Evacuating 35 homes in Shenango Township Sunday night was a necessity after a chemical spill caught fire on Interstate 80 when a tractor-trailer transporting the material wrecked on the bridge over state Route 718, state police trooper William Peterson said.

"It was a precautionary. We had to isolate for 800 meters in all directions," Peterson said Tuesday, adding that Shenango Township police and the Mercer County Emergency Management Agency were a big help.

Archie W. Lucas, 63, of Akron, who was driving for Yellow Transportation Co., was taking the load from Cleveland to New York when his eastbound rig, which was towing two trailers, slid out of control on the ice-covered road. The rear trailer rolled onto its side and caught fire, state police said.

The trailer was hauling a hazardous material, tert-butyl hypochlorite, which was released into the air through smoke from the fire, police said.

Peterson said the truck was carrying 5 pounds of the chemical, which could be harmful, even fatal, if inhaled, he said.

John Nicklin, Mercer County assistant EMA director, said the chemical was bound for a research lab in New Jersey.

Neither Nicklin nor Peterson knew what the chemical is used for. Freda Tarbell of the state Department of Environmental Protection was also unsure of its use. She said DEP usually gets called to help determine the best way to deal with such things as runoff to minimize the environmental impact and to protect human health.

"When we get there we work together with local authorities for best strategy to handle it," she said.

According to Ave Tolentino of the University of California at Berkeley's Department of Environment Health and Safety, tert-butyl hypochlorite is used in various chemical experiments and reactions. It can be used in research, has some industrial uses and can be used as a reagent or additive.

A professor at the university said if the chemical hadn't caught fire, it would have been about as dangerous as household bleach.

Yellow Transportation privately contracted with a hazardous materials team to pay for the cleanup, Nicklin said.

DEP is following up, Ms. Tarbell said.