A dozen children taking swimming lessons in a Dublin pool needed medical attention after the area was contaminated by toxic fumes.
The young people and one adult were taken to hospital from the Clayton Hotel Liffey Valley on Thursday.
In a statement the hotel said it would carry out a “thorough investigation” into what happened.
Irish national broadcaster RTÉ reported that a number of children were found to be suffering “respiratory distress and irritation”.
The Dublin Fire Brigade (DFB)’s chief fire officer Greg O’Dwyer told RTÉ the incident was caused by “overmixing” of chlorine and another chemical in the pool’s plant room.
“There were swimming lessons with 24 children in the pool at the time and some parents,” he added.
23 people in total were medically assessed at the hotel, having complained they felt unwell.
He said firefighters wearing “gas-tight suits” entered the scene and “isolated the chemical”.
In a statement, DFB said units were sent from across Dublin City, including from Tallaght, Dolphins Barn, Donnybrook and Dun Laoghaire fire stations.
“Specialist HazMat appliances from Donnybrook and Dun Laoghaire fire stations” attended and “set up a decontamination zone”, they said, adding there was no danger to the wider public.”
In a statement, Dalata Hotel Group which owns the Clayton Hotel Liffey Valley, confirmed “an excess chlorine incident” had occurred.
“A thorough investigation of the circumstances surrounding this incident will take place as soon as possible.
“Our immediate thoughts are with those impacted and their families. We are doing everything in our power to support them and the emergency services.
“Out of respect for those impacted and their relatives, we will not be commenting further at this time,” they added.