When you can't get house parts, try auto and computer parts.
Get some 120mm fans (or an array of 80mm, etc), and some engine air filters.
Don't change the filters too often:
"It is common, however, for servicing to occur when the filter appears dirty. Engine air filters are designed to actually increase their efficiency by using this initial layer of dust as an added filter layer. Initial filter efficiency is usually approximately 98% but increases to more than 99% by the
end of the service life of the filter.
Therefore, changing an air filter before the useful service life is achieved can result in premature engine wear."
Thank you for bringing this up ... in industrial settings, filters are "seasoned" for some period before they can be expected to perform at their published ratings.
Many air and water filters' performance continues to rise over their lifetime - the downside being decreased flow rate.
If you are filtering infectious agents with a water filter, I would be reluctant to run them past their lifetime because you could have things living in the filter and filter housing, etc., but if you're just dealing with particles, your filtration should continue rising over time.
I did this a couple years back. 2x 120mm fans and a rolled up filter in between with a cardboard box as the frame. I never thought it could do much, but also never left it running very long.
I've been begging for a good use of old computer fans...
Get some 120mm fans (or an array of 80mm, etc), and some engine air filters.
Don't change the filters too often:
"It is common, however, for servicing to occur when the filter appears dirty. Engine air filters are designed to actually increase their efficiency by using this initial layer of dust as an added filter layer. Initial filter efficiency is usually approximately 98% but increases to more than 99% by the end of the service life of the filter.
Therefore, changing an air filter before the useful service life is achieved can result in premature engine wear."
https://www.fueleconomy.gov/feg/pdfs/Air_Filter_Effects_02_2...