I hooked up the return grill near the cat's litter box to a HEPA filter with a MERV 11 prefilter (the manufacturer stopped making the MERV 12 claim). Guess what the MERV 11 prefilter didn't catch? The HEPA filter is now a strong yellow color... I'm only mildly allergic to that cat, but that's stuff I didn't need to breathe deep into my lungs. The small particles caught by the HEPA filter would have otherwise stayed suspended in the air for a long time...