Is it possible to lower the cost of clean air without sacrificing effectiveness?

A few months after I published my DIY tests, there were already Taobao stores up and running, selling DIYs even cheaper than my 200 RMB. I was intrigued. If we could really lower the cost of clean air, that’s a win!

But we have to be sure these cheaper machines are as effective. So I ordered the cheaper machines, tested them systematically, and found they performed much worse (2).

A New Way to Lower the Cost of Clean Air

But that doesn’t mean there aren’t ways to make HEPAs even more affordable. Recently I tested one way to do that: if we can shave 1 millimeter from the size of the HEPA, we can save 6% on the price. This new size still covers the fan opening, but does it still work as well?


To test this question, my collaborator Anna ran 10 overnight room tests with the 29 mm in her Beijing apartment and compared it to my earlier tests of the 30 mm HEPA in the same 15m2 room.


Like in my earlier tests, I calculated effectiveness as the percent reduction in particulate from the start of the test to the average of the last four hours. Here’s what one of those tests looks like with the reduction calculation laid out. The blue line is indoor 0.5 micron particles; the red line is outdoor PM 2.5



On average, the new 29mm HEPA reduced 0.5 micron particle levels in the room by 86% and 2.5 micron levels by 91%. These results were almost identical to my prior results with the 30mm HEPA.



This new HEPA lets us lower the cost of clean air without sacrificing any effectiveness. Win! We just lowered the price of Smart Air HEPAs from 80 RMB to 75 RMB.

HEPA Cost Comparison

Next I took that price and compared it to the two biggest brands out there, Blue Air and IQ Air. (Also check out the long-run cost comparison .)

Nerd Note on Replication

As a side note, this test is now the third series of room tests I’ve published with the Original DIY (early tests; 200-day longevity test). Add that to Dr. Saint Cyr’s tests, and I’d say that’s a satisfying amount of reproducibility.

For fellow data nerds, I’m posting the original data and more details about the test below.





Outdoor PM 2.5 Fluctuations

I like these long tests more than the common 20-minute tests (such as CADR tests) because this mimics how most people use purifiers. However, one drawback of these long tests is that outdoor PM 2.5 levels sometimes change over the course of 8 hours. If outdoor PM 2.5 drops, that can artificially inflate effectiveness. If outdoor PM 2.5 rises, that can artificially decrease effectiveness.

These changes should cancel out over 10 tests, but it’s worthwhile to re-run the analysis while excluding days with large outdoor changes. Among these 10 tests, two days (9/25 and 11/10) had large outdoor PM 2.5 changes. Without those two days, the averages were almost identical: 84% on 0.5 micron particles and 91% on 2.5 micron particles.

Average Outdoor PM 2.5 During Tests

It’s also worthwhile to check out bad outdoor PM 2.5 was during the tests to see if these days were representative of normal Beijing air. The average outdoor PM 2.5 was 116 micrograms. That’s about 20 micrograms higher than Beijing’s average PM 2.5 over the last five years (according to my analysis of the US Embassy’s PM 2.5 data). Thus, if anything, these tests are tougher than the Beijing average.




Is a 30 RMB HEPA Possible?

After I published directions for how to make your own purifier, people asked me: “Which HEPAs should I use? Is this one trustworthy?”

That’s the type of question that makes a nerd like me happy because it means we need to get more data. So I ordered HEPAs from every manufacturer I could find, and I tested them all. After all the testing was done, I found I could ship the HEPAs that worked best for 80 RMB, which was cheaper than the 110 RMB HEPAs I was buying–quality and price!

Can HEPAs be even Cheaper? 

Later I found HEPAs for 20 RMB wholesale. I was excited. If HEPAs are that cheap, we can make the DIY even cheaper!

But the test data was terrible. These HEPAs weren’t anywhere close to getting 99% of particles, so I passed on them. If didn’t want to use it in my home, why would I want to ship them to other people?

A 99.97% HEPA for 30 RMB?

