CHICAGO (AP) _ An explosion triggered by a mattress fire ripped through a fourth-floor apartment Tuesday, propelling three firefighters out windows and injuring eight, officials said.

''It's called a flash-back,'' said Fire Department spokesman Frank Szwedo explained. ''It has the same properties as an explosion.''

The firefighters were extinguishing what had started as a mattress fire when oxygen reached the smoldering material, causing a blast with such force that it apparently sent the firefighters out the windows, Szwedo said.

The most severe injury was sustained by Joseph Gaska, 39, a firefighter who fell four stories, officials said. Gaska was in critical condition with second-degree burns and abdominal injuries, and was undergoing surgery, said Michael Reese Hospital spokeswoman Holly Rea.

No residents of the South Side apartment building were reported injured, Szwedo said.

Five firefighters suffered burns to the head or neck areas, another had multiple fractures and another had a broken ankle and possible back injuries, Szwedo said.