INEOS Styrolution said it was detecting elevated benzene levels on Friday, May 31.
In a notice through Sarnia-Lambton Alerts, the Tashmoo Avenue site said benzene emissions at the facility were being detected above 0.021 parts-per-million.
The company said emission reduction efforts will be identified and implemented if there is an abnormal operating condition at the site.
Monitoring data was also being verified and the source of elevated emission was being actively investigated.
The Aamjiwnaang First Nation shared with community members on Thursday evening that it was informed by INEOS that the company would be loading four railcars on Friday with "off-spec material."
"Railcar loading is a standard process and part of their normal operations," Aamjiwnaang said in a message from its notification system. "The off-spec material is a mixture of benzene, styrene and ethylbenzene. They will continue to monitor emissions and will stop the process if their monitors are reading any levels of benzene. Emissions should be very limited as the vapour outlets on the cars are connected to the incinerator at the rack. The purpose of this process is to de-inventory some of the material and decontaminate part of the equipment in the ethylbenzene unit in preparation to take tank MT109 out of service."
Aamjiwnaang said INEOS agreed to provide its four-week work plan going forward, so the First Nation can notify the community in advance.
Community members are advised to continue to exercise caution and avoid the baseball diamond and playground areas when levels are elevated.
-with files from Melanie Irwin