About 200 residents were evacuated from the town of Portland Mills after a train carrying toxic chemicals derailed Monday at the edge of the Allegheny National Forest.
Four train cars were knocked off the tracks, spilling a combination of sodium hydroxide and sulfuric acid.
The combination formed a hazy mist that lingered over the Clarion River and Portland Mills, a town about 60 miles southeast of Erie.
No injuries were reported, and firefighters sealed the leaking tank car of the Buffalo & Pittsburgh train on Tuesday morning.
Ed Thorwart, a spokesman for the Ridgway Fire Department, did not know what caused the accident or when residents would be allowed to return home.
The railroad runs between state Route 949 and the Clarion River, and cities downstream were notified about potential river contamination.
Emergency crews brought in huge quantities of lime to neutralize the acid.
If inhaled, sulfuric acid can cause respiratory, skin, eye and ear irritation. In greater concentrations, exposure could be deadly.