Pennsylvania ? 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 to flee. At least three residents were believed to be injured.

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

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.