General assumptions: At the end of your visit in the test facility you'll have to disable local security system. It is not difficult to accomplish and shouldn't take much time.
Security system terminal can be found in one of rooms under the main security rooms. By default you should encounter two guards there (screen above).
You may act here in two main ways. First method is to sabotage fuse box in an adjacent room and then attack each guard by a surprise. Another option is to wait until guards leave this area. The latter one is better of course especially that you do not have to worry that guards will return to the terminal room.
Now there is only one thing left - accessing the door leading to the decontamination. In order to do this, return to the starting place (the elevator). Decontamination door can be found on the right (screen above).