Local residents in Menasha, Wisconsin, were evacuated from their homes on Monday morning after a chemical leak and explosion at a paper mill.
The blast at the SCA Tissue plant is thought to have been caused when an "organic polymer" leaked as it was being unloaded, and then came into contact with another chemical, sodium hypochlorite — a highly concentrated form of household bleach.
The two chemicals reacted, creating heat, giving off gases and causing an explosion that blew off part of a roof of the plant, FOX 11 reported.
Both substances are used in cleaning water from the paper making process.
"Upon our arrival and further investigation we did note there was a vapor cloud that was emanating from the building; also there was some liquid on the ground, outside the building, which also had some vapors coming off of it," said Assistant Chief Mike Sipin of the Neenah-Menasha Fire Department.
Fire department officials ordered more than two dozen homes to be evacuated as a precaution, but residents were allowed to return to their homes in the afternoon.
Hazardous-materials crews attended the scene and the spill was contained before it could reach the sewer system.
An investigation into the incident has been launched by the Occupational Safety and Health Administration (OSHA).
SCA said that only a small part of the plant was affected by the blast.
Its vice president of communications, Amy Bellcourt, told FOX 11 that the company will evaluate what happened. "If any changes in how we handle deliveries of these materials are required, we will certainly make them," she added.