cost-effective

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?

Method

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.

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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

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Results

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.

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Conclusion

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.

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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.

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2

DIYer Makes Cannon Quieter

Bill is an expat living in the hills northwest of Beijing, and he uses his DIY Cannon to cut the particulate in his home.

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But Bill wanted to improve on the biggest downside of the Cannon–it’s a bit loud on high:

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So Bill designed a sound dampening box for the Cannon out of foam blocks:

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There are openings in the front and back to let air through, although my guess is this lowers effectiveness by at least a few percent.

To further cut down on noise, Bill wrapped the Cannon with a blanket:

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With all that padding, Bill measured a 6 decibel drop in noise! If you have a Cannon already, here’s an idea of what that’s like: That’s 2 decibels quieter than the regular Cannon on low (52 db).

If the noise gets to you and you don’t mind a craft project, I sanction it! However, keep in mind two things:

  1. Monitor the fan temperature. Make sure your design does not allow the fan to overheat.
  2. I suspect this will lower effectiveness by at least a few percentage points. I can’t say more precisely without testing it.

Great work, Bill!

 

4-4

Are DIY purifiers still cheap in the long run?

When I published tests of the DIY compared to the expensive machines, a couple people on Zhihu (China’s Quora) asked if you really save money in the long run with the DIY once you calculate in the cost of changing the HEPA. After all, the IQ Air HEPA costs 1,782 RMB, but you can use it for more than a year.

Long-Term DIY HEPA Costs

At that time I didn’t have an answer. To get an honest answer, I needed to do tests in the real world, but that took almost a year to complete. (That’s more than I can say for the numbers IQ Air and Blue Air give. If filters last 6 months in Sweden, are they going to last 6 months in Beijing?)

Now it’s a year later, and I have that data. Smart Air co-founder Gus ran his Original DIY for eight hours a day and tracked what percentage of particulate it removed from the air each day with a Dylos particle counter.

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Based on that data, I recommend changing HEPAs after 140 days at 8 hours per day (about 1,000 hours of use). Since that HEPA costs 80 RMB, that averages to .57 RMB per day and 208.6 RMB per year.

To get an idea of what that means, we can compare that to the cost of drinking a bottle of water a day:

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Big Brand HEPA Costs

I’m highly skeptical that HEPA recommendations for Sweden can be mapped onto China. But to be conservative, I’ll assume in my calculations that their numbers work the same in China.

I calculated costs for the Blue Air 203 and IQ Air Health Pro Plus using the same conditions I used for the Original DIY longevity test:

  1. Highest setting
  2. 8 hours of use per day
  3. Extend those costs over a year
  4. Include the cost of pre-filters but not carbon filters

Blue Air

Blue Air’s HEPA costs 300 RMB and lasts 6 months. That works out to 1.67 RMB per day and 609.6 RMB per year.

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IQ Air

IQ Air’s HEPA is more expensive: 1,782 RMB. On the sixth setting, it lasts 4,968 hours. If you use it 8 hours a day, that works out to 2.87 RMB per day and 1,047.4 RMB per year.

However, we have to add the cost of the pre-filter because the HEPA lifespan depends on the pre-filter. The pre-filter costs 645 RMB and lasts 2,016 hours on the sixth setting. That works out to 2.56 RMB per day and 934.3 RMB per year.

IQ Air grand total: 1,982 RMB per year.

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Conclusion

The long-term HEPA cost for the Original DIY is 66-89% less than the Blue Air and IQ Air.

It should be noted that these results may not map onto the Cannon. I’m still working on the Cannon longevity test, so we’ll need to wait until that test is done.

As always, I’m writing more details about the data below for fellow nerds.

DIY HEPA Costs

DIY HEPA costs are based on the 200-day longevity test in real Beijing air. All 200 days of original data are available in that post.

IQ Air Costs

The IQ Air cost calculations are based on a table from the North American IQ Air authorized retailer. That data is available here. I welcome other nerds to double check my calculations!

One thing to note: the calculations above are based on the sixth setting. If the IQ Air runs on setting three, the effectiveness will be lower, but the HEPA will last longer. In my calculations, I totaled the costs for one year one setting three: 901 RMB. That’s lower than the calculation above (because it’s processing less air), but it’s still more than four times the cost of the DIY HEPA.

Blue Air Costs

Calculating costs for the Blue Air is more difficult. That’s because Blue Air doesn’t seem to publish longevity recommendations for different settings or how many hours you can use the HEPA. Instead, Blue Air says you should replace the HEPA after six months.

Thus, I sent Blue Air an email asking how many days I should replace the HEPA if I run my machine eight hours a day. A Blue Air representative told me six months and told me I would void the warranty if I used it more than six months. Thus, I used their recommendation in my calculations (original email).

If Blue Air is wrong and the HEPA lasts long at eight hours per day, the calculation changes. I have alternative calculations here.

30元滤网靠谱吗

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:

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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.)

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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.

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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.

