Johor DoE director Dr Famey Yusoff said an initial investigation revealed no traces of illegal dumping or scheduled waste discharges at any premises nearby.

Fahmey said an investigation team, together with personnel from the hazardous materials unit and the Pasir Gudang City Council (MPBG), monitored the area, but air quality and samples from the village’s water pipes and rivers were normal.

He said his department had not yet been able to determine the cause, but would continue to investigate.

“We are using drones to determine if it is illegal dumping or discharges from factories,” he said.

Famey said the department was also investigating a used cooking oil processing factory operating illegally in the village, which was initially suspected of causing the pollution.

However, he said no trace of illegal dumping or scheduled waste was found, and the water pipeline near the factory was not connected to the river.

He said the city council had taken steps to close down the factory.

On Tuesday, 24 students and two canteen workers at the school complained of dizziness and vomiting due to chemical odour at about 3.15pm.

Two teachers also complained of similar symptoms.