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Both the inhaler testing community and manufacturers of the associated equipment work diligently to reduce sources of testing variability [1, 2] because these efforts ultimately benefit patients by improved product reliability and bene_t regulatory agencies by greater assurance of safety and efficacy [3]. 18 April 2024 Inhalation Introduction Both the inhaler testing community and manufac- turers of the associated equipment work diligently to reduce sources of testing variability [1, 2] because these efforts ultimately benefit patients by improved product reliability and benefit regulatory agencies by greater assurance of safety and efficacy [3]. Careful improvements to the methods provided in the phar- maceutical compendia [4, 5] in the past decade have assisted with these efforts by capturing concepts such as solenoid valve opening time, preferred short tub- ing lengths, and a lower limit on the flow resistance coefficient for the flow-regulating valve. However, oddly missing from the pharmacopeial compendia and from the archival literature are flow resistance data for the glass microfiber filters that are common to the delivered dose uniformity (DDU) testing protocols described in the United States (USP) [4] and European (PhEur) Pharmaco- peias [5]. Likewise, this information is absent from descriptions of historical cascade impactors (CIs) that make use of a so-called "back-up" filter to cap- ture any remaining particulate matter passing the last impaction stage. ese impactors include the Andersen CI variants [4, 5] and the reduced (abbre- viated) impactor configurations that are based on the ACI geometry [6]. Indeed, chapter <601> of the United States Pharmacopeia specifies neither fil- tration particle efficiency nor filter flow resistance. Rather, its guidance simply states that the filters acceptable for testing the total dose of pressurized metered-dose inhalers (pMDIs) and dry powder inhalers (DPIs) are fabricated from glass fibers, stainless steel fibers or microfiber polypropylene [4]. Microfiber filters function by a combination pri- marily of interception and inertial capture for particles larger than about 0.3 μm aerodynamic diameter and primarily by Brownian diffusion for smaller particles [7]. Electrostatic (electret) filters have intrinsic electric charges associated with their fibers that enhance capture of particles of all sizes, Variability in air-flow resistance of filter cartridges and glass-fiber filter discs common in the testing of inhalers Laboratory data at representative inhaler testing conditions Daryl L. Roberts, PhD a and Jolyon P. Mitchell, PhD b a Applied Particle Principles, LLC b Jolyon Mitchell Inhaler Consulting Services Inc. Nomenclature F V – face velocity; the volumetric flow rate immediately upstream of the filter under test divided by the area of the front face of the filter (cm/sec) Q – volumetric flow rate immediately upstream of the filter (L/min) ∆P – pressure drop across a filter cartridge or filter disc (Pa)