how to choose best air filter air purifier guide

Four Steps To Choosing The Best Air Purifier

“What’s the best air purifier on the market?” It’s a question that many of us might ask. There are thousands of air purifier models to choose from, so how can you find the best air purifier that’s right for you? This article covers those steps in a slightly unorthodox way. What’s … Read more

New DIY Purifier Design Increases CADR 15%

After I made my first DIY purifier, a design magazine called me up to interview me. I said, “You realize this has no design in it, right? It’s just a fan and a filter.” Now I actually have a design improvement! I’m calling it the DIY 1.1.  The Problem The … Read more

Activated carbon air purifier VOC

Does Adding a Carbon Layer Reduce Particulate Effectiveness?

I recently published tests showing that carbon actually removes VOCs. But adding that carbon layer raises a question: adding carbon means the fan has to power through yet another layer of resistance. Does that make purifier less effective at removing particulates?

Test Method

To test this question, I ran 10 room tests with the Cannon and 10 tests with the Original DIY in the same 15m2 Beijing apartment as my earlier tests without a carbon layer.

 

Dylos particle counter pollution test

 

I measured how much particulate it removed with a Dylos particle counter and compared the particle counts (1) before I turned on the purifier at night and (2) the average of the last four hours before I woke up in the morning.

 

Air purifier test laser particle counter

 

Carbon vs. HEPA Results

With the additional charcoal layer, the Cannon particulate effectiveness dropped 1-2%. Thus, charcoal has a very minor negative effect on the Cannon.

DIY Air purifier test carbon filter HEPA

However, the Original DIY had a harder time powering through the extra layer. Its 0.5 micron effectiveness dropped 19%, and 2.5 micron effectiveness dropped 15%.

DIY purifier activated carbon test

Conclusion: Carbon Tips

For people who need carbon (and that may not be everyone), I would recommend adding the carbon to the Cannon, but I would think twice about adding carbon to the Original. One workaround is to have one Original fan running with a HEPA and another running with a carbon filter.

As always, I’m posting the raw data and more details on the test for fellow nerds below.

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Thomas First DIY Air Filter Setup

A New Way to Test Whether the DIY Works

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.

DIY Purifier Test Mongolia PM2.5 Dylos

However, I recently bought a second laser particle counter, so my collaborator Gus suggested another method:

  1. Run one particle counter in the bedroom that has the purifier
  2. 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.

 

Particle settling speed velocity

 

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: 1

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:

DIY Purifier Test Control Room

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:

Indoor Air Pollution PM2.5 Versus Outdoors

(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.

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DIY Cannon: Better, Faster, Stronger

Ever since I started my DIY filter experiments, I’ve been wondering whether I could create a super DIY by using a stronger fan. In the past three months, my collaborator Anna has helped me test a half dozen different fans. She now has a home full of fans:     … Read more

PM2.5 Particle Size Virus

The Really Small Stuff

An open-data DIY air purifier test found that even simple, homemade purifiers can reduce particulate, but can they capture the smallest particles? This question is important because it’s the smallest particles that can penetrate deep into our lungs and even enter our bloodstream. To test how effective the DIY purifier … Read more

Particle Counting in the Atlantic

Particle Counting and the DIY workshop made it into the Atlantic. Hooray for getting the word out about how to protect your health in China without shelling out big money.     More workshops are in the planning stages. Stay tuned.

How to Make a DIY Air Purifier

How can we make a homemade DIY air purifier?  Air purifiers can be extremely expensive.  But they don’t need to be. After researching how different air filters work, I concluded that a HEPA filter is all that’s needed to make an effective air purifier. Given that most air purifiers sell … Read more

Smart Air is a social enterprise that creates simple, no-nonsense air purifiers and provides free education to protect people’s health from the effects of air pollution. We are proud to be the only certified B-Corp dedicated to fighting air pollution.