Abstract: A filter unit having a central member of substantially rectangle cross section and made up of one or more interconnected sections and having ends slidably fitting into end members of the same cross section. The central member is formed with ventilating apertures in front and rear walls thereof through which air can flow, and a filter medium is positioned in each section so that the air must flow therethrough. Filler strips are formed on and extend the length of the upper and lower edges of the central member and slidably fit in slots in the upper and lower edges of the end members so that said unit has continuous upper and lower edges in common upper and lower planes and extending the length thereof.