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The mask that a combine wears, is simply a mask, with built in voice synthesizing technology, as well as possible aiming aids or heat detection. Combine units are altered humans (trans-human) that have been altered, and in episode 1, you see a combine without the main armour on a screen, showing that their face is reasonably unchanged. Therefore, behind a combines mask, is a normal human face. Unless your talking about the elite units, which are probably far more changed, maybe with only one eye. Metropolice are just normal people, with no alterations. The mask is based off of an existing gas mask, and one of these was used in the live action movie created by fans.