Iron Lung Achievement
Hold your breath for a cumulative time of half an hourTo hold your breath, press while you are aiming through your rifle scope. This will slow your heart rate and steady your aim along with zooming in a bit. This will unlock over the course of your two playthroughs but if for some reason it doesn't, find yourself a nice hiding spot and keep holding your breath until the achievement unlocks.
If you are prone (laying down) as you do this, your breath will regenerate faster.
This cumulative stat is not tracked in Career Stats but your progress is shown at the bottom of the screen in increments every so often during the game.