X-ray Microscopic Inspection of a Multistage HEPA Filter

With the global COVID-19 pandemic and the recent focus on preventing and limiting the spread of respiratory diseases, we chose to focus this month on the examination of multilayered air filtration media. The filter studied in this work is intended as a home HEPA filter capable of reducing contaminants in small rooms. In addition to the HEPA filter, a pre-filter and activated carbon filter are also present within the sample. The pre-filter serves to remove large contaminants — thick dust clumps, hair balls, and other organic matter — prior to reaching the HEPA filter. The large porosity in the pre-filter allows small particles to pass through with ease while the larger particles are prevented from fouling the much less porous HEPA filter. This pre-filtration allows for a longer service life for the filter through reduced contamination of the HEPA layer. The activated carbon layer is added to adsorb organic odors within the home, deodorizing the air as part of the purification process. Finally, the HEPA filter is designed to remove most contaminants larger than 0.3µm and represents the final portion of the filter device the air encounters before being recycled into the room.

X-Ray Microscopy Imaging of Air Filtration Media

When it comes to examination of filtration media, a choice is often required as to what is the best scale for the imaging and analysis to take place. For our work this month, we chose to work at two different scales to examine different features of the filter. First, we took an overview of a subsection from the entire filter assembly using the SkyScan 1273. We next utilized the high-resolution imaging possible on the SkyScan 1272 to examine a single porous region from an activated carbon pellet.  As can be seen from the digitally cut view in Figure 1 above, the SkyScan 1273 collected detailed information on each of the components combined into the multilayered filter assembly.

Figure 2: Volumetric rendering of a filter assembly subsection showing the pre-filter

As shown in Figure 2, the thin, highly porous pre-filter is the first layer air reaches upon entering the filter. We observe many contaminants present in this outermost layer of the filter, as expected.

Figure 3: Clipped volumetric renderings of the filter assembly, showing the sample without the pre-filter (top) and a cut view from within the activated carbon region (bottom)

Using CTVox, we can digitally cut away the pre-filter layer to reveal the underlying structure (Figure 3). The next region within the filter assembly is a polymeric honeycomb style mesh containing activated carbon pellets. A thin mesh is visible, used to restrict the activated carbon pellets into their respective honeycomb style holes within the larger plastic mesh. It is interesting to note that the pellets are free-flowing within each hole and do not fill the entire volume.

Figure 4: Clipped volumetric renderings of the filter assembly showing the sample without the pre-filter and activated carbon layers

Digitally removing both the pre-filter and activated carbon filter, we are left with the pleated HEPA filter shown in Figure 4. To increase surface area, the manufacturer pleated the filter by introducing regularly spaced folds. Higher surface area allows more air to be filtered in a shorter time period and also may extend the operational lifetime of the filter. From this view, we are looking at the surface where the air first makes contact after leaving the two previous filter layers. We observe a buildup of contaminants on this surface as well.

Figure 5: Colorized volumetric rendering of the pleated HEPA filter

Adding in some color to the image allows us to better visualize the presence of contaminants trapped by the HEPA filter during several months of operation (Figure 5). From this view, small orange spots are present on the inflow side of the filter that air strikes after moving through the pre-filter and activated carbon filter. These same orange spots are not present on the outflow side of the filter, which demonstrates the efficiency of the filter in removing contaminants of this scale.

Figure 6: Photograph of fouled filter section imaged in this study

Our observations from micro-CT imaging are in line with our expectations based on the visual appearance of the sample (Figure 6). Obvious contamination is visible within the pre-filter while the HEPA filter still appears clean on the outflow side.

Figure 7: Colorized volumetric rendering of the pores (orange) within an activated carbon pellet (gray)

After removing one of the activated carbon pellets from within the central region of the filter, we imaged the pellet at remarkably high resolution using the SkyScan 1272. While the SkyScan 1273 is our largest and most powerful desktop instrument, the SkyScan 1272 provides the highest resolution in our desktop series. After imaging a region of the pellet with a voxel size of 700nm, we used CTAn to map the pores and calculate their size distribution. CTVox then allowed us to overlay the color-coded pore data onto the original scan data to produce Figure 7. Larger pores are indicated by darker orange colors while smaller pores show up in lighter orange shades.

Figure 8: Calculated activated carbon pellet pore distribution from SkyScan 1272 data

From the analysis results, a quantitative map of pore distribution was produced (Figure 8). The average pore diameter was determined to be 8.7µm and the total porosity was 31% with 95.5% of the pore volume open and accessible to adsorb odors from incoming air.



For this work, the SkyScan 1273 allowed us to quickly image an assembled filter structure and differentiate between each of the individual filter components with great detail. We even are able to locate contaminants which were filtered out by the final HEPA stage filter. The SkyScan 1272 allowed us to examine one specific component, an activated carbon pellet, in great detail with a voxel size of only 700nm. This fine detail image unlocked the ability to examine the porosity of the carbon pellet. We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our image of the month series next month.

Scan Specifications


Sample Multilayered Filter Activated Carbon Pellet
Voltage (kV) 40 50
Current (µA) 300 200
Pixel Size (µm) 22 0.7
Rotation Step 0.2 0.1
Scan Time (HH:MM:SS) 02:50:36 11:07:07

These scans were completed on our desktop SkyScan 1273 and SkyScan 1272 systems at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using Dataviewer, CTVox, and CTAn.

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