Particle Counting

1. How to make a filter for 166 RMB.
2. Tests showing that the DIY Cannon removes as much particulate as an IQ Air.
3. Live demonstration.

Need clean air?

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

About me.

Test: Do you have to run your purifier all day?

Some purifier companies recommend running your purifier all day. But is that really necessary? If so, isn’t that a big waste of your filters and energy? 


To get to the bottom of it, I set a Smart Air Cannon on a timer to turn on for two hours everyday in a 13.5m2 Beijing bedroom:

I put a particle counter in the room to take measurements every minute. I did the test while I was on vacation, so there was no influence of me opening and closing doors.


After six days, I came back and saw how long it took to the Cannon to clean the air each time it came on. Here’s what six days of data looked like for the small .5 micron particles:

Over the six test days, the air in Beijing became progressively worse. But on each day, it was clear when the Cannon turned on and off. The dropoffs were sharp, showing the Cannon was working quickly.

I averaged over the six test days to find out how long it took the Cannon to clean the air on average.


On average, the Cannon cut .5 micron particulates in half in 10 minutes. By 20 minutes, it removed 80%.

Bottom line: Powerful purifiers like the Cannon clean the air very quickly, so I see no need to run the purifier while I’m not at home. 

Can I turn it off while I sleep?

The data can also answer another question a few people have asked me: “I don’t want to hear the fan while I sleep, so can I run it for an hour and then turn it off while I sleep? 

In the tests, the air got dirty very quickly after the Cannon turned off (even though the windows and doors were closed). Dirty air is entering our homes constantly, even though we can’t see it.

Bottom line: I do NOT recommend turning the purifier off while you sleep. 

Open Data

As always, I’m publishing the raw data and more details on the methods below. I’ll also be publishing data from similar tests in a much larger room (30.5m2) and for tests when I’m at home and moving in the room.

Read More

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.  

Read More

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.