Why a Weaker Filter Actually Makes a Stronger Purifier (HEPA H12 vs. H13)

Why does Smart Air’s latest air purifier, The Sqair, use a weaker filter–an H12 HEPA compared to the H13 HEPA filters out there? Tests of different filter grades revealed that weaker filters sometimes make stronger purifiers.

Of course, purifier companies like to market their filters as top grade. Lots of purifiers on Amazon and Taobao tout their “true” H11, H12, and H13 HEPA filters.

But which one is the best? It’s easy to fall into the trap of thinking the higher the grade the better. Lots of “experts” argue that the real key to identifying a good purifier is to look for a high-grade HEPA or a “true” HEPA.

Reality is far more interesting than this simple idea.

Myth: The higher grade the filter, the better the purifier.

Even if weak furnace filters can capture nano particles (they really can), it’s still better to use a higher-grade filter, right?

Why use a 99.9% filter when we could use a 99.999% filter!? (There really are 99.999% filters, by the way).

The problem with this logic is that the higher-grade filter comes at a cost. That cost is air flow.

Higher-grade HEPA filters will capture more particles, but they’ll let less air through. Thus, there’s probably a sweet spot in the tradeoff between air flow and particle capture.

Is H13 HEPA better than H12?

I started Smart Air based on the idea of using data to question received wisdom, so we set out to run real-life experiments to test this theory. We had the perfect opportunity arise when we were designing our brand-new air purifier – The Sqair.

To find out the answer, we fitted the same Sqair with two different grades of filters: H12 and H13.

We then ran 6 effectiveness tests (CADR test methodology) to see which one performed best.

The results were the opposite of intuition! The H12 filter gave 27% more clean air than the H13 filter, even though it captures fewer particles on a single pass (99.5% of 0.3µm particles as opposed to 99.97% in the H13).

Why Is A Weaker Filter More Effective in The Sqair?

Even though the H12 filter captures fewer particles, it still managed to clean more air. How could that be?

The answer lies in the air resistance of the H13 filter. Imagine you’ve got yourself two tea strainers:

It’s fairly obvious the fine strainer will do a better job of capturing tea leaves. The coarse strainer will drain the tea much quicker, but with the downside of perhaps letting a few more tea leaves through than the finer strainer.

Let’s say the fine strainer takes twice as long as the coarse strainer. In that case, we could put a second coarse tea strainer under the first to capture even more of the tea leaves.

That’s exactly what happens with an H12 filter. The weaker filter lets a little more air pollution through on the first pass, but it processes more air. Once the air passes back through the filter a second time, it’ll capture another 99.5% of particles. In two passes, the H12 filter can capture 99.75% of the particles! (99.5% on the first pass, and 99.5% of the remaining 5% of particles on the second pass).

Testing H12 vs. H13 HEPA Filter Air Flow

One way to double check this theory is to use an anemometer (fancy name for a tool that measures air speed) to test the speed of the air coming out of The Sqair.

On the high setting, the H13 filter gave an airflow of 2.42m/s, and the H12 filter gave an airflow of 2.98m/s. That’s 23% higher air flow, which explains the higher overall effectiveness of the H12 filter.

P.S. But what about nanoparticles?

The Sqair HEPA captures nanoparticles too—thanks to the nerd miracle that is Brownian motion. Check out the weird and fascinating science that explains why.

P.S. Great! Does that mean H12 filters are cheaper too?

When we were developing The Sqair, we asked exactly the same question. Turns out H12 and H13 filters use exactly the same manufacturing process. It’s just a question of how tightly packed the fibers in the filter are. The result is that the H12 filters cost no less than H13 filters.

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