GLASSBORO — A South Jersey elementary school was evacuated Thursday after a "foul odor" made students and staff sick.

Authorities said the school later tested positive for higher-than-normal levels of carbon monoxide, a colorless, odorless and deadly gas.

The air at the Glassboro Intermediate School made about 30 people feel nauseous and lightheaded, according to Glassboro Police.

All students were brought by bus to the Dorothy L. Bullock School, where they spent the rest of the school day.

Police said the parents of children who were ill were notified.

It was not clear Thursday afternoon what produced the carbon monoxide or the odor.

Carbon monoxide is produced by burning fuel. Indoors, the gas can be produced by improperly vented heaters, stoves and fireplaces.

Symptoms of carbon monoxide poisoning are often likened to the flu with headaches, dizziness and confusion, weakness, upset stomach and vomiting, and chest pains, according to the U.S. Centers for Disease Control and Prevention.

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