Should air purifiers have the filter on the front or back?

When I made my first DIY purifier, I put the filter on the front:


Within a few weeks, several people told me I was an idiot. Why’d you put the filter on the front?

That led me on a long, unexpected journey. It turned out that I wasn’t the only one interested in breathing clean air in China, so I started a small social enterprise called Smart Air to ship these low-cost purifiers to fellow-air breathers who don’t have $1,000 to blow on an IQ Air.

Many people have a strong intuition the filter should go in the back

Here’s a more-respectful-than-usual suggestion on my live test video:



I’m fascinated by how many people have this intuition. But when I push further, most people struggle to come up with a good reason. For some people, I think it’s purely an intuition about having air come from a pure source. But here are the two most coherent reasons I’ve heard:

Claim 1: It “protects” the fan.

This is probably the best reason I’ve heard. Yes, putting the filter behind the fan will prevent dust from hitting the motor and collecting on it. I think there’s a small, non-zero benefit of that, although I haven’t seen anyone actually test it.

Gut check: people run “unprotected” fans (we just call them “fans”) all the time, and it doesn’t seem to cause much harm.

Claim 2: It improves performance.

This is an odd idea to me. I’ve asked several physics PhDs, and they’ve said it shouldn’t make a difference. The amount of air coming in is equal to the amount of air coming out.

But who knows? Maybe there’s something about the air flow process that I don’t understand. I don’t like when people point to a theory and assume that proves the matter, without actually testing it. So I went out and tested it!

It’s hard to test with that first fan I used (above) because putting the filter on the back would require me to tape around the back screen. That would alter the fan, so we can’t compare apples to apples.

Fortunately, I tested lots of different fans.


And eventually I settled on this tube-shaped fan I call the Cannon.


It’s perfect for this test because I can easily put the HEPA filter on the front or back. So that’s what I did! I ran 20 tests of 0.5 micron particulate in my apartment in Beijing.
I also had a third-party lab test the two versions in a CADR test.

The results? In both tests, putting the HEPA on the intake side actually did slightly worse! Here are the more-controlled CADR test results.

Filter Front Back Effectiveness Efficiency Test

I was surprised. I thought they’d perform the same.

Now this next part is hard for me to understand, but the Smart Air team is fortunate to have an aeronautical engineer. Here’s an important detail about the Cannon fan I didn’t mention:

DIY air purifier filter Cannon


The fan isn’t in the middle. Instead, it’s about 2/3rds of the way down the tube. And according to our engineer, it helps to have more distance between the fan and the filter so that a column of pressure can build up.

Again, I still haven’t grasped the physics, so I invite anyone with more expertise to add their explanation here. But two independent datasets show that putting the HEPA on the front leads to higher reductions in particulate.

If the fan were in the exact middle, we hypothesize that performance would be equal with the HEPA on the front or the back.

Conclusion: Putting the filter on the front or back of the fan should provide the same reductions in particulate, as long as the distance from the fan is the same.

Input-Output Testing

I think this is a fascinating demonstration of how I bumbled into a good design. I didn’t know much about air flow dynamics, but I did lots of input-output tests before I chose this setup. In other words, I was agnostic about what was going on inside the fan, but I tested different fans, measured the output I cared about (reduction in particulate), and that led me to a good design—even though I couldn’t explain why at the time.

Speculation: Does turbulent air flow explain why “filter first” is worst?

Now we’re not 100% sure on this, but based on research on guide vanes, putting a filter before the fan will make the air going into the fan turbulent.

Purifier Guide Vanes

Making the fan operate in turbulent air flow (as opposed to laminar flow) has two consequences:

A. The efficiency of the fan is reduced.

B. The fan will be noisier.

However, we don’t know whether this effect would be large enough to be meaningful.

Putting the filter after the fan means turbulent air (from the fan) is going into the filter. We’re unsure what effect that has on the efficiency of the filter.


The Sqair air purifier Kickstarter

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Ignacio Pena

Hi, I tried to make my own and I placed the filter on the front. It turns out that 99.5% of the air gets kicked back and only 0.5% passes through the filter. What is wrong? The filter belongs to an air filter machine for home use (around 40 watts) and my fan is 52 watts!

This is exactly what we found with the DIY1.0 purifier. What we did was build a ‘shroud’ which we placed on the back of the purifier to stop the air getting kicked back. You can read more about how this works in this post on the DIY1.1 air purifier

Ignacio Pena

If you take a fan and while working place a hand in front of it and another in the back of it, the strength of flow is much larger on the front. It is therefore more efficient to place the filter in front of the fan. Just don´t use it to hard so that the filter doesn´t get damaged, seal anything between the filter and a bit more that where the fan is and clean the fan once every 6 months. The only question I have is how far the filter must be from the fan for better results. Any… Read more »

Great insight Ignacio! I’ve had exactly the same thought. Whilst it often feels like air going into a fan is much less than air going out of the fan (I think it’s to do with the way we ‘feel’ air), the truth is that the amount of air going into the fan is exactly the same as the amount of air coming out of the fan. If it wasn’t the case, then the air would be ‘disappearing’ into some black hole inside the fan! Regarding your question of how far the filter should be from the fan. We’ve run some… Read more »