An HME (heat and moisture exchanger) (10) having a septum (or filter) (15) formed by pleating a sheet (150) of material capable of retaining and releasing heat and moisture. The HME (10) is characterized in that walls (15a, 15b, 15c) of the septum (or filter) (15) are coated partly or completely with sealing material to define at least two exposed areas (A1, A2) through which inhaled gas (F1) and exhaled gas (F2) flow respectively. Moreover, each sheet (150) of material is divided into identical units (150a) "fan-folded", in use, one on top of another to form a first set of passages (150b) through which only the inhaled gas (F1) flows, and a second set of passages (150c) through which only the exhaled gas (F2) flows.
