The explosion happened at Veolia ES Canada Industrial Services Inc. on Scott Road north of Churchill Road/Highway 40, according to Sarnia Police Services.
In addition to one person being critically injured, Bradley said three suffered serious injuries and one has minor injuries.
Bradley added that the fire wasout and the scene was "under control."
Sarnia Fire Rescue Services, Sarnia Police Services, Lambton Emergency Medical Services and Lambton OPP are on the scene. An Ornge air ambulance has reportedly arrived at Sarnia Chris Hadfield Airport.
Scott Road between Churchill Road/Highway 40 and St. Andrew's street is closed. All motorists are being asked to avoid the area.
It is unclear at this time what caused the explosion. Nearby residents reported hearing at least one big bang in a discussion on a Facebook page for alerts in the area.
Veolia Environmental Services's Sarnia location offers industrial cleaning and maintenance services, according to its website.
The company's industrial cleaning and maintenance services include chemical cleaning, emergency response for spill containment and clean up, hydroblasting, tank cleaning and separations, and vacuum services.
The Ministry of Labour has been notified of the incident, according to Sarnia police.