Eight people – including four children – were taken to hospital after a chlorine leak at Craigie Leisure Centre this morning.
The injured swimmers are understood to have inhaled chlorine gas after a malfunction in one of the centre’s indoor pools.
Another seven people were treated at the scene and 51 Mullaloo Heights primary school students, who had been taking swimming lessons, were assessed by paramedics.
The four injured children have been taken to Princess Margaret Hospital while two of the adults have been transferred to Sir Charles Gairdner Hospital.
The final two adults were taken to Joondalup Health Campus.
It is understood none of the people hospitalised have serious injuries and were taken via ambulance as a precaution.
The City of Joondalup, which owns the leisure centre, said emergency services had inspected and declared the facility safe, but as a precaution it would remain closed indefinitely.
WorkSafe is investigating the incident.
The triple-zero call was first made to St John Ambulance about 11am.
Fire-fighters from Duncraig, Joondalup, Osborne Park and Wangara attended the scene.
A senior Department of Mines and Petroleum dangerous goods officer is on site and investigating.