Four people were taken to hospital in Montreal late Thursday afternoon following the leak of a hazardous substance into the water system.
Residents in a downtown area were advised not to use tap water, even if brought to a boil.
A Montreal ambulance official says the affected residents live in the same building in the Ville Marie district and suffered from nausea and diarrhea. Two other residents were treated by paramedics but not taken to hospital.
The city has not yet said what has tainted the water, but Jean-Francois Coornaert says firefighters had identified the product as magnatrol, a biocide used in air conditioning towers to eliminate bacteria.
A spokeswoman for the city says in addition to not consuming tap water, residents were advised to not use it to wash or clean dishes.
Fire hydrants in the area, bordered by Bleury, Beaver Hall, Rene-Levesque and Saint-Antoine streets, have been opened to flush water from the system.