I live next to a busy dual-carriageway and until recently, the windows haven't exactly been what you would call sealed. This means that the various particulates emitted by vehicles can make their way into my home. Corralling them so we don't inhale that nastiness is a good thing.
I've been interested in getting an air filter for a while, but they have a price point that is too high for the function they provide. However, recently Big Clive posted a video about a set of 3d printable adapters for cylindrical filter elements that you can just fix a standard computer fan to. As I have a 3d printer, I set about the task.
You can see that a few months of more or less 24/7 filtering has pulled a lot of junk out of the air.
Now that the windows have been replaced with actually viable frames, they should cut down on a lot of the crud that makes it inside.