The worker, whose name was not released, was airlifted to a London, Ont., hospital with severe burns after the explosion at Veolia ES Canada Industrial Services in Sarnia on Saturday.
Four other workers remain in hospital.
Ministry of Labour spokesman Bruce Skeaff says a dust collection system located outside the building exploded at about 2:40 p.m. Saturday, causing significant structural damage.
There were eight employees inside the tank car maintenance facility at the time of the blast.
The road (Scott Road) remains closed in the area while the investigation is completed.
Veolia released a statement saying it is "fully co-operating with the Ministry of Labour and other local authorities to investigate its cause."
"The employees of Veolia are deeply concerned for our co-workers, and our thoughts are prayers are with them at this time."
(BlackburnNews.com, The Canadian Press)