An explosion at a South Carolina pharmaceutical packaging company July 22 sent seven workers to the hospital and triggered a safety investigation, according to The Columbia (S.C.) State.

Six Holopack International Corp. employees were treated and released. One was flown to a hospital that specializes in burns, company and fire officials told the paper.

The flash explosion occurred about 10 a.m. in a warehouse at the back of the German-owned plant. Somehow, dust from plastics being ground ignited, Battalion Chief Cleveland Geter of the Columbia Fire Department told reporters.

"We just don't know what caused a dust explosion,'' Geter said. "It's unusual."