[dredmorbius]

The HEPA filters are the expensive component. Adding more filters doesn't improve net effectiveness.

Cardboard is the vastly more cost-effective option. And taping a HEPA filter directly to the back of the fan effective and proven.

[sliken]

Actually it does. You'll get higher CFM through the system and it will take 5x longer to saturate the filters with dust. The cost per month will actually be similar, granted you'll have a higher startup cost with 5 filters instead of 1.

[dredmorbius]

The time-to-saturate is a wash when you factor in per-filter rates. You're achieving a 4x longer life per filter, but multiplying that by the 4x filters you are using.

The net filtration rate per filter will fall as that's a function of flow rate * filtration size.

Yes, the four- or five-filter version will probably have a slightly better overall filtration rate ... but that's really not the principle constraint here. A single-filter design will drop particle density by ~90% in about 30 minutes. That's sufficient. It's doubtful the final particle density measurement will be much reduced (though if someone's got the data I'd be interested in seeing it), or that this reduction is clinically significant from a health perspective. Remember that you're likely going outside the filtered area, so achieving perfection inside but spending a substantial fraction of your time in unfiltered or far-more-poorly-filtered air, isn't much of a win.

Meantime, you're tying up 3-4 filters that you don't really need, and which others could make effective use of, for a very low marginal imrovement in your own experience.

Where the filters themselves are in low supply -- typical for a region where a wildfire has errupted, and particularly true where filters aren't a high-volume item as with Australia presently -- designs which economise on filter use are a net social benefit.