A PhD student and Fulbright scholar based in Beijing has a great little Tumblr blog that shows, with data, that you can skip the expensive air purifiers and make your own for about $30.
This post goes through the details of how to make the air purifier, and where to buy a proper filter. The principle is embarrassingly simple: Buy a typical box fan, stick a HEPA filter on the front of it, breathe clean air — no kidding.
From the blog: From my perspective, filter companies like IQ Air are taking advantage of how little we know about air pollution and the fact that you need expert instruments to tell whether the filter is working or not. When consumers don’t know how to assess the products we buy, we often use price to tell us whether the product is good. That happens with expensive wines all the time. I’m convinced you can breathe safe air in China for far less than filter companies want you to believe.
In a follow-up video, the author of the blog clearly illustrates that the air coming out of the filter is virtually absent of particles 2.5 micrometers and above (the stuff most often measured when you see those PMI 2.5 ratings in the news). It also greatly reduces 0.5 micrometer particles.
But it’s one thing to measure the air quality coming out of the filter, and quite another seeing if it effectively scrubs the ambient air in an apartment. The short answer: yes, yes it does. The longer (more chart-happy) answer here.
Not only does the DIY air purifier perform well, tests indicate that it may actually out perform the expensive alternatives — at the very least it is just as good at a fraction of the cost.