Normally, I would agree with that being the intent of the design, but abnormalities do crop up every now and then resulting in the unknown, right?
Certainly, about 2+ years ago Exide battery recycle plant in Muncie had a failure in the building where the the batterys are ground up and the lead smelting begins with the air filtration system.
Instead of the vents to the outside closing when there was a problem they opened and flooded the surroundings with noxious gas.
All employees in the plant when working wear full face respirators. None do outside of the plant.
They went bankrupt and last summer were bought by a affiliate of Atlas Holdings.