Imperial Oil is not expecting emissions from its Sarnia site to exceed government guidelines following an operational upset Tuesday morning.
The upset was reported around 11:15 a.m., resulting in a darker than normal plume from the stacks at the refinery.
The Ministry of Environment, Conservation, and Parks has been notified.
"Testing during similar past events indicates emissions have not exceeded health-related air quality standards during those occurrences," read an update from Imperial.
Personnel have been working to return the site to normal operations. Elevated flaring is possible for several hours and increased noise may be heard offsite.
