Gatineau firefighters were on scene at an apartment building Tuesday morning where several residents reported feeling sick Monday night.
About 27 residents were forced from their apartments at 24 Rue Arthur Buis at around 8:30 p.m.
Air quality tests are ongoing at the building where one unit is undergoing renovations. Investigators believe that unit may be the source of the substance.
No chemicals have yet been found.
Fire officials said residents will not be able to return to their homes until the substance making people sick is identified.