EDMOND, Okla. (AP) - Fire officials say a dust cloud collided with a kerosene space heater and caused an explosion at an Edmond manufacturer that injured four workers.

Edmond Fire Maj. Kelly Lewis tells the Norman Transcript that maintenance workers at NOV Tuboscope were trying to unclog a machine Thursday morning when a pipe disconnected, releasing a polymer dust cloud. The cloud reached an industrial space heater that had an open flame and ignited.

Lewis says four employees were burned in the explosion. One was treated and released from the hospital, while three others were hospitalized.

The company says the Edmond facility has 23 workers and that it's been shut down as the company investigates the blast. The Edmond site coats pipe with corrosion prevention material.