A corrosive liquid overflowed from a tank at a chemical plant in western Pennsylvania on Saturday, evaporating into a toxic cloud that snaked along the ground and forced about 2,500 people to flee. At least three residents were believed to suffer respiratory problems.

A material called oleum, similar to sulfuric acid, leaked from a tank at the Indspec Chemical Corp. plant in Petrolia, about 40 miles northeast of Pittsburgh, plant manager Dave Dorko said.

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 3 miles to evacuate. Tarbell said residents were cleared to return home early Sunday after officials from the federal Environmental Protection Agency monitored the air and determined that no acid remained.

About 250 people went to shelters in nearby Karns City, North Washington and Bruin on Saturday. Red Cross officials said they expected to shelter only 50 to 100 people overnight.

Dorko said all 30 employees of the plant, which produces a bonding agent for the tire industry, were evacuated and no injuries were reported. Authorities said three residents were taken to hospitals with apparent respiratory problems, but it was unclear whether the problems were related to the leak.

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.

The plant will remain closed until the cleanup is completed.