Simply strap a HEPA air filter to a desk fan for $30:
I tested it over and over—hundreds of days, with a control room, with a stronger fan, against the big brand purifiers that I borrowed from my rich friends. Eventually, I saw enough data that I was convinced. This $30 DIY purifier was removing significant amounts of tiny particulate from my Beijing bedroom. I wanted to tell the world that those $1,000 purifiers were ripoffs. I made all the data and testing methods open source. I wrote up the instructions for how to make one.