Early tests did not indicate dangerous levels of any chemicals around the plant in Geismar, about 20 miles southeast of Baton Rouge, but area residents were told to remain indoors with doors and windows closed, said Jean Kelly, spokeswoman for the state Department of Environmental Quality.
She said helicopters took three or four people Thursday from the plant owned by The Williams Companies Inc., and ground ambulances took 30. Kelly did not have other details about injuries.
Seven victims were taken to Baton Rouge General Medical Center and more were expected, the hospital said in a news release on its website.
Workers scrambled over gates and ran from the plant when they heard the explosion, which shook the ground and was followed by a "ball of fire," Daniel Cuthbertson, a fuel truck operator with CBI construction based in Taft, Calif., said later at a police roadblock.
The company said in a news release on its website that the flow of chemicals to the fire had been cut off by 10 a.m.
Louisiana State Police Capt. Doug Cain said the explosion occurred around 8:30 a.m. Thursday at the plant. It makes ethylene and propylene - highly flammable gases that are basic building blocks in the petrochemical industry.
A thick plume of black smoke rose from the plant. At a roadblock several miles away, where family members waited anxiously to hear about loved ones, flames were easily visible above the trees.
Kelly said an ethylene fire at ground level reportedly was waning several hours after the explosion.