South Carolina governor Mark Sanford announced a state of emergency in Aiken county after Thursday's accident.
He urged "everybody in the effected areas to stay inside with the doors and windows shut and the ventilation off".
He added that residents should "keep the television or radio on so they can stay up to the minute with the latest information".
The train carrying chlorine hit another train that was parked.
Police said about 5400 people in a 1.6-km radius around the wreck was evacuated.