A blast resistant window structure capable of withstanding typical car bomb blast pressures of 100 psi or more and of resisting leakage of chemical or biological agents is described, the window structure including a frame hermetically enclosing two glass panels in confronting relationship defining an air gap therebetween, each glass panel having a thin layer of polymer film on each surface thereof, and a pressure relief valve in the frame for releasing air pressure from within the gap in response to blast pressure imposed on an outer surface of one of the panels.