An explosion at a chemical plant in the Houston area Wednesday shook nearby homes, generated black smoke seen for miles and injured two workers.

A tank at the American Acryl plant in the Houston suburb of Pasadena exploded just before 9 a.m., Seabrook police Lt. Sean Wright said. The plant is near Seabrook.

TV coverage showed holes in a tank at an industrial complex. Debris could be seen near mangled, blackened equipment.

Officials in Pasadena and Seabrook ordered residents to remain indoors after the explosion because the tank contained toluene, a toxic petroleum byproduct used in paint and gasoline, said Rosie Torres, a spokeswoman for the Harris County Office of Homeland Security and Emergency Management. That order was lifted late Wednesday morning.

Roads around the plant were closed but reopened late in the morning after a fire started by the explosion was brought under control, Torres said.

Two workers hurt in the blast were taken to an area hospital as a precaution for inhalation injuries, said Kelli Gregory, a spokeswoman for American Acryl. They were treated and released, she said.

The cause of the explosion is still being investigated, Gregory said.

The Occupational Safety and Health Administration has opened an investigation into the blast, agency spokeswoman Elizabeth Todd said. No other explosions or similar incidents have been reported previously at the plant, she said.

Half of American Acryl's 50 employees were working when the tank exploded. Operations at the facility were stopped for the day, and Gregory said she was not sure when work would resume.

The 55-acre plant built in 2001 makes acrylic acid, which is used in such products as diapers, nail polish and household cleaners, she said.

American Acryl is a joint venture between Philadelphia-based chemical manufacturer Arkema Inc. and Japanese subsidiary NA Industries, Inc.

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