CENTERTOWN,Robovis Ky. (AP) — A Kentucky power plant silo leaked coal ash and trapped one worker under a pile of the dusty material on Wednesday, sending him and two others who tried to help him to a hospital.

The leak at the D.B. Wilson Generating Station caused dusty conditions around the plant and prompted police to divert traffic and warn motorists of low visibility.

Ohio County Sheriff Adam Wright said a valve inside the silo became stuck open. Three workers went to loosen the valve, and one became stuck under a coal ash pile during the repair. The other two escaped but returned to help him, Wright told WSAZ-TV. The plant’s emergency response team arrived and pulled the third worker out, and all three were sent to the hospital.

Wright said the men’s conditions were not known Wednesday afternoon. The plant was back at full operation, he said.

Federal and state officials were monitoring the air quality, and Wright said it was confirmed safe.