Swimmers have been taken to hospital after being exposed to "chemical fumes" that left them "struggling to breath".
The gym-goers fell seriously ill while in the infinity pool at Athelis Club & Spa, in Warrington, at around 3pm on Thursday, 1 September.
Emergency services rushed to the scene to find 11 people struggling to breath, with staff suspecting there had been a chlorine leak.
The building was evacuated and nine people were checked over by ambulance staff, two of which were taken to Warrington Hospital. Their condition is described as "minor".
Cheshire Fire & Rescue Service believe the treatment of the water in the pool could have caused the swimmers to fall ill.
A spokesperson said: "Crews were called after reports of a number of people becoming unwell as a result of exposure to chemical fumes in the swimming pool area.
"Wearing protective equipment as a precaution, they assisted staff in the evacuation of members.
"The cause is believed to be related to treatment of water in the pool."
Athelis Club & Spa say the rooftop pool had a "technical issue" and it would re-open to gym members on Friday, 2 September.
A spokesperson said: "Following a technical issue with our rooftop infinity spa this afternoon, we followed our standard health and safety procedures and closed access to the spa to carry out necessary checks and repairs."
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