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Inhalation April 2024 23 the importance of testing under conditions realistic to inhaler testing and emphasizes the stark lack of air flow resistance measurements in the open literature. Two relevant questions for personnel who are testing inhalers should be whether one manufacturer's grade is the same as another and whether so-called "simi- lar sounding" filter types actually perform similarly. Some available filters clearly declare a "grade," such as 934AH, and others have a similar name without nec- essarily declaring its "grade," such as filters with an "A" in their product name (which we call 'type A' for the purposes of this discussion). To explore this issue fur- ther, in Figure 4, we have plotted the pressure drop/ face velocity results for the two filters named "Grade 934-AH" and also presented similar information for the three filters that appear to be of the same "type A" within a given manufacturer's product line. e two filters of Grade 934AH exhibited pressure drop values that were within approximately 10% of each other. However, the "type A" filters showed a wider range overall; specifically, the Tremont Grade A had approximately 25% higher flow resistance than the other two filters with an "A" designation (Whatman GF/A and Sartorius MGA). It is important to appreciate that we are not implying by these data that the filter product manufacturers may have misrepresented performance characteristics HEPA-grade cartridge filter offered by TSI Incor- porated (Figure 3). e pressure drop for each filter followed a similar, slightly parabolic shape. In this second-order poly- nomial curve fit (equations 1 and 2), the linear com- ponent was dominant: Pall Breathing Filter ∆P=2.2231 * Q * (1+0.00724 * Q ) (1) TSI HEPA 1602300 Filter ∆P=14.598 * Q * (1+0.00426 * Q ) (2) In Figure 3 and these equations, the pressure drop has units of Pascal, and the flow rate, Q, is the flow rate at the inlet to the filter and has units of L/min. We observed that the pressure drop of the Pall breathing filter measured at 60 L/min, 191 Pa, was quite close to the sole pressure drop figure given in the Pall spec- ifications of 2 cm H 2 O gauge at 60 L/min (Table 1; 2 cm H 2 O = 196 Pa). Filter disc media We observed that for the nine types of glass-fiber disc filters tested, the measured pressure drop could be fit readily to a second-order polynomial (equation 3), much like those for the cartridge filters. We chose the face velocity as the independent variable, rather than volumetric flow rate, to allow the results to apply more readily to filters made of the same material but with a different disc diameter than the 47-mm- diameter discs we tested: ∆P=A * F v * (1+B * F v ) (3) e Tremont Grade 934-AH had the lowest flow resistance and Tremont Grade A had the highest flow resistance (Table 3). ere are two filters where pressure drop values are given in the manufacturer specifications (Table 2), namely the Tremont grade A-E and the Pall 61631 A/E filters. Neither of the face velocities in the man- ufacturer specifications are in the range that we tested nor in the range encountered in inhaler testing. Nev- ertheless, curiosity demanded that we compare our data to the manufacturer specifications by extrapolat- ing to the face velocities mentioned by the manufac- turers. For the Tremont A-E filter, the manufacturer gives a pressure drop of 360 Pa at a face velocity of 5 cm/s. e pressure drop calculated from equation 3 for this filter at a face velocity of 5 cm/s is 295 Pa. For the Pall 61631 A/E filter, the manufacturer gives a pressure drop of 70 kPa at a face velocity of 1,000 cm/s. e pressure drop calculated from equa- tion 3 for this filter at a face velocity of 1,000 cm/s is 175 kPa. So, this "necessary" exercise of comparing to the manufacturer specifications simply points out Figure 4 Pressure drop-face velocity profiles comparing Grade 934AH and "A-type" disc filters 1 Tremont Grade A 2 Sartorius MGA 3 Whatman GF/A 4 Whatman 934AH 5 Tremont 934AH 1 2 3 4 5 Pressure Drop (Pa) 25000 20000 15000 10000 5000 0 Face Velocity (cm/sec) 0 50 100 150 200