An incident at Imperial's Sarnia Manufacturing site was reported early Thursday afternoon through the Aamjiwnaang Notification System.
A Community Awareness Emergency Response (CAER) alert was issued by the company.
It said there was an equipment malfunction during the start-up of a process unit.
In a Facebook post the company said plant sirens sounded at about 12:45 p.m. when a malfunction caused a release of light hydrocarbon to the air that contained a small amount of hydrogen sulphide. Imperial personnel responded and readings from perimeter air monitoring were non-detectable.
The all clear was issued at 1:39 p.m.
The environment ministry was notified about the incident and Imperial was investigating the cause.
