Eight people were taken to hospital for “precautionary checks” after cleaning chemicals were incorrectly mixed at a health club.
Staff members and a family of four were taken to hospital from the David Lloyd club at Pride Park in Derby.
A spokesman for the club said it had reopened after the incident on Saturday morning, but the pool remained closed.
A Derbyshire fire service spokesman said workers at the pool area “had incorrectly mixed chemicals”.
“Seven people who were in the pool or the area, plus one maintenance worker were conveyed to hospital for precautionary checks,” the fire spokesman said.
A David Lloyd Clubs spokesman said: “We have very strict health and safety policies in place to ensure the safety of our members and team, and will be launching a full investigation to understand how this incident happened.”