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 …

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Thomas First DIY Air Filter Setup

A New Way to Test Whether the DIY Works

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.

DIY Purifier Test Mongolia PM2.5 Dylos

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

  1. Run one particle counter in the bedroom that has the purifier
  2. 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.

 

Particle settling speed velocity

 

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: 1

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:

DIY Purifier Test Control Room

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:

Indoor Air Pollution PM2.5 Versus Outdoors

(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.

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DIY air purifier effectiveness blueair iqair philipps cheap god air purifier

Directly Comparable Tests: DIY vs Blue Air

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

DIY Air Purifier Cheap

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:

  1. The rooms were different.
  2. 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).
  3. 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.

DIY Purifier Test Method PM2.5

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.

DIY Air Purifier PM2.5 Test

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:

DIY Purifier Test BlueAir Philips IQ Air PM2.5

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:

Price versus Effectiveness DIY Air Purifier Philips Blueair

(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.

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

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 …

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Are Particle Counters and Government Machines the Same?

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 …

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Does the Air Conditioner Bring in Dirty Air From Outside?

A question that I get asked often (and that I have always wondered about) is whether my wall-mounted air conditioner is bringing in dirty air from outside. If so, it’d be safer not to use it, especially on really bad days.

air conditioner

My short answer is no. To explain why, I’ve got three points of evidence:

 

1. How air conditioners work

Regular wall-mounted air conditioners in China do have a unit outside connected with tubes to the inside, but that tube is not bringing in outside air. It’s passing coolant, and letting heat escape outside.

So where does the air it’s blowing come from? If you look around your air conditioner, you’ll probably discover that it works like mine: it brings air from the top, runs it over the cooling coils, and blows it out the front. It’s recycling indoor air, not bringing in outdoor air.

 

2. Tests of the air coming out of the air conditioner.

I’ve held my particle counter up into the air coming out of my AC unit, and it’s no different from the ambient room air (See a live test here). I’ve also compared that air to outside air on very dirty days, and the air coming out of the AC is nowhere near as dirty as outside air.

(I did this test when I had just turned on my AC. If the AC were bringing in dirty air and I were to test the exhaust after I had been running the AC for a long time, then my whole room would be dirty, not just the exhaust.)

 

3. Tests of the ambient room air before and after turning the AC on

Results? AC makes basically no difference.

Here’s what happened in one test after turning the AC on:

And here is the average effect over 7 different tests in my bedroom.

In each test, I ran my particle counter for 30 minutes to get a baseline. Then I turned on the AC for 30 minutes. Here I’m comparing the numbers just before I turned the AC on and 30 minutes later. As you can see, there’s basically no effect. If anything, the larger 2.5 micron particles go down slightly. My guess is this is because the coarse plastic filter in the AC unit captures up some large particles.

 

Conclusion: If it’s hot outside, don’t sweat it. Use your AC.

Central AC: I should note that these tests are of wall-mounted AC units in China. Central air conditioning may work differently.

For those interested, I’m pasting the detailed data below.

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Comparing Air Purifiers

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 …

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DIY Filter Compared to Expensive Filters

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

In earlier posts (1, 2), I showed that you can make effective an DIY filter to clean air pollution out of your home. But just because it 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.

 

Comparison with Dr. Saint Cyr’s Test

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 filter. Here are the results:

DIY Filter comparison

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. 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:

DIY filter cost per reduction

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: A DIY Filter can be as effective

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:

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