Update: Eight people have been taken to hospital after they were exposed to potentially hazardous hydrogen sulphide gas near Fremantle this morning.
Those affected were at the Austal shipyards on Clarence Beach Road, Henderson, and it is believed they had been working on the bottom of a boat when the substance leaked out.
Austal said a personal gas monitor alarm on one of its workers was triggered by the gas at 10.30am.
Four firefighter crews were called to the scene and set up decontamination equipment.
None of the workers was seriously injured and they were taken to Sir Charles Gairdner, Fremantle and Rockingham hospitals for medical checks.
They have since been discharged from hospital.
Workers returned to the site at 12.30pm after gas readings were taken by fire crews.
Hydrogen sulphide is a colourless gas with a characteristic odour of rotten eggs.
At very high levels it can have neurological effects including memory loss and dizziness.
WorkSafe has been notified of the incident.