But with only 75-85% infectious aerosol capture efficiency, how can MERV13 furnace filters in Corsi-Rosenthal boxes often outperform "True/Medical" HEPA H13 that have 99.97% capture efficiency even for the most challenging .3um particles?
A proper diagram can help visualize both CADR & filter efficiency. The square polka-dot patch stuck to each filter represents the total # of particles removed, and blue outflow arrows represent removal rate. Due to large filtration area and least restrictive airflow, the MERV13 pair capture more total particles leaving 250 cfm cleaned outflow stream. MERV13 do leave a substantial (red arrow) output portion dirty, but this just remixes back into the room like a bypass for 25% of the inflow.
The two HEPAs leave really tiny slivers of red contamination in their output streams, but because of the lower filter area & restricted airflow, their blue cleaned outflows, or CADR, are also smaller (e.g. 120cfm for H12 and 100cfm for H13).
The recent controversy over whether Levoit Core are "True" HEPA H13, in a complaint brought by Dyson to BBB, is really an argument about whether Levoit's output contamination is the 0.03% super tiny red sliver, or the 0.5% really tiny red sliver.
For room cleaning, both slivers are going to remix into an already contaminated room within a meter or two. Compared recurring contaminants like ongoing breathing, both slivers are negligible. What matters to most consumers is that Levoit's blue output CADR is relatively large at low noise & power, meaning it's capturing a lot of particles quickly. Dyson's complaint is thus a gotcha over Levoit's poor quality control around a HEPA efficiency spec that some companies heavily market, but doesn't matter for everyday room applications. (It does matter for sterile semiconductor cleanrooms & medical laboratories HEPA was invented for).
Dyson (& others like Alen) tend to make flashy but not very large H13 99.97% purifiers. They don't report official CADR --because theirs are surprisingly low?! To reach CR box competitive CADRs quietly, HEPA H13 need high RPM (loud) fans, or filters as big as a small file cabinet (e.g. SmartAir Blast or Coway Airmega ProX). And then they cost >> $500.
For a personal HEPA within a meter of your face, the tiny red slivers of HEPA E11/H12/H13 ARE a potential advantage over MERV13. A HEPA can create a bubble close to the fans much purer than the MERV13s that leave 25% of contaminants each pass. Past a meter from the fans, room remixing erases this advantage. Tiny personal HEPAs (< 10cfm) like the Pure Enrichment don't have the cfm to make a substantial clean bubble anyhow. The SmartAir QT3 begins to with 25cfm on high, holding a 35% reduction 2-3 feet from the fan: https://smartairfilters.com/.../qt3-portable-air.../
Flow collimators can help focus the airstream so the purity bubble is narrow and projects slightly farther from the fans. We are experimenting with these for the Exhalaron.
In conclusion, for room purifiers, filter grade & capture efficiency for the target particle size (Smoke, Dust/Virus, or Pollen) don't matter much, while CADR is key. Lower filter grades like H12, E11 or MERV13 can usually reach higher CADR with less noise and energy than "true" HEPA H13.
For personal HEPAs that may be directed at the face, MERV13 may be less appropriate. E11, H12, or H13 grades can create a purer local bubble, assuming their airflow is sufficient and laminar enough to project the purer air the desired distance in front of the fans.