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Can DIY Air Purifiers Clean Large Rooms?

2020-09-252014-12-31 by Thomas Talhelm

Can DIY air purifiers clean large rooms? Hundreds of hours of Smart Air tests have found that DIY air purifiers cut fine particulate, reduce smoke, remain effective over 200 days, and even capture chemicals like formaldehyde. But an important practical question is whether DIY purifiers are effective enough to purify large rooms.

DIY Air Purifier Search

To answer this question, I tested every fan I could find, as long as it had a flat front. I was looking for a fan that put out a lot of air.

After getting all the data, of one fan stood out above the rest. I called it “the Cannon.”

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. I wanted to know how well a DIY air purifier can clean under real-world conditions.

DIY Air Purifiers Clean Large Rooms: The Test

I tested the Cannon on its high setting six times in a 30.5m2 living room–a large, open space. To track pollution, I measured 0.5 and 2.5 micron particles using a Dylos laser particle counter. Each test lasted at least three hours.

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

Besides testing in a big space, this experiment extends the findings 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–PM2.5 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.

DIY Air Purifiers Clean Large Rooms Effectively

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.

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.

Bottom Line:

With a fan as strong as the Cannon, DIY air purifiers clean large rooms effectively, reducing 0.5 micron particles 92% on average.

Smart Air

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

HEPA Longevity Test: 170 Days

2020-07-152014-09-06 by Thomas Talhelm

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 …

How Long Do Smart Air HEPAs Last? Longevity Test Day 130

2020-07-152014-07-11 by Thomas Talhelm

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 …

Are All DIYs the Same? Poor Results from Huanwo Lantian

2020-07-272014-06-06 by Thomas Talhelm

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. The Huanwo Lantian (还我蓝天) 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 Smart Air co-founder 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.

DIY Purifier Test 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.

Imitation DIY Purifier Results

The Huanwo Lantian DIY removed 21% fewer particles 0.5 microns and above and 11% less 2.5 micron particles than the Original DIY.

Is It The Fan?

The Huanwo Lantian fan is slightly smaller than the Smart Air Original, so one explanation could be that the fan is just moving less air. Anna tested that by strapping the Huanwo Lantian HEPA onto our Original fan.

Here’s what that test showed.

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 Filter?

The second possibility is that the Huanwo Lantian 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.)

The Huanwo Lantian HEPA captured 7% fewer particles that the Smart Air HEPA. Thus, the major source of the poor performance of the Huanwo Lantian seems to be the quality of the filter.

Bottom Line:

In room tests, the Huanwo Lantian DIY removes 21% less particulate than the Original DIY, and the data suggests that the reason is that it uses a lower quality HEPA filter.

Smart Air

Now, don’t get me wrong. The Huanwo Lantian DIY purifier is making the room air cleaner. It demonstrates yet again that DIY air purifiers work. I’d rather have a Huanwo Lantian 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

2020-07-152014-05-20 by Thomas Talhelm

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 …

A New Way to Test Whether the DIY Works

2020-07-152014-02-10 by Thomas Talhelm

So far, I’ve been testing air purifiers by taking a baseline measurement of particulate pollution in a room, and then turning on the purifier and testing whether the counts drop. I’ve used that method to test the DIY and more expensive machines.

However, I recently bought a second laser particle counter, so my collaborator Gus suggested another method:

Run one particle counter in the bedroom that has the purifier
Run another particle counter in a different room that does NOT have a purifier.

Two Problems Solved

The benefit of this method is that the control room represents the counterfactual–what would have happened if we hadn’t turned on the air purifier. That can answer two types of “what if’s.”

1. Fluctuations in outdoor air pollution

If a northwest wind hits Beijing and makes the outdoor air a lot cleaner, we can separate the effect of the outdoor air fluctuations from the effect of the purifier. In that situation, my old method would artificially raise our estimates of effectiveness. Changes in outdoor air can also artificially lower our estimates of effectiveness if the outdoor air gets dirtier after we turn on the purifier.

In previous tests, I corrected for this by averaging over multiple tests. I also analyzed the data after removing days in which outdoor air pollution fluctuated a lot (for example, I do that sort of analysis in the extra nerd notes here).

But it’s always nice to use different types of tests to make sure an effect is real, so Gus did this experiment.

2. Particles settling in closed rooms

If you close a room, the particles–even really small particles–will slowly float down and settle out of the air.

Thus, if we’re testing in a closed room at night, how do we know the purifier is causing the reduction, and not particle settling?

