The situation is becoming "increasingly critical" after hundreds more children were admitted to hospital. Three people suspected of involvement in the dumping of toxic waste have been arrested.
Malaysia has closed 111 schools after hundreds of people, many of them children, were hospitalized following the suspected dumping of toxic waste in a nearby river, authorities said.
Hazardous fumes spread across Johor in the country's south last week after a truck was believed to have dumped the waste, causing symptoms including nausea and vomiting.
Education Minister Maszlee Malik said the situation was getting "increasingly critical" and ordered the schools to close after 500 pupils, teachers and others received treatment after inhaling the fumes, with over 160 people admitted to hospital.
Initial reports suggested the poisoning was far less widespread, but as hospital admissions climbed, Malik raised the number of schools to be closed from 13 on Wednesday. Up to nine people were receiving intensive care treatment.
After students and teachers first reported breathing difficulties on March 7, forcing the closure of two schools, a second wave of poisoning hit within hours of the reopening of the schools on Monday. The next day, 260 people were rushed to the hospital.
Three men were arrested earlier this week over the dumping, with some reports claiming the poisoning to have been caused by methane.
One of the suspects is the owner of an illegal tire recycling factory, according to German news agency dpa.
The suspects are reported to face five years' jail time for breaking environmental protection laws.