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.

Affordable clean air is coming to Hebei!

Smart Air is coming to Shijiazhuang for our first-ever Hebei workshop! I’m excited that we’re finally getting the word out beyond Beijing that 200 kuai is all you need to protect yourself against PM 2.5.

Date: Sunday, September 21st, 8:00-9:30pm

Location: Chocolate, #7 Heping Donglu, Rongjingyuan Bldg. 20, Unit 1, Suite 2202

Phone: 15076117308


Come listen for free or RSVP to take home a Smart Air purifier (200 RMB)!


HEPA Longevity Test: 170 Days


When I started Smart Air, a lot of people asked me how long the HEPAs last. Several people criticized the DIY on Zhihu because they said you’d probably have to change the HEPA so often that it’d end up being more expensive than the expensive brands. 

At the time, I really wanted to pull out a nice round number, but I couldn’t think of any way to answer the question without getting hard data first, so I started doing tests (well, actually Gus did). At 90 days, we found it worked as well as in the beginning. At day 130, we found a 4% decline. Now we’ve finished 170 days!


Gus used the Original DIY and the same HEPAs we ship from Smart Air every night in the 12.3 m2 bedroom in his Beijing apartment. Gus used the same method as my previous tests to calculate effectiveness—the percentage reduction of particles .5 microns and above from the room air overnight. 

To smooth out the variability in any single datapoint, I averaged the effectiveness over each 10-day period. (More details on the methods here.) Here’s what a single test day looks like:



At day 100, the effectiveness dropped by about 4%. It stayed at around that level, until day 140, when it dropped 5-10%. After that point, the total effectiveness has bounced between 65-80%. 



In this test with real-life Beijing air, the Smart Air HEPA lasted 100 days at about 8 hours a day at full effectiveness (729 hours to be precise). People who want every percent of effectiveness should change HEPAs after 100 daysPeople who don’t mind the 4% drop, I’d recommend replacing it by 140 days (1,028 hours). 

For now, the test continues! I’ll post the final results when we finally run this HEPA into the ground.

As always, I’m posting the raw data as a download here, and there are more details on the methods in my earlier post.

Clean air is coming to The Bookworm

Still haven’t equipped yourself with clean air? Catch our newest workshop at The Bookworm!


Sunday, September 14th, 7:30-8:30pm

Location: The Bookworm, Building 4, Nan Sanlitun Road, Chaoyang District, Beijing

Cost: 250 RMB, includes 1 Smart Air purifier and a drink from The Bookworm.

RSVP: Click here to RSVP (limit 30).


What’s in a workshop?


Do masks work? Are electrostatic filters safe? We explain what the latest science says, and we show data on exactly how much particulate pollution filters remove from the air in Chinese apartments.



We show you how to build your very own air purifier. Assembly is quick and easy, and we provide all the tools, fans, and HEPAs. In about 15 minutes, everyone has an air purifier they can take home to zap PM 2.5.



Test your new filter with our laser particle counter. We’ll stick around for as long you like to answer questions and continue the discussion.


P.S. Can’t make the workshop but still want a DIY kit? Order via PayPal at or Taobao at We’ll ship the materials to you for no extra charge.


Who’s breathing smart air? 
Destination of orders shipped from Taobao 9/2013 through 7/2014.

Shanghai Sunday 7/27: Workshop @ UVa China Office


My university is helping host a clean air workshop next Sunday in Shanghai! It’s a UVa event, but all are welcome. 

Sunday, 7/27, 3pm

UVa China Center

Shanghai, Xuhui District

Fuxing Xilu, #55, Room 101

This event is open to the public: RSVP


Two options:

Option 1: Bring a purifier home.

Take home a Smart Air purifier and cut the PM 2.5 in your home right away! Cost: 200 RMB (the same as on our website). Pay at the door.

RSVP here.

Option 2: Free Knowledge

All are welcome to come and learn more about clean air, whether masks really work, and how you can protect yourself without giving thousands of kuai to air purifier companies!


What’s in a workshop?

1. Air pollution knowledge! I’ll cover the basics about air pollution, whether masks actually work, and how to protect yourself.

2. Build your own Smart Air purifier. It takes just five minutes, and you can bring it home to remove PM 2.5 from your home right away.

3. Use Smart Air’s laser particle counter to test your new purifier on the spot.

Smart Air is Coming to Hong Kong!


LOCATION UPDATE: The Hive @ Wan Chai

Before now, the farthest south we’ve brought our message about clean air was Hangzhou. Now Smart Air and The Hive are teaming up to bring affordable clean air for Hong Kong’s first-ever DIY air filter workshop:

Date: Thursday, July 17, 6:30pm

Location:  The Hive, Wan Chai

Cost: 300 HKD (includes a Smart Air DIY purifier and a drink from The Hive)

RSVP here

The Hive @ Wan Chai

21st Floor, The Phoenix Building
No. 23 Luard Road
Wan Chai
+852 3568 6343


What’s in a workshop?

1. What everyone needs to know about air pollution

We teach the basics about air pollution, its effects on our health, and the science behind air purifiers.

2. Clean air to take home

Build your own DIY air purifier—it takes just five minutes. Then bring it home to remove PM 2.5 from your home right away!

3. Live tests

Use a laser particle counter to test your new purifier on the spot!


How Long do Smart Air HEPAs last? Longevity Test Day 130

How long do Smart Air HEPAs last? My quest to give a solid empirical answer has now completed 130 days! Gus is still dutifully tracking the effectiveness of the HEPA he’s been using in his bedroom since January (with a trip to the US in the middle), calculating the percent reduction in particulates every night. 