Conclusion

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…)

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How Big of a Room Can the Cannon Clean?

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After I tested every fan in China I could find with a flat front…

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…the results of one fan stood out above the rest. Because of its appearance and how much butt it kicked, I called it “the Cannon.”

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In Anna’s 15m2 Beijing bedroom, it crushed particulates. It removed 97% of the small .5 microns particles.

But how well can it do in larger rooms? Most companies estimate this using flow rate, but those calculations depend on how well houses are sealed and how dirty the air outside is. Since air outside is dirtier in China, I think it’s necessary to do real-world tests.

Methods

Fortunately, I moved into a large 4-bedroom apartment, with a 30.5m2 living room. I tested the Cannon six times on high and tracked particulates with a Dylos particle counter. Each test lasted at least three hours.

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I calculated the percentage reduction in particles from the first hour to the last hour.

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This test also serves as an important extension of the earlier room tests because:

  1. These tests were run in the daytime. Several people have the intuition that pollution goes down at night because people are less active and fewer cars are on the road (but the data shows that intuition is false–PM 2.5 pollution is worst in the middle of the night in Beijing).
  2. These tests were run while people were moving around in the room and opening the door to the outside. This is more conservative than the nighttime tests because the Cannon has to fight influxes of outdoor air.

Results

Over six tests, the Cannon removed 92% of .5 micron particles and 89% of 2.5 micron particles even with people moving around and opening doors.

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In previous tests, 2.5 micron reductions were usually slightly larger than .5 micron reductions, so it’s a little surprising that the 2.5 micron reduction was 3% lower than the .5 micron reduction. My guess is that this is because people were moving in the room, and human movement affects the 2.5 micron readings much more than the .5 micron readings.

Conclusion

The Cannon can clean rooms at least 30.5 m2, which puts it above the 22.3 m2 Blue Air recommends for their 3,6000 RMB 203 model.

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

 

(more…)

3-10

Does Carbon Really Work?

I wrote early on that my number one concern in China is particulate pollution. It’s on so many people’s minds that nowadays even the guy I buy chuan’r from knows the word “PM 2.5.”

HEPAs do an amazing job at removing particulates, but particulates aren’t everything. Gases like volatile organic compounds (VOCs) are so small that they slip through HEPAs. “VOC” is a big category, including things like benzene and formaldehyde. VOCs can cause cancer, throat irritation, dizziness, and other not-fun side effects.

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And for homes that have VOCs problems, we’re supposed to use activated carbon filters. But do they actually work?

When I published the instructions for how to build your own purifier, I wasn’t comfortable recommending activated carbon because I hadn’t tested it, and I didn’t want to just trust what the big filter companies say. So this year, I set out on a journey to test whether carbon actually works.

Method

I soon learned that gas testing is not easy. First off, “harmful gases” is not a natural category. You can buy a particle counter that will detect all particles of a certain size, but there is no detector that will detect all gases. Instead, you need one for each type of gas, and that is not cheap.

In this case, my scientific curiosity cost me $3,542 for this Industrial Scientific Ibrid MX6. It detects VOCs, carbon monoxide, sulfur dioxide, hydrogen sulfide, and nitrogen dioxide. It uses a photo-ionization detector to measure VOCs from 0-2,000 parts per million with a resolution of .1.

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Next, I needed a source of gas pollution. Interestingly enough, my apartment didn’t have enough VOCs to register on the MX6–nor did 8 other Beijing apartments I tested. (That speaks to whether purifier companies should scare people into thinking that everyone needs carbon.)

Because I didn’t have detectable VOCs in my home, I had to go out and find VOCs to pump into my room. I chose cigarettes because they are known to emit VOCs like benzene and formaldehyde.

In each test, I burned 3 Marlboro cigarettes in an enclosed 3.78 m2 porch. At the same time, I ran the Cannon on high for five tests with a composite carbon filter on the front:

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I ran four control tests with a fan but no filter. That way the room still has air flow, but no carbon. The cigarettes burned out after about 15 minutes, and I left the fans on for another 30 minutes.

Results

Looking at the Cannon + carbon alone after the cigarette extinguished, the VOCs dropped. Here is the data from one test:

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From there we can zoom out to include the time the cigarette was burning and the control fan-only condition. From there, it becomes clearer that the carbon was removing VOCs above and beyond just having a fan on.

Averaging across all of the tests, the VOCs reached a maximum of about 1 ppm while the carbon was on. Without the carbon, VOCs reached 1.5 ppm.

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After the cigarette burned out, the Cannon cleared the air of VOCs in 5 minutes to just over 15 minutes on average. Without carbon, the air still had VOCs after 30 minutes.

Averaging over all of the tests, the carbon removed 38% of VOCs by the time the cigarettes burned out compared to the fan-only condition. The Cannon removed 68% after another 5 minutes and 100% by 20 minutes after the cigarettes burned out.

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Conclusion

The results showed that carbon effectively removed VOCs. Thus, I’ve decided these filters are ready for game time. I’ve started shipping these tested carbon filters via Taobao and PayPal for 70 RMB.

