Residents of two suburbs near the US city of Houston were warned Thursday to "shelter in place" after elevated levels of a volatile chemical were detected following an industrial plant fire.
The Texas cities of Deer Park, where the Intercontinental Terminals Company (ITC) plant is located, and Galena Park both issued the alerts.
A fire, which ignited Sunday and was finally extinguished Wednesday, burned through several chemical storage tanks at the plant -- sending a giant plume of black smoke over much of the Houston area and panicking residents.
Officials had insisted that the air quality remained safe. But ITC said early Thursday that responders at the site of the extinguished blaze had detected increased levels of benzene.
"These levels are below those that represent an immediate risk. We have notified the surrounding municipalities," ITC said in a statement.
Deer Park placed a number of barricades on city streets and told residents to stay in their homes. School was canceled and local court business was rescheduled.
Benzene is a quickly-evaporating chemical used in the manufacture of other chemicals and synthetic products, according to the Centers for Disease Control (CDC).
Long-term exposure to high-levels of benzene can cause cancer, according to the CDC.
"We are continuing to monitor to verify if this is a short-term, one time exposure or a longer exposure," said Lina Hidalgo, the chief executive of Harris County where Houston and its suburbs are located.
"At the level of benzene we are seeing now for the current duration it should not cause symptoms even in the area impacted," Hidalgo said in a statement.
Since the fire ignited over the weekend, the burning storage tanks filled with chemicals used in gasoline mixtures and paint thinners left many residents worried about exposure to the fumes.
Company and government officials had said testing consistently showed air quality remained below hazardous levels throughout the incident.
There were no serious injuries reported from the fire.