conditioning equipment 40 has an air inlet 54 in communication with the plenum 52 and draws air into the air conditioning equipment such that a reduced air pressure is created in the plenum 52 forming a negative air pressure plenum the negative air pressure within the plenum draws air from the room through the air ducts and plenum into the air conditioning equipment as shown by arrow 56 once conditioned the air is directed through air duct 60 to distribution tubes 62 where he air passes through the hepa filters 50 and back into the clean room the negative pressure plenum above the clean room helps prevent any contamination from entering the clean room through the ceiling since the interior pressure of the clean room is higher than the air pressure in the plenum if any air leaks were to exist in the ceiling the direction of air flow would be from the room interior to the exterior if a negative pressure plenum above the ceiling is not desired it is possible to connect the air conditioning equipment intake 54 with the side wall module air ducts through an enclosed duct such that the return air from the clean room does not freely circulate in the space between the ceiling and support deck the lower end of a module 18 is shown in fig2 with a vertical opening 64 through inner panel 24 for the air intake 66 shown in fig3 the air intake 66 serves as an air scoop having a vertical air intake opening.