Patch the Gasmask.

While wearing the Gasmask, when taking damage the Gasmask itself will also take damage and a visual of this will be on screen with a crack in the mask/screen. When this appears, if you're not playing on Ranger Hardcore, a message will appear on screen telling you to press to repair your mask as you will now be exposed to radioactive elements. Do this to put some sticky tape over the hole, unlocking the achievement.