KARNS CITY, Pa. -- Three people were taken to the hospital with respiratory problems and 2,500 residents of four Butler County communities were evacuated from their homes last night when a dangerous chemical was accidentally released at the Indspec Chemical Corp. plant in Petrolia.

Shortly after 9 p.m., Petrolia Fire Chief David Blair said the emergency was over but that a low-lying cloud containing the chemical oleum, which is similar to sulfuric acid, had not dissipated enough to allow evacuees to return to their homes.

Chief Blair said he was hopeful that residents wouldn't have to spend the night at three shelters that had been set up for the emergency. He said it all depended upon whether the wind picked up to clear the fumes.

"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.

She said a team with the federal Environmental Protection Agency arrived at the scene from its Wheeling, W.Va. office to conduct air monitoring.

Dave Dorko, manager of the Indspec plant that produces a bonding agent for the tire industry, said the incident occurred at 4:30 p.m. when a tank containing the oleum overflowed. Alarms were sounded and emergency procedures were immediately undertaken, he said. The 30 employees working at the plant at the time were evacuated and no one was injured.

"Safety at the plant is our primary concern for our employees and the community. We didn't do a good job of that today and we apologize to the community for that," Mr. Dorko said last night. "This is something you never want to see happen at your plant. We take our responsibility very seriously."

Emergency personnel went door-to-door alerting residents to the evacuation, Chief Blair said.

The evacuation area was a 3-mile radius from the plant in the mostly rural area containing about 2,500 residents. The affected area included Parker, Fairview, Washington and Concord townships.

Shelters were set up at Karns City High School, where a high school dance had to be canceled, the Bruin Fire Hall and the North Washington Fire Hall.

Chief Blair said about 250 people reported to the shelters and that the remaining residents most likely stayed at hotels or with relatives.

Two elderly women and a 44-year-old man with a medical condition were taken to Butler Memorial Hospital with respiratory problems. It was unclear whether their problems were caused by the chemical leak, Chief Blair said.

Mr. Dorko said last night that the leak was contained and that employees wearing hazardous material suits were back in the plant performing cleanup duties.

No further evacuations were planned as of midnight, but residents in the path of the noxious cloud 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.

Dale Pinkerton, chairman of the Butler County commissioners, said that while the situation "could have been very, very serious," he was pleased with the emergency response.

"Things went very well. I grade it up at the top."