Toddlers taking part in an Aquatots class were evacuated from a swimming pool due to contamination from air bubbles “containing high levels of chlorine”.
The swimmers in the class for two and three-year-old children, held on Saturday morning, were all rushed from the Philip Southcote School site in Addlestone, Surrey, to hospital as a precaution.
There were worries of the children suffering "irritation to the lungs, drowsiness and vomiting".
In an email, Aquatots told parents it believed droplets of chlorine in pipework from the pool dosing system had escaped into the pool, Surrey Live reported.
Aquatots wrote: "The air bubbles, once in the pool water, rose to the top of the pool water surface and into the air space in the pool room building, effectively creating a level of strong-smelling chlorine gas.
"The swimmers in the pool suffered irritation to the lungs, drowsiness and vomiting, due to inhaling the chlorine-concentrated air that became present in the pool building."
The email explained that a full investigation was yet to take place which would find exactly what happened.
Quick-thinking staff opened up doors to dilute the air and all swimmers were ushered out of the room.
Emergency services were also called when the hospital reported that it could have been down to a possible gas leak, according to Aquatots, but the teams closed the incident after carrying out tests to the pool building and the water.
By 1pm all swimmers were discharged from hospital and allowed to go back home, Aquatots reported.
The swim class provider added: "We are extremely sorry for the anxiety, trauma, upset, and suffering that the incident may have caused to our young swimmers and families.
"We have maintained contact with the swimmers since the incident to ensure they are all OK.
"Thankfully, the hospital has said, they expect there to be no long-lasting effects and that the incident, although traumatic for the young swimmers, was a mild respiratory irritation that had affected everyone in the pool building.
"The school headmistress and other staff arrived at the school immediately when notified of the incident. The staff are distraught that this incident happened, and have been very cooperative both at the time of the incident and the follow-up."
The incident has been referred to Surrey County Council and the Health and Safety Executive (HSE).
An HSE spokesperson confirmed it is aware of the incident and "making inquiries".
According to Aquatots, an independent pool engineer also carried out checks at the site on Tuesday.
The email added: "In an effort to further assure you, the pool water quality and chemical levels are checked and recorded daily by the school staff, as per industry standards.
"Copies of the records were checked on Saturday by the Aquatots office manager and by the emergency services. All chemical levels were found to be satisfactory."
Philip Southcote School head teacher Mary Rome clarified the incident does not affect sessions prior to October 23.
She said: "We immediately worked with the swimming school and emergency services to address the situation. Children and adults in attendance went to hospital as a precaution and discharged the same day."
"We are working with Surrey County Council and HSE as a matter of urgency to investigate how this happened and to prevent this happening in the future, the pool will remain closed while that is done," she added.
Echoing her comments, a Surrey County Council spokesperson said: "We're aware of an incident on Saturday morning at Philip Southcote School and are supporting the school and working with them as they investigate what happened, with the pool staying closed until that work is completed."
