About 2,500 people who fled when a corrosive liquid overflowed from a tank at a chemical plant and evaporated were allowed to return home Sunday after authorities determined that no toxins remained in the air.

A material called oleum, similar to sulfuric acid, leaked from a tank at the Indspec Chemical Corp. plant on Saturday and evaporated into a toxic cloud, plant manager Dave Dorko said. All 30 employees of the plant in Petrolia, about 40 miles northeast of Pittsburgh, were evacuated and no injuries were reported, he said.

Authorities went door to door to warn the 2,500 people living within 3 miles to evacuate. About 250 went to shelters in nearby Karns City, North Washington and Bruin.

Authorities were concerned about the potential for respiratory damage and skin burns, said Freda Tarbell, spokeswoman for the state Department of Environmental Protection. She said residents were cleared to return home after officials from the federal Environmental Protection Agency monitored the air and determined that no acid remained.

Authorities said three residents were taken to hospitals with apparent respiratory problems, but it was unclear whether the problems were related to the leak.

Residents who were in the path of the dense cloud Saturday night 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, which produces a bonding agent for the tire industry, will remain closed until the cleanup is completed.

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Information from: Butler Eagle, http://www.butlereagle.com