An in-line modular air filtering system for ductwork. A housing formed as a body and one or more caps has an intake opening at a first end and an outflow opening at a second end. The cap and body may attach to one another at a junction containing a seal with a stepped cross-sectional configuration. A set of retaining clamps may be used to secure the cap on the body. One or more replaceable filter assemblies may be retained in the housing for filtering air passed therethough. A filter assembly may include a seal having the stepped cross-sectional configuration for securing in the housing and sealing the cap and body junction. Filter assemblies may have differing porosities and/or different filtering mechanisms and may be stacked for applying multiple treatments.