The Control Room Purifier Test

Gus set up one particle counter in his room and one in his kitchen:

He let the particle counters run for several hours, and then a timer turned on the Original DIY in his room. (The kitchen had no air purifier.) Here’s what happened:

The difference between the bedroom and the kitchen air quality can approximate the effect of the air purifier. It looks like Gus would have been breathing 16,000 of these 0.5 micron particles in the air in his bedroom if he hadn’t turned on his DIY purifier.

And it’s pretty clear that the kitchen air quality (where we don’t have a purifier running) is following outdoor air quality:

(Be aware that I’m overlaying these two lines on the same graph, but the Y-axes are different. This is NOT saying that indoor air is as bad as outdoor air. Indoor air is usually cleaner than outdoor air.)

Conclusion

Similar to earlier tests, the double particle counter test shows that the DIY purifier is removing particulate pollution from the air.

As always, I’m including more details for fellow data nerds below.

DIY air purifier effectiveness blueair iqair philipps cheap god air purifier

Directly Comparable Tests: DIY vs Blue Air

2020-12-152013-12-09 by Thomas Talhelm

I’ve wanted to know for a long time whether DIY air purifiers are as effective as the Ferrari filters.

In an earlier post, I compared my data to the tests of Dr. Saint Cyr (whose excellent posts inspired me to look into filters in the first place). But I noted that the comparisons were far from perfect because:

The rooms were different.
The Cyr post did not specify how long the tests were (and that can make a big difference if you’re looking at times under an hour–see this time comparison).
The Cyr post did not describe the particle counter or particle size.

How We Tested The Air Purifiers

But now I finally have directly comparable data! That’s because two kind souls donated a Blue Air 203/270E (3,600 RMB) and a Philips AC4072 (3, 000 RMB). That means I could finally test the DIY against expensive brands in the same room, for the same amount of time, with the same particle counter.

To do that, Anna ran 11 overnight tests with the Blue Air and 9 tests with the Philips. As always, I calculated effectiveness as percent reduction in particulates from the room air. Anna tested the air before she turned on the air filter, and then set a Dylos DC1700 laser particle counter to take hourly measurements of the air in her 15m2 room.

Anna used the highest setting on each purifier. (I’m putting the original data and more details about the methods for fellow nerds at the end of this post.)

And (drumroll!) here are the results:

The Cannon removed as much particulate as the expensive machines. Not bad for 450 RMB!

Yet all four filters were making the room air significantly cleaner. For particles 2.5 microns and above, all four removed over 90%. For particles 0.5 microns and bigger, all four removed over 80%. I’m not the first person to say: All you need to significantly reduce the particulate pollution in your home is a simple HEPA filter.

Based on the data, here’s how much you’re paying for each percentage reduction in 0.5 micron particles:

(And that’s not counting the cost of the exorbitantly priced replacement filters.)

Recently, a Chinese news article claimed air filter companies are making “falsely inflated profits.” That fits with this data showing that the Cannon removes as much particulate as the Blueair, yet costs about 10% per percentage of 0.5 micron reduction. Similarly, the Original DIY removes 4% less 2.5 micron particulate and 6% less 0.5 micron particulate than the Blueair on average, yet the Blue Air costs more than 16 times as much.

Bottom Line: Air Purifier Test

You can remove particulate pollution from the air in your home and pay far less than a Blueair or Philips

Smart Air

What About Gas Pollution?

Now, as I’ve said before, particulates are not everything. There are also gases like ozone and nitrogen dioxide (although I’m less concerned about those). Here’s how to know whether your home has serious gas pollution. If it does, tests show that activated carbon removes gases like formaldehyde and benzene.

Open Data and Methods

As always, I’m posting the original data and detailed methods for fellow nerds. Don’t believe me. Check out the data and decide for yourself.

Q&A: Do ACs Bring in More Dirty Air?

2020-07-152013-10-08 by Thomas Talhelm

tmthyliu 提问: Just built a DIY air filter, pretty excited to have clean air in the house! I was wondering though, how does this work when the air conditioner is on? Does it make a difference at all? I don’t know how ACs work but it seems they pump more …

Are Particle Counters and Government Machines the Same?

2020-07-152013-10-03 by Thomas Talhelm

My particle counter is a beast–I’ve loved it. But I’ve always wondered how the counts of laser particle counters like mine: …compare to the measurements of the huge stationary air quality monitoring stations that governments use, like this one in New Zealand: If you look at …

Is your air conditioner bringing in dirty outside air?

2020-07-152013-08-24 by Thomas Talhelm

Is your air conditioner bringing in dirty outside air? Here’s a live test with a particle counter. See the average results over multiple AC tests here.