The Combine

[Mujuro]

I believe the metro-police are the lowest rank of combine forces, being just pure humans who like to beat and terrorize other people ("Pick up the can!" and so on). If you talked to the lady in the line at the beginning of the game long enough, she'll say, "I'm thinking about joining Civil Protection just to get some decent food." As you progress as a metro-cop, you gain new ranks (just like in the military) and gain new rights and whatnot. The only downside to this is that you must go through a tedious process which turns you into the actual Combine race. This is just a theory though...

*Attached is a screenshot of what seems to be the respirator bit sticking out once you exit the train station.

 

[Asknoone]

Yes, its true, i think most people know, but still nice to be reminded for people who come looking for info on the storyline.

Although theres a flaw in it, its not a tedious process to become combine because the metrocops are complete humans, they havnt been touched with combien tech, they just volunteeers for overwatch.