Officials first learned of the leak at 10:15 a.m. ET on Saturday.
They swiftly evacuated homes and factories within one kilometre of the plant on Marie-Victorin (Highway 132), and closed the highway in both directions. A confinement order was also issued for people in the vicinity but living outside of the security perimeter.
On Sunday, city manager Sébastien Roy said people would be allowed to return to their homes throughout the morning, but that the 400-metre security perimeter would be maintained. Highway 132 remains closed as a result.
Kronos makes pigments for paint and other chemical products. Titanium tetrachloride is commonly used to produce titanium dioxide — a paint pigment.
Up to five tonnes of the product escaped the plant after a piece of equipment broke.
White vapour poured out of the facility's ventilation shafts during the leak, the titanium tetrachloride mixing with the ample snow to form hydrochloric acid.
Hydrochloric acid can cause irritation to the eyes, skin and mucous membranes, as well as breathing difficulty in some.
Five people were medically treated for minor injuries following the leak.