Imperial says it "erred on the side of caution" and sounded its plant sirens at 5:04 a.m. Saturday for a potential hydrogen sulfide release.
Company spokesperson Kristina Zimmer says alarms detected low levels of hydrogen sulfide in the area of the refinery's sulfur plant, located east of the Vidal Street and Kenny Street intersection.
The wind was northeast at the time.
Zimmer says the levels were contained to the sulfur plant, and site perimeter monitoring showed no readings were detected.
As a result of sounding the sirens, the City of Sarnia's Emergency Operations Centre was activated, but plant personnel were able to troubleshoot a piece of equipment in the sulfur plant, and the issue was resolved.
The all-clear was sounded at 5:38 a.m.
The Ministry of Environment was also contacted.