Does everyone need carbon?

Purifier companies have a financial incentive to convince people to buy carbon filters. They make more money if they can scare people into buying carbon. But does everyone need carbon?

I don’t want to scare people into buying carbon. Why? In most homes, my MX6 detected zero VOCs. I only found VOCs in homes that had an obvious source of pollution such as remodeling or smoke. And in all of the places where the MX6 detected VOCs, I was able to notice a chemical smell. That makes sense, since lots of VOCs have noticeable smells, like benzenetoluene, and formaldehyde.

Now, I don’t want to say that VOCs are never a problem. Photo ionization detectors like the MX6 are not the most sensitive test type out there. I bet I’d pick up small amounts of VOCs if I sent air samples to a laboratory.

However, scientists have used fancier methods and found similar results. For example, scientists in Hong Kong tested homes and found that most non-smoking homes did not have un-safe levels of VOCs. We need more tests like this.

For now, I will not be using carbon in my home, but I think it is right for people whose homes have:

1. Recent remodeling

2. Recent painting

3. Smoking

4. Odors

5. Nearby sources of gas pollutants (such as living near a factory)

6. Symptoms such as inflammation and asthma

Do I still need the HEPA?

Yes. Carbon is designed to get organic compounds, not particulate in general. I wouldn’t have even tested this, but Anna accidentally forgot to attach the HEPA and unknowingly ran a regular particulate room test with carbon only.

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The results weren’t pretty–far below the 95% reduction with the added HEPA. Thus, I do not recommend using carbon only.

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


Gas Test Methods

During the gas test, all of the doors and windows were closed. Because the test was in an enclosed porch, there were several windows, which means the room was less well sealed than an interior room.

During the test, I burned three Marlboro cigarettes pointing straight up in an ash tray on the floor. The gas detector was on a small ledge at just above waist height, only a few feet laterally away from the cigarettes. The purifier was placed a couple feet from the cigarettes in the center of the porch with the air stream pointing upward.

Limitations

Burning three cigarettes in a small room is extreme. With the exception of airport smoking lounges, few places will have this much smoke in one place. However, the test does demonstrate that carbon reduces VOCs.

Many people have asked me how long carbon filters last. Unfortunately I don’t have a good answer to that. The carbon filter I used was still effective after several of these powerful smoke tests, which should be far more VOCs than a normal person would face, but I cannot say more than that.

VOC Info: The PID lamp has an eV rating of 10.6. eV ratings determine what types of VOCs PID machines can detect. With a rating 10.6 eV, this machine was probably reliably detecting benzene, which has an ionization potential of 9.25 and perhaps toluene, which is at 8.82.

This PID lamp is not designed to detect formaldehyde, which has an ionization potential of 10.88. However, I later conducted tests with formaldehyde samples and found that the MX6 could detect it, and an engineer at Industrial Scientific told me he thought it was plausible that the machine would pick up on formaldehyde, although not 100% reliably.

Detector Info: The MX6 came with a pump to draw in air (as opposed to versions that passively sample air). During the test, it was outfitted with the filter designed to prevent particles from entering the machine. I zeroed the machine if necessary before the test.

Early Tests: As I started doing the tests, I was still figuring out how to do the tests–it seems simple when you look at the results, but tests like this are not easy! For the first three control tests, I had not yet decided to let the MX6 run for 30 minutes after the cigarettes burned out, so data is limited for those three occasions. However, the conclusions are similar with those three tests excluded.

Control Condition: In the control condition, I used a Honeywell fan on the low setting to approximate the air flow of a Cannon on high with a filter on it. In other words, a filter lowers the air flow, so the Honeywell may be a better approximation of the amount of air flow during the experimental condition.

In addition, I ran tests with an IQ Air Health Pro Plus (1) with the filters removed and (2) with the pre-filter only as additional control tests. The results were similar to the Honeywell fan-only tests. I’ll use the IQ Air tests as the control condition in tests of the IQ Air’s ability to remove VOCs in the future.

Gas Test Data

Below is a summary of the gas tests. Because the raw data file is large, I’m making it available as a download here.

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Data: Carbon-only test

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Anna only ran the carbon-only test for one day because it was an accident. More tests would give a more reliable estimate of the effectiveness of using carbon only, but because carbon isn’t designed to get particulates in general, I’m hesitant to subject anyone to that much dirty air. On the upside, the outdoor PM 2.5 was quite stable that day, only moving up 4 micrograms from start to finish.

 

 

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.

Method

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:

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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.

Results

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

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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!

Conclusion

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!

净化器需要全天开着吗

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?

Method

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:

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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.

Results

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:

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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.

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Conclusion

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.

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(more…)

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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.

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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?

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Methods:

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.)

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Results: 

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:

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

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

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Conclusion:

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:

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

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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:

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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.

(more…)

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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!

Methods: 

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:

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Results: 

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%.

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Conclusion:

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 days. People 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.