A group of school children have been taken to hospital after apparently falling ill within minutes of each other at Toronto’s Art Gallery of Ontario Friday.

One child fainted shortly after entering the Frida Kahlo exhibit at the gallery around 11 a.m., while four others complained of headaches and nausea.

Emergency medical services were called and all five children were taken to the Hospital for Sick Children to be examined. They are said to be doing fine.

Because of concerns about possible carbon monoxide poisoning, the special exhibition area of the AGO building was evacuated while the air quality inside was tested.

Air readings suggested nothing abnormal, Toronto Police report.

The AGO in a statement said testing confirmed that the gallery’s “air quality is excellent throughout the building.”

The exhibit was subsequently re-opened.

It’s still unclear what caused the children, aged 10 and 11, to become ill.

“Police Hazmat and Fire Hazmat have been inside the AGO and done a thorough testing in there. There were no indications of any high levels of anything there or anything out of the ordinary,” Toronto Police Sgt. Ian McLaughlan told reporters.

“So right now we’re looking at whether they might have passed something, perhaps some idling equipment or idling bus and picked up a source of contamination there," he said, noting the group had previously come from the subway.

Alicia Vandermeer, the director of Visitor Experience at the AGO said police examined the special exhibits area before re-opening it to the public.