman said: “I’m not sure what’s happened but I was swimming. the alarm went off and we were evacuated. It looks serious but I don’t anything. I just want to go home but my car keys and clothes are inside. I’m not sure when we can go back in.”
Nicola Caine, from Bradford, has come on holiday to Derby.
She said: “Kids are on holiday so the pool was very busy.
“We were in the pool for an hour or so and then got whistled out.
“We don’t know why but there are lots of checks going on. The emergency services have been great and made sure we are ok. Nobody is telling us anything so not sure when we get back.”
There seems to be a relaxed atmosphere despite the array of emergency vehicles at the scene.
People wrapped in foil are smiling and chatting with loved ones.
There appears to be a few children who have been evacuated.
Moorways said on Friday 31 July
‘We are temporarily closed and will provide an update in due course.’
