Does the DIY purifier work? Test results (1, 2) give a conclusive answer, but this series of pictures gives a visceral answer. I took a picture of the HEPA about every seven days for four weeks. Glad that black gunk’s not stuck in my lungs instead.

In static form, here’s Week 0 and Week 5:

Thomas Talhelm

Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

Here’s what else you should know about:

What combination of air filters do I need to protect against everything?

How to Make a DIY Air Purifier

The Data-Backed Most Cost-Effective Office Air Purifier