EMPLOYEE WAS HANGING VENTILATION FAN WHEN FAN SLIPPED OUT OF HANGER. FAN CAME OUT OF FRONT HANGER WHILE TIGHTENING JUNCTION BOX TO SIDE OF FAN. WHEN THE FAN SLIPPED COMING OUT OF FRONT HANGER IT TIPPED (FRONT) DOWN STRIKING EMPLOYEE IN CHEST/RIB AREA. EE WAS TRANSPORTED BY AIR TO HOSPITAL.