passport4adragon 提问: Hi, I’m in Shanghai and recently put together your DIY purifier and bought the same fan and filter you suggested – I think it’s working! A couple of questions – 1. I bought the exact same filter you did on Taobao – how often should it be replaced if I run the purifier every night? 2. The filter came with 3 extra thinner, foamy filter pieces (if you revisit the Taobao page, I think it’s item #1, I don’t know about #3) – should I stick that in between the actual filter and the fan, or not? Thanks!

Great questions!

  1. I don’t have good data on how often the filter should be replaced. I’ve been replacing mine after the filter turns dark, but that’s just based on my intuition, not data. What I need to do is use the particle counter to test effectiveness over time and figure out at which point it starts to lose effectiveness. Hopefully I’ll have more on that later!
  2. You’re right! The filter comes with some “pre-filters.” I don’t use them, but you could use them. On the plus side, they’d probably help your HEPA last longer. On the downside, I’d be worried the pre-filters would cut down on the air flow too much, but I haven’t tested that possibility with the particle counter.

More Comparisons

Dr. Saint Cyr recently pointed me to great tests of air purifiers from the Shanghai Consumer Protection Bureau:


The reason this type of research is so badly needed is that Western research (like this report from Consumer Reports) focuses on allergens, not general industrial air pollution. Allergens are probably more relevant for most Americans, but for those of us living in China, particulate air pollution is the real problem. And a lot of smog is smaller than pollen:


Thus, I was excited to see research that focused on PM 2.5 here in China (results here, in Chinese). In removing PM 2.5, 17 out of 22 models removed more than 90% of PM 2.5 in just 20 minutes in a 30-square-meter room. Pretty good!

Yet even the cheapest of the “non-famous” brands cost about 1,500 RMB. For that price, you could make 9 of my DIY filters, and that’s before eating the cost of proprietary replacement filters (US$200 a piece at IQ Air). Even with these cheaper brands, I still think consumers lack a truly affordable way to get clean air. Using the consumer bureau’s results, I calculated how much you’re paying for each percent reduction in PM 2.5.


Now, their tests weren’t perfect. The biggest downside I see is that they used cigarette smoke as the PM 2.5 source, rather than outside air. It’s not clear to me if cigarette smoke behaves the same as smog in filters. Also, many filters were less effective at removing the formaldehyde that they released in the room. That said, I don’t know how much formaldehyde is in the air normally.

But these results from Dr. Saint Cyr do use ambient air pollution in China (as do mine 1, 2). In tests with doors closed and the filters at their highest settings, Dr. Saint Cyr’s two Alen Air filters, Blueair, and IQ Air all got rid of at least 95% of PM .3. But the Alen Air A375 will set you back US$500 or RMB 5,000 imported in China. Here’s how to get the same results for 166 RMB.

Particle Counting in the News

Science in the news! The word on clean air seems to be getting out. Particle Counting was written up in a few places recently:

James Fallows graciously wrote about Particle Counting as one low-cost safeguard while living in “toxic” China, partly in response to New York Times Beijing correspondent Edward Wong’s personal account of worries about living and raising a child in China’s pollution. (Ed also recently tweeted a shout out to Particle Counting.)

As usual, Reddit was ahead of the curve and posted about it on their China page, as did the expat mag The Beijinger. Particle Counting also made the Sinocism newsletter.

(Update: Quartz wrote up a piece on the “MacGyver” solution to air pollution. The Middle Kingdom also mentioned Particle Counting in a post detailing more affordable ways to protect yourself from air pollution.)

I’m currently working with the folks at the Beijing Energy Network to host a workshop (for those in Beijing) where people will be able to make their own filter to bring home. More details to follow.

Here’s to hoping this helps people get clean air without breaking their piggy banks!


Independent Confirmation

Independent Confirmation. I’ve always suspected that what I’ve “discovered” is so basic that people must’ve known about this before. Lo and behold, doctors at the University of Michigan recommend that people who suffer from allergies make a purifier for $25 with a furnace filter and a box fan.

Getting independent confirmations of results is always important, and their live tests with a particle counter are convincing. However, their test is with a MERV filter, which catches less than HEPA filters. The upside is that they let more air flow through. I suspect that’s why his numbers are low, but not as low as my tests with a HEPA.

DIY Compared to Expensive Filters


I now have directly comparable data with the DIYs, Blue Air, and Philips: Direct comparison tests.

In earlier posts (1, 2), I showed that you can make an effective DIY air purifier to clean Chinese air pollution out of your home air. But just because the DIY filter is effective doesn’t mean it’s as effective as the Ferrari filters. How effective is the DIY filter compared to the expensive filters?

I’ll attempt to answer this question from several angles in different posts. Here’s one method: comparisons with the published data from the blog of the Beijing doctor Dr. Saint Cyr (which was an original inspiration for me that air purifiers could actually reduce air pollution). He ran tests of an 11,000 RMB IQ Air and a 6,000 RMB Blue Air. I used the same calculation of effectiveness and compared his results for the pricey machines with tests of my DIY purifier. Here are the results:


To make the comparisons more precise, I compared my filter results only to the tests he reports from the IQ Air and Blue Air on max power in his smaller room.

Dr. Saint Cyr calculated reductions based on outside air quality, which I do not think is the best method. I think it makes more sense to calculate reductions based on the room air before and after using the filter because my room air is significantly cleaner than outside air, even before using the filter (more on that later). But to make the data comparable, I calculated effectiveness using Dr. Saint Cyr’s method.

