The mask that a combine wears, is simply a mask, with built in voice synthesising 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 there 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.