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 an IQ Air.
3. Live demonstration.

Need clean air?

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

About me.

Is Guangzhou’s Air Safe?


Beijing’s air pollution is the most famous in China, but that can make people in some other cities think their air is good—at least, not as bad as Beijing’s. But is the air in other cities safe? 

In 2013, Shanghai had newsworthy air pollution that convinced many people in Shanghai that air pollution was a problem there too. But what about Guangzhou? Guangzhou doesn’t have winter heating. Does that mean its air is safe? 

To answer this question, I analyzed all of the hourly data from 2014 from the American consulate in Guangzhou. Year to date, that data covers 4,572 datapoints. Then I calculated what percentage of those had PM 2.5 readings about the WHO 24-hour upper limit of 25 micrograms (which is about 77 on the US AQI scale).

Before I give the answer, take a guess at what percentage exceeded the WHO limit: 

1. 10%

2. 25%

3. 50%

4. 75%

5. 90%

Answer below


To date, 76.9% of the readings exceeded the WHO upper limit. The average reading was 53.4 micrograms—over two times the WHO limit. 

Breathe safe, Guangzhou!

New Tests! Can a 450 RMB DIY remove as much particulate as a $1,000 IQ Air?

When the “airpocalypse” struck Beijing, I got out my credit card to give $1,000 to IQ Air. But the price just didn’t seem right, and I soon learned that HEPA filters are what capture particulates (even in the expensive machines), and they were invented way back in the 1940s. So I made my own and started doing tests.


Could a fan and a HEPA remove as much particulate as an IQ Air? The IQ Air is one of the most expensive purifiers on the market, and it advertises “the cleanest air guaranteed.” On top of that, it uses more than twice as much energy as the DIY Cannon, so surely it can capture more particles?


Thankfully, a kind soul donated his IQ Air Health Pro (8,000 RMB), and I ran 11 room tests on the highest setting. Then I compared it to a Blue Air 203/270E (3,600 RMB), Philips AC4072 (3,000 RMB), Original DIY (200 RMB), and Cannon (450 RMB).

I tested all of the purifiers in the same 15m2 bedroom in Beijing, with the same particle counter, for the same amount of time (overnight). To calculate effectiveness, I compared the number of .5 micron and 2.5 micron particles before turning on the purifier (baseline) to the average of the last four hours before I woke up. (More methodological details here.) 



Over 11 tests, the IQ Air removed 91% of the .5 micron particles and 95% of the 2.5 micron particles. That’s good, but not any better than the Cannon: 


I was particularly surprised because the IQ Air was noticeably louder than the Cannon:


With this new data, we can use my earlier tests in the same room to compare the DIY to the three major brands: 



All of the filters significantly reduced particulates, but the 450 RMB Cannon removed as many particles as the highest-performing big brand. Even the 200 RMB Original was only 6% behind the Blue Air on the .5 micron particles and 4% behind on the 2.5 micron particles. 

More money = more effectiveness?

Among the different brands, there seems to be no relationship between price and particulate removal. The cheapest name-brand purifier was the Philips, and it removed the most particulates:


That’s exactly what Dr. Saint Cyr found in his fit tests of pollution masks


According to his data, 6 RMB 3M masks like the kind I use now outperform the 349 RMB Respro masks I used to use. With the 3M 9501, I can buy 58 masks for the price of one Respro AND block a higher percentage of particles:


Bottom line: clean air doesn’t have to cost an arm and a leg, whether it’s in your home or strapped to your face.

As always, I’m posting the raw data and more details on the testing methods below for fellow nerds. And in a follow-up post, I’ll talk about the other important ways to compare purifiers: HEPA replacement costs, gas removal, and extra features.  

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Smart Air is going on tour! Guangzhou and Hong Kong

Hong Kong

Date: Sunday, September 28, 7:30pm

Location:  The Hive @ Wan Chai

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


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

First-Ever Workshop in Guangzhou!

Date: Monday, Sep. 29, 7pm (English)

Date: Tuesday, Sep. 30, 7pm (中文)

Location: Ricci Creative Eats 

Cost: 250 RMB (includes a Smart Air DIY purifier and a drink from Ricci)

RSVP English Workshop

RSVP Chinese Workshop

Shop 015B, G/F, Popark Mall

63 Linhe Zhong Road, Tianhe District

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.

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