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Ashford's Stour Centre was evacuated after a chlorine chemical leak, but has now re-opened.
Fire crews were called to the leisure centre in Tannery Lane after six people were hurt and taken to the William Harvey Hospital after breathing in the chemical.
A fire service spokesman said the four men and two women were given oxygen therapy after the chlorine leak, which is thought to have been accidental.
An ambulance spokesman described them as the 'walking wounded', who are suffering with eyes watering and itchy throats. Their injuries are not thought to be serious.
Around 200 members of staff and customers were evacuated while firefighters dealt with the problem and the whole area was cordoned off by police tape.
Cllr Jim Wedgbury, an ex-firefighter, was one of the evacuated members who was stuck for a while because his keys and clothes were inside.
He said: "I was using gym equipment. I cycled in this morning and was in the gym for two hours, I was just about finished when the alarm went off.
"The staff evacuated all the customers who stood outside and then the fire brigade arrived.
"I understand there's a chemical leak in the plant room and fumes may have entered the building."
There were believed to be three ambulances, five fire engines and two police cars at the scene at one point.
A witness said the alarm went off, some customers were checked over by paramedics and police were handing out water bottles.
A woman and a toddler who were evacuated were seen outside in their swimwear and wrapped in silver emergency blankets.
Lauren Pope, 20, from Hythe, was in the gym at the time with friends when she heard the alarm going off.
She said people were looking around before the instructors came in and told them they needed to leave.
She said: "We thought it was a practice fire alarm, but then we were sat outside and saw the fire engines arrive. It is quite shocking to see how many engines arrived."
Lauren and her friends were told it would be around a two hour wait.
Police also asked waiting users if they had any medication inside in case they needed it.
A spokesman for the Ashford Leisure Trust said a full internal investigation will be carried out.
She added: "We would like to thank the Emergency Services and Ashford Borough Council for their support and also our customers for their patience."
The leisure centre re-opened at 3pm.
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