You'll open a door at the end of the Contagion mission and Lesnitsky will have a gun to Anna's head and demand that you take off your gas mask. DO NOT shoot at him. Hold
(assuming your control scheme is the same as mine) to take off your mask and unlock this achievement (as well as gain one of several CRUCIAL moral points to work toward the Redemption achievement at the end of the game).
Check out the video below for a full playthrough of the game showing all collectibles, miscellaneous achievements like this one, and the Moral Points I obtained (I skipped a lot of them) in order to get the Redemption achievement. The Contagion mission starts at timestamp 1:23:59