Thanks for the article! Using incense for testing seems like a good approach. And yes, the lack of difference between "taped" and "non-taped" is definitely int

Thanks for the article! Using incense for testing seems like a good approach. And yes, the lack of difference between "taped" and "non-taped" is definitely interesting. I'm wondering if this might be in part because of your choice to use thin and small HEPA filters instead of the more common thicker and full size MERV filters. Intuitively, what matters is the product of the efficiency of the filter and the amount of air passing through the filter. It's possible that because the resistance of the small HEPA filters is so high, the unfiltered air passing around the filters generates a lower pressure in front of the filter than the non-moving air after the back is taped. I recently built up a couple similar box fan filters, but using 20"x20"x5" MERV 15 filters bought "used" off Amazon for about $20 each. Note that the total surface area of the folded filter matters for air resistance, not just the height and width of the filter. That is, depth matters, because all else equal the folds increase the amount of filter area. I sealed well, because I thought it would matter. I removed the back plastic grill to reduce resistance a bit more, and also added a cardboard cowling on the front of the fan, based on the research here: https://www.texairfilters.com/how-to-improve-the-efficiency-of-the-box-fan-and-merv-13-filter-air-cleaner/ https://www.texairfilters.com/how-to-improve-the-efficiency-.... The idea is that with a high resistance filter on the back, in the absence of cowling you get unhelpful recirculation from the front corners of the fan, although maybe this recirculation might actually be helping reduce pressure in front of the filter in your taped example? I haven't done any air quality measurements, but I think the system is working the way I'd expect. Noise at low speed is tolerable when in another room, but I only use higher speeds when leaving the house. I'd be interested in more details about your particulate monitor, especially if you are satisfied with it and if it's available in the US.