Abstract: An air filter unit of the HEPA type having a filter core of zig-zag folded media enclosed by a four-sided frame having two side casings and two end casings. Each end casing includes a recess extending into the inwardly facing surface thereof and bounded on all four sides by marginal edge portions of the end casing. The end flaps of the folded media are adhesively secured to the inwardly facing surfaces of the side casings and the recesses in the end casings are filled with an adhesive initially in a flowable state and hardenable to a solid state. The zig-zag edges of the folded media are disposed entirely within the adhesive, as are tenons extending from the ends of the side casings, thereby forming a mortise type joint between the side and end casings. A plurality of channels, preferably of dove tail cross section, extend across and communicate with the recesses in the end casings.