Here's something I've been wondering...I've seen Gordon wear the HEV suit, but never the actual helmet. Does he wear the helmet as well? He has telescopic vision and headcrabs can't mount his head, so he must wear it, right?
Thought about that too. Perhaps it is a localized force field generated by the collar ring? Also, those glasses have a built-in H.U.D..
BTW, the collar force field also contains the controlled atmosphere needed to survive the toxic gases and breath on Zen, BUT it functions as a two-way sound damper with a defective (Acme)mic, therefore Gordon cannot verbally communicate. "...You're a man of few words..."