Issue link: https://www.e-digitaleditions.com/i/1518209
24 April 2024 Inhalation Filters designated "A/E" as part of their product name is another situation in which users might wonder if filters are interchangeable. In this context, Figure 5 shows that the Tremont Grade A-E filter and the Pall 61631 A/E Glass Fiber Filter have substantially different flow resistances. A superficial inspection of published micrographs reveals that the two filter products appear to be quite different in terms of fiber density (Figure 6). However, without knowledge of their flow resistances, because the names are quite similar due to use of the letters "A" and "E" that are likely to be interpreted primarily as designating qual- ity, a user might expect similar flow resistance behav- ior. Our data, however, show that such assumptions can be rather misguided, at least with respect to air-flow resistance. We therefore encourage users to determine filter air-flow resistance as part of qualifi- cation, especially if a change of filter supplier or type designation takes place. e apparatus for testing flow resistance is simple (as shown in Figure 1 and its description) and is within the technical abilities of all practicing inhaler testing laboratories. Effect of the means of holding the filter Roberts [12] previously reported the pressure drop profiles of four of the filter media tested in the cur- rent study, namely the Whatman Grade 934AH, the Tremont Grade 934AH, the Tremont Grade A-E and the Advantec GA-55 filters. However, these tests took place in a reduced Next Generation Impactor configuration (rNGI). e measurements were also over the more limited flow rate range of 30 L/min to 60 L/min. Roberts reported that the pressure drop profiles could also be fitted to the second-order polynomial described by equation 3. Table 4 shows the curve fit parameters "A" and "B" for the current test data and for these previous test data. Note that the pressure drop for each fil- ter media is smaller when the media is tested in a DUSA as opposed to the rNGI configuration. e higher flow resistance in the rNGI configura- tion is a strong argument for the hypothesis that in their published specifications. However, we observe that often manufacturers are silent on the subject of air flow resistance, a variable of importance, especially in connection with DPI testing. e user therefore remains uncertain as to the impact of the contribu- tion from filter resistance to the accuracy of his or her inhaler performance measurements. Given these cir- cumstances, it is a logical outcome that current users would choose not to change suppliers or filters for fear of introducing some unexpected testing results. Fur- thermore, in addition to the risk of non-availability for any particular filter product, there is the potential for added cost if out-of- specification inhaler results arise that are linked with filter resistance variability following an investigation for cause. In this explor- atory study, we have not examined filter resistance variability, an effort that would require the testing of a statistically meaningful number of discs taken from any given box of filters. Figure 5 Comparison of Grade A-E and A/E filters Pressure Drop (Pa) 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0 50 100 150 200 Face Velocity (cm/sec) Pall 61631 A/E Tremont Grade A-E Figure 6 Approximate fiber density of Tremont Grade A-E and Pall 61631 A/E glass-fiber filters Tremont Grade A-E Pall 61631 Grade A/E