The air filter room is next door to the air conditioning room and would clean up the air as it entered the installation. At least one large fan would draw the air in through the air intake at the top of the emergency exit and filter out any radioactive or other pollution before passing it on to the air conditioning unit. Plans were drawn up for the air filters to extract poison gas as well as particles but the delay in developing suitable filters meant that these were never actually installed. In emergency use the system was expected to be able to do a complete air change in the bunker every 10 minutes or so. The filter plant is no longer evident but there is a filter pack still under the floor.
The air intake with its canopies is still present at the top of the emergency exit blockhouse although it is all bricked up to prevent access. More detail of this structure, from the inside, appears later.
