Story related, cannot be missed.
After the Chapter finishes loading, turning the lights on and then head towards the next area. Open the door to the left, grab the Gray Bag (Radiation Checker Meter) that you see on top of cardboard boxes on the right of the room. Now grab the NBC Suit off the rack and then leave Level 3, the achievement will unlock.
**All credit goes to themindisacity for the help!**