An odour detected at the Shell Canada plant Wednesday resulted in traffic control being set up and a shelter-in-place order being issued for Corunna.
Shell spokesperson Kristina Zimmer says they received an initial call from one of their neighbours at about 4pm Wednesday and started an investigation that included air sampling.
There was no community notification until after 6pm when sirens started wailing and the precautionary shelter-in-place was issued, advising residents in a portion of Corunna to go indoors, close all windows and doors and turn off ventilation systems.
The company reported that elevated benzene levels had been detected south of the Shell manufacturing centre on St. Clair Parkway.
Zimmer says the all clear was issued at Shell at 8:10pm but air monitoring was ongoing and the investigation was continuing.
"Shell has located the source of the odour and at this time we are investigating further and trying to assess exactly why this would have occurred," says Zimmer.
The environment ministry was notified.
Winds were northerly and the city of Sarnia was not included in the shelter-in-place.