Comment Umm except they did (Score 1) 182
You can see it right in the Slashdot thread where people link to Youtube videos of this being done a long time ago. The idea of "stick a filter on a fan" isn't new because that's PRECISELY what air filters are. They don't make any bones about it.
So, why doesn't everyone do it? Well because it turns out DIY isn't much cheaper if you want it to work well. When I first heard of the idea, on Youtube, I said "let's try that!" Went and got out one of my box fans, bought a filter at Home Depot, and taped it all together. Well I quickly discovered two flaws with this system:
1) Furnace filters aren't near as good as HEPA filters, particularly not the multi-stage system good units have. It did reduce particle count, but not a ton. I could buy better filters, for sure, but then the cost goes up. Buying a filter with a large surface area like good units have gets quite expensive, much like the filters they have themselves.
2) A regular fan is not well suited to the static pressure you get trying to push through a filter. It had very little airflow. Better sealing would have helped some of that, but of course that's more money and effort, but part of it is you just need a good fan. That is again one of the things that the high end units have. They have a fan particularly made to deal with high static pressure, and a case made force lots of air through the filters.
So easier, and not really any more expensive in the long run, to just buy a filter unit. On the cheaper end of things, there's Austin Air units. Basically you pay $250 for a metal box with a powerful fan in it, and another $250 for a huge filter. Every 5 years or so, you pay another $250 for a new filter. Not the world's best filtration stats, but pretty good, better than furnace filters, and cheaper when you look at how long it lasts. At the high end that's IQAir. $1000 for a unit, but the unit and filters last a long time. I particular their HEPA filter can last a decade or more, provided you replace the prefilters when it tells you to.
That's the thing here: This is NOT a new idea, people were talking about it online a minimum of 6 years ago, and it is also not a hard idea since it is literally doing what the filters do. They are simple devices, they have diagrams of how they work on their websites. The cost is in a good fan, and good filters.
Finally, with regards to particle count, you need to be careful that you specify what size and have a counter that can deal with it. One of the reason IQAir units cost so much is they filter extremely tiny particles, like clean room level, viruses and so on. That is much harder to do than larger stuff. Now maybe you don't care, ok fair enough, but don't try and act like it is the same level of performance. You can very well get cheaper air filters that don't filter as well, or as small a particles.