Ambulances were seen outside the 321-million-euro headquarters of the European Council, dubbed the "Space Egg" because of its futuristic oval interior shape, and five people were taken to hospital, they said.
"There was a bad mix of chemical products on the kitchen level. That caused fumes which made 15 people sick," Pierre Meys, a spokesman for Brussels emergency services, told AFP.
He said the symptoms included "sore eyes and vomiting".
"A technical issue affecting the ventilation in the kitchens of the Europa building, producing noxious fumes in the kitchens, has led to a number of kitchen staff falling ill," the council said in statement.
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"The Belgian firemen and medical services were brought in today to investigate the situation and evacuate kitchen staff who felt ill."
Staff were evacuated to the neighbouring Justus Lipsius building "applying the principle of safety first" it added.
"This incident will not prevent next week's meeting of the European Council from going ahead," it said.
The Europa building opened in January after a series of delays.
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