Abstract: A modular air cleaning system having at least one filter module having a closed plenum box provided with male and female air flow porting structure, one of which porting structure provides an air inlet and the other of which provides an air outlet, each of the porting structures is in substantially cylindrical form and projects outwardly from the box and each has a section of substantially the same diameter lying adjacent the box, a shoulder on each of the sections peripherally circumscribing the same at a short distance outwardly from the box and adapted to engage end portions of a flexible hose for preventing its withdrawal from the sections, the male porting structure having a reduced diameter segment lying outwardly of the adjacent section and adapted to telescope within the female porting structure of another plenum box, and an annular seal within the female porting structure adapted to engage the segment and form a substantially gas-tight seal thereagainst.