The prime minister arrived at the Senai airport at 5pm.

He is expected to visit the Sultan Ismail Hospital in Johor Bahru and speak to reporters later at the Bilik Gerakan Media.

About 900 people are said to have been affected by the toxic fumes from chemicals illegally dumped in the river which emitted flammable methane and benzene fumes.

No deaths have been reported but 12 victims are being treated at intensive care units. They are in stable condition.

Bernama earlier reported that 506 people had sought treatment with 166 hospitalised.

The chemicals dumped in Sungai Kim Kim are believed to be scheduled waste needing proper disposal because of their hazardous nature.

The owner of a chemical factory in Kulai was arrested on Sunday in a joint operation mounted by the Department of Environment and the police.

This was followed by the arrest of the owner and a worker of a shredded waste factory in Taman Pasir Puteh, Pasir Gudang, yesterday.