A recommendation to evacuate 2,500 people in four Butler County communities due to an accidental toxic chemical release Saturday afternoon from the Indspec Chemical Corp. plant in Petrolia was lifted at 3 a.m. today.

Based on air quality tests and meteorological information from state and federal environmental agency officials, Butler County Emergency Management officials lifted the evacuation notice after the low-lying cloud containing oleum, also known as fuming sulfuric acid, dispersed.

Many people elected to stay inside their homes and "shelter in place," and many others stayed with friends and family outside the evacuation zone. But about 200 people went to three shelters set up in Karns City, North Washington and Bruin.

Three people were taken to Butler Memorial Hospital with respiratory problems during the evacuation, but it is not known if their problems were caused by the chemical cloud.

The state Department of Environmental Protection also said that the chemical residue precipitated from the oleum cloud at the plant when workers sprayed it with water drained into storm sewers and into the south branch of Bear Creek, killing fish downstream from the plant for approximately 1,000 yards.

Oleum is used by Indspec to make resorcinol, a chemical compound used in the tire and rubber industries. Indspec is the world's largest and North America's only resorcinol producer. When it is spilled as a liquid it reacts with the water vapor in the air to create a dense cloud that hugs the ground and has a sulfuric smell, like rotten eggs.

The oleum spill occurred at approximately 4:30 p.m. Saturday when a 38,000-gallon tank inside the Indspec plant overtopped and spilled through a vent on the top of the tank. The tank is one of five tanks containing oleum at the plant.

Dave Dorko, Indspec plant manager, said there is no indication of equipment failure and the tank and piping are intact. He said he didn't know how much oleum had spilled.

An investigation and cleanup are continuing but he said the plant expects to be open and operating normally with a full shift of 160 workers Monday.

Freda Tarbell, a DEP spokeswoman, said department workers are at the Indspec plant today to monitor the clean up and determine what caused the spill.

"We want to know exactly what happened and how the company responded," said Ms. Tarbell, who indicated the company has not had other spill problems of this magnitude in the past.

When the incident occurred, alarms were sounded and emergency procedures were immediately undertaken, he said. All of the 30 employees working at the plant at the time were evacuated and no one was injured.

Emergency personnel went door-to-door alerting residents to the evacuation, Petrolia Fiore Chief David 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.

Shortly after 9 p.m., Chief Blair said the emergency was over but that a low-lying cloud containing oleum had not dissipated enough to allow evacuees to return to their homes.

Early this morning the U.S. Environmental Protection Agency moved through the neighborhoods around the Indspec plant with mobile air monitors and did not detect any acid or residue of the oleum vapor.

More details in tomorrow's Pittsburgh Post-Gazette.