Why a Weaker Filter Actually Makes a Stronger Purifier

Why does Smart Air’s latest air purifier, The Sqair, use a weaker filter–an H11 HEPA compared to the H12 and 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.

Let The Data Speak: Which HEPA Filter Works Best?

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: H11 and H12.

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

The results were the opposite of intuition! The H11 filter gave 27% more clean air than the H12 filter, even though it captures fewer particles on a single pass (95% of 0.3µm particles as opposed to 99.5% in the H12).

Why Is A Weaker Filter More Effective in The Sqair?

Even though the H11 filter captures fewer particles, it still managed to clean more air. How could that be?

The answer lies in the air resistance of the H11 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 H11 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 95% of particles. In two passes, the H11 filter can capture 99.75% of the particles! (95% on the first pass, and 95% of the remaining 5% of particles on the second pass).

Testing H11 vs. H12 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 H12 filter gave an airflow of 2.5m/s, and the H11 filter gave an airflow of 3.15m/s. That’s 26% higher air flow, which explains the higher overall effectiveness of the H11 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 H11 filters are cheaper too?

When we were developing The Sqair, we asked exactly the same question. Turns out H11 and H12 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 H11 filters cost no less than H12 filters.

---