Eleven people, including 5 children under the age of 10, needed to have hospital treatment after caustic soda was added to their food at a fast food restaurant in Bunbury instead of salt.
Two adults needed to be flown to Perth for further treatment and 3 of the children had to stay in hospital.
Caustic soda is often used as a detergent or drain cleaner and when it comes into contact with the skin it causes chemical burns and has been known to cause permanent blindness if it comes in contact with the eyes.
The local forensic police are looking into how caustic soda happened to be inside a salt shaker at the restaurant.
WorkSafe WA are investigating the incident.