The explosion at the Sayankkhimplast plastics factory, suspected of having been caused by a build-up of dichloroethane fumes, occurred at around 5:34 a.m. Moscow time (1:34 a.m. GMT). Eleven fire engines and five rescue units responded to the scene.
A local emergencies ministry official said the fire had been contained and there was no further danger.
He said that dichloroethane, which is used to manufacture vinyl chloride, is extremely noxious, and serious environmental contamination was avoided only because the fire was quickly brought under control.
Under normal temperatures, the substance is not highly volatile. However, despite official assurances that the blaze was promptly extinguished, witnesses report seeing heavy smoke rising from the factory for some time.
Sayanskkhimplast is Russia's largest manufacturer of polyvinyl chloride resin, producing some 250,000 tons annually, or 60% of the total.