Authorities were concerned about the potential for respiratory damage and skin burns, said Freda Tarbell, spokeswoman for the state Department of Environmental Protection.
Authorities went door to door to warn the 2,500 people living within 2 miles to evacuate. Residents were taken to shelters in nearby towns, and officials said they did not know when they would be allowed to return.
Red Cross officials said they expected to shelter only 50 to 100 people overnight.
No injuries were reported at the plant, where the spill was contained and crews were trying to clean it up. An official at Butler Memorial Hospital, where authorities said three residents were taken, declined to comment.
The dense cloud was reported moving slowly west Saturday night, close to the ground. No further evacuations were planned, but residents in the path were told to remain inside, to close doors, windows and vents, to shut down any system that draws air from the outside, and to put out any fires in fireplaces.
Petrolia is about 40 miles northeast of Pittsburgh.
Information from: Butler Eagle, http://www.butlereagle.com