Imperial says the plume from the main stack at its Sarnia manufacturing site may be darker than normal over the next few days.
The company is in the process of restarting the coker unit and main boiler following planned maintenance.
Imperial says emissions aren't expected to exceed government guidelines and the Ministry of Environment and Climate Change has been notified.
Members of the public who would like more information can call Imperial's 24-hour plant operations line at 519-339-5666.