Thus I wasn’t surprised a couple weeks ago when I saw a store on Taobao selling HEPAs for 30 RMB and claiming that my HEPAs are 暴利 (aggressively overpriced). They claim that their HEPAs get 99.97% of particles, and if that’s true, maybe these were the holy grail of HEPAs I was looking for all along!

So I ordered two online and put them to the test. The first shock was seeing that it doesn’t have a frame:


That makes it harder retain its shape, but it might still be effective without a frame, so I put it through the tests.

Air Outlet Test

First, I tested it by putting it on the Cannon and testing the air coming out of the HEPA with a Met One Aerocet 531S. (The Met One is useful because it has a pump to regulate airflow. In air outlet tests, the particle counter is sitting in a stream of air, so using a pump maintains constant readings.)


The results weren’t pretty. Smart Air HEPAs got over 99.9% of particles, but the 30 RMB HEPA was below 90%–far below their claim of 99.97%.

Air Speed

But particle effectiveness isn’t everything. A HEPA in the 80% range might work better if it has better air flow. In that case, maybe the HEPA could process the air more times and clean the room air as well as a real HEPA.

To test that possibility, I put each HEPA on the Cannon and used a tool to measure air speed (fancy name “anemometer”). I placed the anemometer on the HEPA at four locations (left, right, top, bottom) and took the average air speed.


Again, the results weren’t pretty. So not only was the 30 RMB HEPA getting far fewer particles, it was letting much less air through.


Quality HEPAs for 30 RMB are still a dream. They’re not useless, but using this 30 RMB HEPA would expose people to significantly more particles.

I still hold out hope that manufacturers will be able to innovate cheaper HEPAs without sacrificing quality, but I haven’t seen those HEPAs yet.

Is the Taobao Store Owner Being Dishonest? 

The 30 RMB HEPA store makes claims that their HEPAs get 99.97% of particles, and the data clearly contradicts that, so it’s tempting to think that they’re lying.

But are they? I don’t know what’s in their mind, but my guess is that they simply didn’t go through the hassle of buying a particle counter and testing the HEPAs. I suspect that half of what seems like dishonesty on Taobao is actually just sloppiness.

As usual, I’m posting the raw data below. (more…)

Longevity test Day 200

HEPA Longevity Test: Day 200

Longevity test Day 200

One of the biggest questions when I started doing the DIY was how long the HEPAs last. Rather than just give people a nice-sounding answer, I wanted to see what the data says, so Gus has been dutifully turning on his Original DIY every day in his 12.3m2 Beijing bedroom and tracking how effective it is each day in real Beijing air

Around Day 100, effectiveness dropped 4%. Around another Day 170, effectiveness dropped another 5-10%. But for the benefit of science, Gus has risked his own lungs by continuing the test for another 60 days.


Gus turns on the Original DIY while he sleeps each night and tracks effectiveness using a Dylos particle counter. I calculate effectiveness as the percentage decrease in the number of particles .5 microns (and above) over the course of the night.

Here’s what a sample day looks like:


To smooth out variability over time, I averaged the effectiveness over every 10 days. For the full details on the methods, check out my earlier post.


The death crawl continues. At Day 200, effectiveness dipped to just below 50%.


At the risk of Gus’s health (especially given the coming onslaught of winter air), I voted for Gus to stop the test. So the test is over!


In real Beijing air, the Smart Air HEPA lasted at 100 days at about 8 hours a day at full effectiveness (729 hours to be exact). It lasted through 140 days (1,028 hours) with a slight 4% drop, which is when I would change the filter.

Recommendation: I recommend changing the HEPA every 140 days at about 8 hours a day or every 1,000 hours. You can adjust the number based on how many hours you use it per day.

Limitations: We did the test in Beijing, so HEPAs probably last a few more weeks in places with cleaner air (but still not “clean air”) like Guangzhou and Hong Kong.

The results may also be different for the Cannon because it pushes more air. The next longevity test will be with a Cannon!