I reported a few weeks ago that the HEPA was just as effective at Day 1 and Day 90. Now after 130 days and 951 hours of usage, here’s what the HEPA looks like:


Results: At 100 days, the overall effectiveness declined by about 4%. Even so, the HEPA is still removing 80% of .5 micron particles from the overall room air: 



1. On the Original DIY, the Smart Air HEPA lasted about 100 days without losing any effectiveness. That was for a total of 729 hours of usage or an average of about 8 hours a day (7.29 to be precise).

2. Between days 100-130, the effectiveness dropped by about 4%. I think it’s up to you to decide whether a 4% drop is enough to make you change the HEPA.

3. You can adjust these numbers to the number of hours you use the machine. For example, if you run the DIY 24 hours a day, you can get about 30 days with full effectiveness and at least 40 days with a roughly 4% reduction. 

As I’ve argued before, I prefer this type of test over the context-less numbers that some big purifier companies give because (1) it is based on air in China and (2) it gives the concrete number of hours you can use it rather than just the number of months. Of course, there will be some variation across different seasons, apartments, and cities in China.


1. This number doesn’t necessarily apply to the Cannon. On the one hand, the Cannon filter might have a shorter life because it pushes more air and therefore should collect particles faster. However, our Cannon HEPAs have a pre-filter, which extends the life of the HEPA. The next step is to do longevity tests with the Cannon!

2. Gus changed apartments on Day 102, which is not ideal. However, the average effectiveness didn’t seem to be affected by his move (see the points at Day 100 and Day 110). We’ll repeat the tests in the future, and that will give us more reliable estimates. 

For now, the test continues! Stay tuned for the next report.

I’m posting the raw data and detailed methods below for fellow nerds.

Read More

The new Smart Air team gathered at HQ last week, and we took a break from discussing HEPA filters to take a few group selfies on the roof.
Back row (L to R): Gus, Thomas, Ted.
Front: Henry, Anna, Matt. About. 

The new Smart Air team gathered at HQ last week, and we took a break from discussing HEPA filters to take a few group selfies on the roof.

Back row (L to R): Gus, Thomas, Ted.

Front: Henry, Anna, Matt. About

Are All DIYs the Same? Poor Results from 还我蓝天

When I did my first experiments, several people told me not to publish the data. “Don’t give it away for free,” they told me. “Use it to make money!”

I decided then that my main goal wasn’t to make money. I almost got tricked into paying $1,000 for clean air, and I wanted to help people avoid getting tricked too, so I published the data anyway.

Of course, publishing the instructions online has made it easy for people to copy the idea. 还我蓝天 (Huanwo Lantian) was one of the first to follow in our footsteps, selling a DIY filter a few months back. They even use a screen capture of Gus’s appearance on Chinese TV on their shop:
I was curious to see how their filter works, so I decided to order one off of Taobao and put it to the test.

Now I’m in an awkward position because I found that their HEPA was not working nearly as well as my Smart Air HEPAs. It’s awkward because, if I publish the data, will people think I’m just trying to attack a competitor?

In the end, I think it’s better to publish the results and be honest about my conflict of interest. At the very least, I think people have an interest in knowing how well other DIYs work—especially when some of those websites use graphs that are lifted from my site, which can mislead people into thinking the test results are from their machines. 

And as always, I’m publishing my raw data and testing methods at the end of this post, so fellow nerds can replicate my studies.

Method: My collaborator Anna used the same methods as our earlier tests in her 15m2 room. Anna did five overnight tests with the same Dylos Pro particle counter, and I calculated effectiveness as the percent reduction of particles in the room air, averaging the last three hours (more info). Then I compared the results to my earlier tests in the same room.

Here’s what I found: 


Results: The 还我蓝天 DIY removed 21% fewer particles .5 microns and above and 11% less 2.5 micron particles than the Original. 

Is it the fan? The 还我蓝天 fan is slightly smaller than the Smart Air Original, so one explanation could be that the 还我蓝天 fan is just moving less air. Anna tested that by strapping the 还我蓝天 HEPA onto our Original fan.

Here’s what I found: 


Result: There wasn’t much difference. With the new fan, it was getting 4% more PM .5 and 2% less PM 2.5. Thus, the fan doesn’t seem to be the reason.

Is it the HEPA? The second possibility is that the 还我蓝天 HEPA isn’t as good. Anna tested the HEPA by doing air outlet tests with a Met One GT-521, which measures down to .3 microns. Anna tested the air coming out of the HEPA for 10 seconds, and I averaged the results from three tests. (More details at the end of the post.)

Here’s what I found: 


Results: The 还我蓝天 HEPA performed worse, about 7% lower than the HEPA standard. The major source of the 还我蓝天’s poor performance seems to be the quality of the filter. 

Conclusion: In room tests, the 还我蓝天 DIY removes about 21% fewer particles than the Original DIY, and the data suggests that the reason is that 还我蓝天 HEPA is lower quality. 

Now, don’t get me wrong. The 还我蓝天 DIY is making the room air cleaner. I’d rather have a 还我蓝天 than nothing. But the results show that this DIY copycat is cutting corners by using cheap HEPAs.

As always, I’m posting the raw data and more detail on the methods for fellow nerds. 

Read More

Smart Air on PRI/NPR

As a nerd who grew up listening to NPR, I was psyched to see Smart Air on NPR/PRI’s The World today! PRI journalist Rebecca Kanthor visited our Shanghai workshop and took audio with Henry and Gus.