Kuala Lumpur, Malaysia (CNN) More than 400 schools in Malaysia have closed this week after air pollution caused vomiting in dozens of students, authorities said.

Since Monday, 104 children have fallen ill in the southern state of Johor because of the pollution, according to local officials. Most cases were in the state's Pasir Gudang district.

"The department will continuously monitor the situation and assist the schools to ensure the safety and well being of the students are at its best," Johor Education Department Director Azman Adnan said in a statement.

The closures began on Tuesday and were expected to last until Thursday.

They come just months after toxic waste was illegally dumped in a local river, causing 4,000 people to fall ill and more than 100 schools to close in March.

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