Open Data

As always, I’m publishing the raw data–all 200 days!


Types of FiltersTypes of Filters

Premise: This blog is for people concerned about air pollution. There are many valid reasons people want air purifiers: pet allergies, pollen, and asthma. However, these are not what I’m concerned about while living in China. So I assess purifiers solely based on whether they can help protect me from particulate air pollution.

With that in mind, here are the three most common types of filters in portable air purifiers:

  1. UV light filters are designed to kill bacteria. In China, I’m concerned about air pollution, not bacteria, so UV filters are unnecessary. But beyond that, Consumer Reports says that UV filters in most air purifiers don’t even kill bacteria:

The Environmental Protection Agency cautions that air cleaners outfitted with ultraviolet light are unlikely to kill bacteria and mold because they won’t be in contact with UV light long enough to have any effect.


Conclusion: UV filters aren’t what I need.

  1. Activated carbon filters use charcoal screens to catch certain types of chemicals and organic matter that happen to interact with carbon. In other words, these filters catch some things, but not particulate in general. In sum, these will do a little bit, but they aren’t the whole solution.


Activated charcoal is good at trapping other carbon-based impurities (“organic” chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all – sodium, nitrates, etc. – so they pass right through. Howstuffworks.

Conclusion: Carbon filters may help (particularly for certain types of gases that carbon can get), but they don’t target all particulate matter. It would be a mistake to use ONLY a carbon filter to get rid of particulate pollution.

  1. HEPA filters are the solution. “HEPA” sounds fancy, but it’s just a standard that means the filter catches 99% of particles .3 micrometers and above (they also get particles smaller than .3 micrometers–the .3 designation is based on the overlapping point of different types of filtering). That covers a lot of particulate pollution–the most-often cited figures are for 2.5 micrometers.


HEPA filters aren’t rocket science. They work pretty simply: particles get stuck in the fiberglass fibers (using a few types of physical filtering processes). In fact, if you have a vacuum cleaner, it probably has a HEPA filter in it. You can get one for about $10.

Contrast that with the major purifier companies like IQ Air and Blue, which have proprietary HEPA filters with names like “HyperHEPA” that cost $200. These may have benefits, such as a longer lifetime or smaller pore size, but it’s not clear to me that’s necessary or worth 20 times the price.

Bottom line: If your concern is particulate air pollution, a HEPA filter is really all you need. And for that, you don’t need spend 8,000 RMB. You can make your own for 166 RMB.


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 0.5 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 0.5 micron particles and 2% less 2.5 micron particles. 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 0.3 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.



How long do Smart Air HEPAs last? At least 90 days

The most frequent question people ask me these days is: how long does the HEPA last?

This question is important because replacement HEPAs are the biggest long-term cost of clean air. IQ Air charges $370 for its filters. So if you have to replace the HEPA every two weeks, the DIY might not save you money in the long run.

What’s Wrong with the Standard Numbers? I’ve been slow to answer because I wanted good data. Lots of filter companies give a single number (often 6 months), but they don’t say whether those numbers are for European air or Chinese air. If they last 6 months in Swedish air, I highly doubt they’d last 6 months in China’s thick air.

Another problem is that “6 months” doesn’t tell you how much you can use it per day. Is that 6 months for 24 hours a day? 8 hours a day?

A More Empirical Answer: So to get a concrete answer to this question, I think the best way is to get a new HEPA and test how effective it is every day. Over time, we can see when it starts to lose its effectiveness.

It’s been tough to start that test because I’ve been busy running tests of new HEPAs, fans, and other purifiers. But thankfully my collaborator Gus volunteered his bedroom as a laboratory, and we now have data from 90 days of tests.

Limitation: However, before I reveal the results, I discovered a flaw in the study. I was looking at Gus’s data, and I noticed the particle counts in his room before he turned on the purifier were a lot lower than they were in my room. Why?

