Air purifier manufacturers use odd shaped filters to force lock-in. Those proprietary filters are also expensive. And of course they're constantly going out of

Air purifier manufacturers use odd shaped filters to force lock-in. Those proprietary filters are also expensive. And of course they're constantly going out of business, since it's consumer electronics, so there's no guarantee at all you can even buy that filter a year from now. (And it's not like the purifier itself is cheap either! The Blue Air purifiers I see people on here talk about start at two hundred bucks. Ten times more than a box fan.) Furnace filters, on the other hand, are standardized and have many vendors. I wouldn't be surprised to see 20x20 filters still being made a century from now.