They are probably storing compressed air in the cabin. You start the bioweapon-defense-mode, the fan raises the pressure in the cabin ( that uses outside air) a

They are probably storing compressed air in the cabin. You start the bioweapon-defense-mode, the fan raises the pressure in the cabin ( that uses outside air) and then shuts off. (Or they store a little bit of air somewhere, perhaps for the breaks, and use that.)