Issue link: https://www.e-digitaleditions.com/i/1518209
Inhalation April 2024 19 Scope Given the paucity of relevant data and the potential importance thereof, we undertook a laboratory inves- tigation to provide insight into how filter flow resis- tance can vary across several similar products available commercially. With these data, shown below, we then discuss ways this variability can be handled in practice by users of inhaler testing methods. We tested two cartridge filters common to the flow- rate-setting portion of DDU testing. In separate measurements, we also evaluated nine common, fiber-glass filter media offered commercially in many disc sizes. e discs relevant to inhaler testing have diameters of 25 mm and 47 mm and commonly serve the purpose of collecting the particulate mat- ter discharged from pMDIs and DPIs, respectively. Our selection of filter cartridges and disc filter media (Tables 1 and 2) was derived from our own experi- ences and also involved discussions with colleagues working in the inhaler testing community. We could not, practically speaking, exhaust all available filter products; but we have included enough examples to highlight variability in filter flow resistance in the common commercial offerings. We cannot and do not claim that this selection is representative of avail- able products. Furthermore, the study of these filters does not constitute a recommendation to use or to avoid these particular filter media for inhaler testing. We also are mindful of the complexity of the start-up kinetics of real-world DPIs when evaluated by the pharmacopeial methods [10, 11]. Our purpose was therefore to create an initial set of experimental data relevant to air flow in total dose and particle size mea- surements to quantify the variability among com- mon glass-fiber filters and to coach inhaler testers how to set up their own in-house testing methods for measuring flow resistance of these filters. e overall effort is aimed at reducing variability in inhaler test results to the benefit of patients and regulators. depending upon their surface charge [8]. is type of filter imposes an additional capture mechanism related to the charge sign and distribution on the filter fibers as well as the charge distribution associ- ated with the incoming aerosol particles. Whichever type of fibrous filter is used, its resis- tance to air flow is a fundamental property affect- ing its particle capture efficiency [9]. At the flow rates associated with inhaler testing, filter resistance is well described by the magnitude of the pressure drop at a given air-flow face velocity across the fil- ter [7, 9]. Filter resistance is especially important for the testing of DPIs following the procedures in the pharmacopeias because it affects the time- dependent flow rate/rise time profile that starts from zero flow initially to the final stable flow rate. e shape of this profile influences both aerosoliza- tion and subsequent transport of the particles from the DPI through the measurement system [10, 11]. e absence of flow resistance and particle capture efficiency specifications in the pharmacopeial meth- ods is likely a fair and reasonable situation for filter manufacturers whose products serve a wide spectrum of user applications. However, it places the responsi- bility on each member company of the inhaler test- ing community to choose carefully and validate filters for its particular product testing purposes. Given that there is also an absence of such data in the manu- facturer specifications for the filter products typically available commercially, practitioners in the user com- munity tend to start by making an educated guess about what filter will fit their purposes. As a result, developers of testing protocols often simply retain the filter products that they have tried in the past or have seen in vendor literature or heard described by col- leagues. is pragmatic approach leaves those evalu- ating inhalers vulnerable to a decision by vendors or manufacturers to cease offering certain specific filter products or to make subtle changes to material sourc- ing, depending upon supply and demand, without notifying end users. Product Name Manufacturer Vendor Specifications BB50T Breathing Filter Pall Corporation, Port Washington, NY, US (owned by Cytiva Life Sciences, a Danaher Company, Washington, DC, US) Betty Mills Company Incorporated 1 San Mateo, CA, US > 99.999% Airborne bacterial/ viral removal efficiency Pressure drop of 2 cm of water at 60 L/min DFC06 HEPA Filter Pentair Engineered Filtration, St. Paul, MN, US TSI Incorporated 2 Shoreview, MN, US HEPA Grade 1602300 HEPA efficiency, 99.97% efficiency for particles 0.3 micron diameter 1. https://www.bettymills.com/bb50t-breathing-circuit-filters-pall-corporation-gelbb50th 2. https://tsi.com/product-accessories/filter-375-inch,-fnpt,-hepa-grade-1602300/ Table 1 Cartridge Filters Tested