Now, the comparisons aren’t perfect. Dr. Saint Cyr’s smaller room is still 6.5 meters larger than mine. He also doesn’t say how long he ran his filters and whether the doors were closed. My tests were with the doors closed and overnight, so the filter had several hours to run.

(Update: via email Dr. Saint Cyr said his tests were with doors closed and testing times of 1 hour+.)

But even if the difference in room size bumps mine down 5% and the others up 5%, that would mean 11,000 RMB and 166 RMB get you similar results. To illustrate that, I calculated a graph of how much you’re paying for each percentage reduction in air pollution:


Of course, the ideal test would use all three filters in the same room with the same particle counter. To that end, I will hopefully get the chance to borrow a friend’s Panasonic filter and test it in my home. I’ll post those results when I get them.

Conclusion: Although the comparisons are imperfect, they show that a DIY filter is at least roughly comparable to expensive filters at a far lower cost. This illustrates my larger point: all you really need to combat particulate air pollution is a HEPA filter. In fact, Dr. Saint Cyr found the same thing in his tests of a much simpler Hunter brand HEPA filter. In a room with the doors closed, his HEPA got 91% of the pollution–similar to what I found.

For data nerds like me, I’m posting more details on my data and methods here:

Test Details:


Purifier Tests

I think you can break the question of whether an air purifier works down into two questions.

  1. Is the air coming out of the purifier clean? This is the easier question to answer, and the test results are as clear as can be. The DIY purifier shoots out very clean air.
  2. But is that enough to actually clean the ambient room air? For example, if you have a really tiny filter and a huge room, the filter could work properly but still not be strong enough to make a difference.

Answering this question is more difficult because you need a controlled environment (you can’t open and close windows during the test), and you need to test the air for a longer period of time. Fortunately for you, I’m a nerd, and I’ve been doing these tests for fun for the past few weeks.


According to my particle counter, here’s what the filter did in an hour:


I’ve also tested the effect by running tests with the particle counter on hourly mode. These tests show that the downtrend continues over several hours:


(Note that the hourly test and minute test are on different days.)

The particle counter also gives data on 0.5 microns – even smaller particles. Here’s what that looks like over eight hours:


Conclusion: The DIY purifier works. You can get clean air for 166 RMB, as opposed to 8,000 RMB as long as you know that a HEPA filter is all you really need to fight particulate air pollution in China.

From my perspective, filter companies like IQ Air are taking advantage of how little we know about air pollution and the fact that you need expert instruments to tell whether the filter is working or not. When consumers don’t know how to assess the products we buy, we often use price to tell us whether the product is good. That happens with expensive wines all the time. I’m convinced you can breathe safe air in China for far less than filter companies want you to believe.

Finally, for data nerds like me, I’m including more details on the tests here:


Test details:

The test above was done starting at 11:30 pm (I’m a night owl) on 6/16/2013, when the outside AQI in Beijing was 230 according to the US embassy’s AQI Twitter feed. (The outside air improved the next day, but results were similar on a later test where AQI actually went up slightly from 195 to 202 during the test. Details to follow here.)

According to comparisons of my particle counter’s tests of outside to US embassy AQIs, an AQI of 230 would convert to about 2,650 on the 2.5 micron count on my reader. (Remember, the particle counter gives the raw number of particles 2.5 micrometers and above per .01 cubic feet. The US embassy takes mg/m3 and converts that to an AQI. Therefore, the numbers are different, although they correlate.)

I did the test in my bedroom with the doors and windows closed. The room is 13.5 meters squared, with two windows.

Astute readers have asked whether I let the particle counter run a bit to get a stable reading before turning on the filter. The particle counter tends to take a 5-10 minutes to get stable readings. To be conservative, I gave it about an hour:


(Update: This question is answered with much more data in my new tests where I left the particle counter on for 6 days, and I turned the DIY Cannon on a timer to turn on every day for 2 hours.)

The spike at the top was when I entered the room to turn the air purifier on and reset the machine, so it may just be noise or it may be the dust I kicked up by walking around. A more stable reading for that time would probably be about 230.

The uptrend prior to turning the filter on may have been because I was running my dehumidifier prior to the tests, and I have some small filters in that. (Yes, for some reason my house gets very humid–at times over 80%–despite the fact that Beijing is a semi-desert. I think it’s a problem with the plumbing system.)

Regardless, this data suggests that the effect of the filter was NOT a confound of calibration.

How to Make a DIY Air Purifier

Based on my research into how different filters work, I concluded that a HEPA filter is all you really need to fight particulate air pollution in China.
Given the fact fancy air purifiers run for 8,000 RMB here in Beijing, and I’m only here for a year, I thought I’d try making my own.
I’ll give details on how well it works later, but here I’ll explain how I made it:

  1. Buy a fan. It’s important that it has a flat front so that you can strap the HEPA filter to it.
    Fans with flat fronts usually have a recessed motor unit, so look for that. Here’s the one I bought for 58 RMB:


  1. Buy a HEPA filter. I bought this one for 108 RMB, and I’ve tested it with my particle counter, so I know it works:


  1. Pull the grating off the front of the fan. It helps if you have pliers for this. Then turn the power setting to 3 and pull the knob off.
    The knob gets in the way of the HEPA. Without the knob, I turn the fan on and off by plugging and unplugging it.


  1. Use string to strap the HEPA filter onto the front of the fan, and you’re done!
    The metal bar in the middle will stick through the filter a little bit. You could saw the metal bar off, but my tests show it doesn’t seem to make a difference.



Total cost, 166 RMB (US$27). Compare that to 8,000 RMB ($1,300) for the fancy purifiers here in Beijing.Your next question is probably, does it work? See a live test here.