The HGU-39/P was designed specifically as a 1st attempt to protect pilots in Nuclear, Biological, and Chemical environments. Back in the 1970's when there was fear of the Russkies attacking with chemical or biological weapons across the European frontier, the USAF needed to protect its forward-based pilots. The HGU-39/P + a full face MBU mask [with clear and tinted visor] + a full chemical suit, although cumbersome and bulky, was how the A-10 and F-4 and F-111 pilots were to get from sheltered space to their aircraft on the flightline - and any environment whereby their canopy was in the open position. It was never given a design consideration as a crew helmet - then or now. The HGU-39/P is absolutely a relic of the cold war. It was later superceded with the current full-face MBU + HGU-55. The HGU-39/P was always designed as a stop-gap interim design until the USAF could get a fully tested and integrated design into production and fielded. Necessity is the mother of invention and the HGU-39/P was born out of necessity - real fast.