Then we realized that Gus’s roommate was running his large Blue Air 24 hours a day. That made the air in his house cleaner than it would be in a normal house. Fortunately for science, the roommate moved his Blue Air out on Day 57, so we now have a normal testing environment.

However, this created two limitations in the data:

  1. It may inflate our estimates of the longevity of the filter because the air was cleaner than it would be in a normal Beijing apartment.
  2. It artificially lowered the estimates of effectiveness for the first 60 days. (At the end of the post, I compare data from when the Blue Air was being used nearby and after it was gone.) This also makes it hard to tell whether there were any slight changes in effectiveness over time.

Gus will start completely new tests after we finish these tests, so we’ll have a better answer later. We debated whether publishing this data would be useful. In the end, I decided that it’s better to give people some idea of the longevity, as long as I make it clear that there are limitations with the data.

Method: 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 particulates from the room air overnight.

To smooth out the variability in any single datapoint, I averaged the effectiveness over each 10-day period.

Here’s an example of what one of the 90 days looks like:


Here’s what the HEPA looks like after 90 days:


Results: After 90 days, the HEPA is still getting as many .5 micron particles as in my earlier tests:


Conclusion: This suggests Smart Air HEPAs last at least 90 days without losing effectiveness. Stay tuned to see how the next 90 days turn out!

Remember: these results can only be generalized to Smart Air HEPAs. HEPAs from different manufacturers vary in thickness and quality.

Number of hours: At the end of 90 days, Gus has used the HEPA for a total of 660 hours, an average of 7.33 hours per day. When we get a

total estimate of the number of hours the HEPA lasts, you can plug in the number of hours you use the filter per day to get an exact number of days for your own use.

For fellow data nerds, I’m posting more details about the tests below.


The Effect of Having a Second Purifier on Nearby: As I described in the main text, Gus’s roommate had a large Blue Air running for the first 56 testing days. My hypothesis was that the Blue Air was artificially lowering the baseline particle counts, which makes it harder to get high-percentage reductions.

We were able to test this hypothesis after his roommate prepared to move to Hong Kong and shipped his Blue Air there ahead of time.

To test my hypothesis, I compared the effectiveness (1) during the 56 days that the Blue Air was running and (2) the 34 days after his roommate stopped using the Blue Air:


The estimates of effectiveness rose to 86%–even higher than my original tests. This suggests that the earlier numbers were artificially low.

It also means that we can’t interpret any changes before Day 70:


The numbers go up when Gus’s roommate stopped using his purifier, but I think it’s wrong to say the DIY became more effective.

What we can say is that, even after 90 days of usage, the HEPA is still working as well as in my original tests.

Can we generalize the results? These tests are great for people living in China because the tests are based on Beijing air. However, the filter may last longer in places with cleaner air (for example, Hong Kong) and shorter in places with dirtier air (for example, Taiyuan).

In addition, fluctuations in AQI could make these estimates longer or shorter. For example, winter air tends to be more polluted.

Dropoff After 10 Days? The dot for the first 10 days is slightly higher than the later days, which suggests a dropoff after 10 days. However, I think that is not the case. In the first 10 days, Gus was still learning how to use the particle counter, and he failed to set it properly or get a good baseline reading on four of the days, so we couldn’t calculate effectiveness. That means the estimate for the first 10 days is less reliable.

What’s Going on at Day 30? The third datapoint (days 21-30) is the lowest. That is probably because the outdoor air was particularly bad in Beijing during that time. The numbers rebound slightly during the next 20 days.

Why HEPAs get better with age (to a point). Many people have the intuition that filters get worse over time, but HEPA filters actually get better at removing particles. That’s because the more you use it, the more particles fill up the pores in the HEPA, and it becomes harder for particles to get through.

The problem is that it also becomes harder for clean air to get through. So eventually, the HEPA will get so full of particles that it won’t let enough air through. That’s how the HEPA eventually dies.

Raw Data: The file is too large to display all of the raw data here, so I’m making it downloadable from my University of Virginia homepage (click here to download).