The cause of the incident at the Philips Oral Healthcare office building in the 1700 block of South Baker Avenue was not immediately known.
A spokesperson for the Ontario Fire Department said the agency's firefighter-paramedics were at the scene shortly after 8 a.m., following a report of a man complaining of chest pain. A total of 24 patients were treated.
News video from AIR7 HD showed an impromptu triage area in front of the building, where emergency responders were speaking with several people, a few of them with blankets draped around their shoulders.
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At least six ambulances were summoned to the location. Nine people suffering from respiratory irritation were transported to medical centers in conditions that were not life-threatening, Fire Department Battalion Chief Scott Roeber said.
Dozens of other individuals were seen in an adjacent parking lot after the office building was evacuated.