About 2,500 residents of four Butler County communities in western Pennsylvania were evacuated from their homes on Saturday night following a dangerous leak of a corrosive material at the Indspec Chemical Corp. plant in Petrolia.

The plant is about 64 kilometers northeast of Pittsburgh. The leak involved a material called oleum, which is similar to sulfuric acid. The plant uses the chemical during its production process for its ability to absorb water.

Three residents were taken to a local hospital with apparent respiratory problems, but officials said it was not clear whether they were related to the leak.

“Of course, the concern here is…the potential for respiratory damage and skin burns, things like that,” said Freda Tarbell, a spokeswoman for the state Department of Environmental Protection.

About 250 people went to shelters in nearby Karns City, North Washington and Bruin and the rest likely went to stay with relatives, officials said.

Currently, a team with the federal Environmental Protection Agency is at the scene conducting air monitoring.

The leak was caused by an overflow from a tank. The material evaporates easily, creating a toxic cloud, Plant manager Dave Dorko said.

The cloud was reported moving slowly Saturday night, close to the ground. Residents in its 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. No further evacuation were planned.