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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.

 

 

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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:

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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:

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Conclusion: 

  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.
  4. 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.Limitations:
    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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    Because there is so much data, I’m making the raw data available here on my personal site. The methods are the same as in my previous tests.

    Basics: Gus did the tests overnight while he slept. The room door was closed during the tests. I calculated effectiveness by averaging the number of .5 micron particles over the last three hours before Gus woke up. Then I divided that number by the baseline measurement before Gus turned on the purifier.

    Effect of Apartment-mate’s Purifier: As I wrote in my last update, we discovered on Day 57 that Gus’s roommate was running a purifier 24 hours a day in his room. This may artificially boost our estimate of the lifespan. It also artificially lowered the baseline particle counts and thereby lowered the estimates of effectiveness for the first 57 days.

    Therefore, the raw data points for days 0-60 would give people a false sense that the HEPA increases in effectiveness after 60 days. To correct for that, I averaged difference between Days 0-57 and Days 58-90. The original graph is in my earlier post, and all of the raw data is available here.

    Outdoor Fluctuation: On Day 94 and Day 113, the outdoor PM 2.5 concentration increased by almost 100 micrograms during the test, so I removed these days from the calculation. Keeping those datapoints lowers those two 10-day estimates by about 3%. This data is retained in the raw data file.

    Room Change: For days 1-101, Gus was in his old apartment, which had a 12.3 m2 bedroom. For days 102-130, he was in his new apartment. Obviously that’s not ideal, but his landlord was taking back his old apartment! We will repeat the tests in his new apartment from scratch after this test ends.

    Calculating room size for Gus’s new bedroom is a little tricky because the ceiling is slanted. The floor area is 13.7 m2–slightly larger than his old room. But the slope of his ceiling gives it a volume of 26.3 m3 compared to 33.5 m3 if it were perfectly rectangular and 30.1 m3 in his old room.

    Thus, his new room may be easier to purify, although I suspect the roof is not very well sealed, so it may leak more than his old room.

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Q&A

joshuaw81 提问: Hi Thomas, just started one of your cannon kits earlier today and it’s already turning gray. Already a fan of your work! Two quick questions for you: 1- I received a foam/fiber square that is white on one side and light green on the other. I’m assuming this is packaging, and not part of the filter? 2- Are these HEPA filters washable? Many thanks!

 

Hi Joshua,

Good questions! We’re actually working on a pamphlet to go along with the cannon that will answer these two questions, so it’ll be clearer in the future! But here are the answers:

  1. What’s the pre-filter for?

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The pre-filter goes behind the HEPA filter, and it catches the larger dust particles so that the HEPA will last longer. We don’t include pre-filters on the Original DIY because the fan isn’t as strong as the Cannon.

  1. Can I wash the HEPA?

Unfortunately, no. HEPAs are not washable. That’s one reason we try to keep our HEPAs as affordable as possible.

Q&A

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.
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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:

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  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:

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  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.

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  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.

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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.