Particle Counting

1. How to make a filter for 166 RMB.
2. Tests showing that the DIY cannon removes as much PM 2.5 as a Blue Air.
3. Live demonstration.

Need clean air?

1. Attend a workshop.
2. Get a kit delivered.

About me.

DIY Workshop: Saturday in Wudaokou


The good folks at Ricci Cafe in Wudaokou are hosting a DIY air filter workshop from:

11:00am-12:00pm this Saturday, April 19th 

(Correction: The workshop has been moved from 1pm to 11:00am due to a scheduling conflict.)

Stop  by and enjoy a banana pancake. (Gus says they’re great.) Then learn how air purifiers work and how you can build your own. Participation fee of 200 RMB includes the cost of your own DIY purifier, so you can take it home and breathe easier right away.

Sign up to guarantee a spot (limit 25 people).

这周六11-12点,Ricci Cafe要帮我们办DIY净化器活动。Ricci Cafe在清华附近的清华科技园科技大厦D坐1层。


Ricci Café is located at 1F Block D, Tuspark, Wudaokou.



Nerd Christmas

What makes a nerd happier than a new particle counter?


I’ve been busy playing with my fancy new particle counter, the Aerocet 531S made by the same people who make the huge government machines

It’s not cheap! About US$3,000. But it estimates mass, so we can estimate AQI. That means our numbers will be easier to understand. 

But first, Ted is busy standing outside the US embassy testing the numbers against the US Embassy’s numbers. That way we’ll know if it’s accurate. 

Scan this with your WeChat and you can add the official Smart Air WeChat account! I’ll post updates about workshops, new machines, and new data.

Scan this with your WeChat and you can add the official Smart Air WeChat account! I’ll post updates about workshops, new machines, and new data.

Smart Air WeChat

Anyone who knows me knows that I’m a Luddite. I bought my FIRST smartphone last August.

So we should be all the more surprised that I just created an official Smart Air WeChat account! Add me, and I’ll keep you up to date on all of our workshops, new tests, and projects. 

ID: SmartAir聪明空气


Smart Air wants to hire a fluent Chinese speaker to be a full-time workshop host. Successful applicants would help people build purifiers and spread the word about affordable clean air in Chinese. It is a full-time, salaried position, including travel. We’re looking for outgoing, smart, and independent people.

-Fluent Chinese

-Good at public speaking or willing to learn

-Full time, would enjoy traveling for workshops in other cities

If that sounds like you or someone you know, email a resume to me at talhelmt (at gmail) or message me on WeChat: talhelmt.


(Photo courtesy of James Le)

Smart Air在找中文DIY净化器workshop的主持人。中国人或者是中文特别好的外国人都行。带薪全职主持人会负责主办workshop,帮助大家了解怎么得到干净空气。包括旅行去其它城市主办workshop。成功的申请者将是外向,smart,独立和擅长于演讲的人。
如果感兴趣,请给Smart Air原始nerd Thomas发简历:talhelmt(at或者微信:talhelmt.

Independent Tests

Beijing-based Dr. Saint Cyr’s tests of air purifiers were one of my original inspirations for the whole DIY project, so I was happy to see that the DIY recently became a part of those tests: 


These are the first independent tests of the DIY, and the results parallel mine. You could also include the tests by doctors at the University of Michigan as independent  "proof of principle," although they used a different fan and filter. The commonality is that all of the tests have shown that a simple filter and a fan can reduce particulate pollution in the home. 

Dr. Saint Cyr’s review isn’t all glowing. He rightly notes that the cannon is noisy, which I’ve also written about (decibel counts and comparisons here). Tests show the cannon is still very effective on the lower settings, so I recommend running the cannon on the somewhat quieter settings. And for people who are sensitive to noise, I recommend the quieter Original.

Saint Cyr writes that we’re still in the early rounds of our design, and I think that’s right. In fact, we’re working on a new model that keeps the high performance with less noise. I’ll be posting more data on that in the coming months.

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.

However, I recently bought a second particle counter, so my collaborator Gus suggested another method: run one particle counter in the bedroom that has the purifier, and run another particle counter in a different room that does NOT have a purifier. 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.

Thus, 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. He 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 PM .5 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. 

Read More

Does Chinese New Year Affect Inside Air?

I’ve posted data before showing that outdoor air quality is strongly correlated with indoor particle counts (r = .71), but Chinese New Year gives nerds like me a great chance to see what happens when we get a momentary shock to air quality. 

The media made a big deal about people cutting back on fireworks this year out of a concern for air quality, and that may be true, but you can still see a strong spike in PM 2.5 as Beijingers rang in the year of the horse: 

Not all that surprising. But what’s more interesting is that you can see a corresponding increase in the particle counts in my collaborator Gus’s bedroom: 

These indoor counts are without a purifier running, so they demonstrate how quickly outdoor air pollution can find its way indoors and how variable indoor air quality can be in a single room over time. Simply put: the worse the air is outside, the worse it is inside. 

A couple of notes for fellow nerds: 

1. The indoor particle counts are not precisely on the hours, so the apparent time lag between indoor and outdoor counts may be exaggerated. 

2. The early spike in indoor PM 2.5 may be because people were moving around the house at that time, which affects the larger PM 2.5 more than the smaller PM .5. 

The Dirtiest Filter I’ve Seen

Matt Myers attended our workshop in September, and he used it straight through December. On December 7th, he sent me a picture of the blackest HEPA (and pre-filter) I’ve ever seen. Yikes! 


Time for a new filter! I’d be happy that black gunk is in the filter and not in my lungs.

joshuaw81 asked: Hi Thomas, just started one of your cannon kits earlier today and it's already turning gray. Already a fan of your work! Two quick questions for you: 1- I received a foam/fiber square that is white on one side and light green on the other. I'm assuming this is packaging, and not part of the filter? 2- Are these HEPA filters washable? Many thanks!

Hi Joshua, 

Good questions! We’re actually working on a pamphlet to go along with the cannon that will answer these two questions, so it’ll be clearer in the future! But here are the answers:

1. What’s the pre-filter for?


The pre-filter goes behind the HEPA filter, and it catches the larger dust particles so that the HEPA will last longer. We don’t include pre-filters on the Original DIY because the fan isn’t as strong as the Cannon.

2. Can I wash the HEPA?                                                 

Unfortunately, no. HEPAs are not washable. That’s one reason we try to keep our HEPAs as affordable as possible.