So far, I’ve been testing air purifiers by taking a baseline measurement of particulate pollution in a room, and then turning on the purifier and testing whether the counts drop. I’ve used that method to test the DIY and more expensive machines.
However, I recently bought a second laser particle counter, so my collaborator Gus suggested another method:
- Run one particle counter in the bedroom that has the purifier
- Run another particle counter in a different room that does NOT have a purifier.
Two Problems Solved
The benefit of this method is that the control room represents the counterfactual–what would have happened if we hadn’t turned on the air purifier. That can answer two types of “what if’s.”
1. Fluctuations in outdoor air pollution
If a northwest wind hits Beijing and makes the outdoor air a lot cleaner, we can separate the effect of the outdoor air fluctuations from the effect of the purifier. In that situation, my old method would artificially raise our estimates of effectiveness. Changes in outdoor air can also artificially lower our estimates of effectiveness if the outdoor air gets dirtier after we turn on the purifier.
In previous tests, I corrected for this by averaging over multiple tests. I also analyzed the data after removing days in which outdoor air pollution fluctuated a lot (for example, I do that sort of analysis in the extra nerd notes here).
But it’s always nice to use different types of tests to make sure an effect is real, so Gus did this experiment.
2. Particles settling in closed rooms
If you close a room, the particles–even really small particles–will slowly float down and settle out of the air.
Thus, if we’re testing in a closed room at night, how do we know the purifier is causing the reduction, and not particle settling?
The Control Room Purifier Test
Gus set up one particle counter in his room and one in his kitchen: 
He let the particle counters run for several hours, and then a timer turned on the Original DIY in his room. (The kitchen had no air purifier.) Here’s what happened:
The difference between the bedroom and the kitchen air quality can approximate the effect of the air purifier. It looks like Gus would have been breathing 16,000 of these 0.5 micron particles in the air in his bedroom if he hadn’t turned on his DIY purifier.
And it’s pretty clear that the kitchen air quality (where we don’t have a purifier running) is following outdoor air quality:
(Be aware that I’m overlaying these two lines on the same graph, but the Y-axes are different. This is NOT saying that indoor air is as bad as outdoor air. Indoor air is usually cleaner than outdoor air.)
Conclusion
Similar to earlier tests, the double particle counter test shows that the DIY purifier is removing particulate pollution from the air.
As always, I’m including more details for fellow data nerds below.
