A cloud of toxic chlorine gas released from a public swimming pool left 25 people poisoned including nine nursery school children having a swimming lesson.

Emergency services went to the site at the Monti Lessini Sports Centre, in the municipality of Bosco Chiesanuova, in the province of Verona, in northern Italy, on Friday, 17th March.

Firefighters, medics, police and even nuclear experts experienced in disaster scenes helped evacuate the pool at around 10am after the gas was released.

The nuclear experts, from an NBCR (Nuclear, Biological, Chemical and Radiological) unit of firefighters were at the scene. The Italian Ministry of the Interior describes the NCBR as “a specialised group of firefighters that is called to intervene in exceptional situations”.

Seriously affected victims were taken to hospitals in Negrar, Borgo Trento and Borgo Roma.

Others were ferried to medical centres in two minibuses, according to local media.

Out of the 25 poisoned, nine were nursery school children aged between 3 and 6 who were at the pool for a swimming lesson.

Local media said that four swimmers were seriously affected by the toxic gas.

The manager of the pool is also reported to be in hospital.

Investigators believe the poisoning happened when pool workers miscalculated the amount of chlorine needed to sterilise the pool.

A massive overdose, according to local media, led to a cloud of toxic chlorine gas rising from the water to poison swimmers.

Chlorine gas was used by the Germans as a weapon in World War I and can be fatal if breathed in.

Victims can experience shortness of breath, blurred vision, burning pain in the nose, eyes and throat and blisters on the skin.