Types of Filters

Premise: This blog is for people concerned about air pollution. There are many valid reasons people want air purifiers: pet allergies, pollen, and asthma. However, these are not what I’m concerned about while living in China. Here I assess purifiers solely based on whether they can help protect me from particulate air pollution.

With that in mind, here are the three most common types of filters in portable air purifiers:

  1. UV light filters are designed to kill bacteria. In China, I’m concerned about air pollution, not bacteria, so UV filters are unnecessary. But beyond that, the US Environmental Protection Agency says that UV filters in most air purifiers don’t even kill bacteria because the air passes too quickly through the light. UV light takes more time to kill bacteria.


Conclusion: UV filters aren’t what I need.

  1. Activated carbon filters use charcoal screens to catch certain types of chemicals and organic matter that interact with carbon. For example, carbon will capture things like formaldehyde and other volatile organic compounds (formaldehyde test; general VOC test). But carbon will NOT capture particulate in general (see the extra test at the end of this article).


Activated charcoal is good at trapping other carbon-based impurities (“organic” chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all – sodium, nitrates, etc. – so they pass right through. Howstuffworks.

Conclusion: Carbon filters may help (particularly for certain types of gases that carbon can get), but they don’t target all particulate matter. It would be a mistake to use ONLY a carbon filter because it would not get rid of enough particulate pollution (see test evidence here).

  1. HEPA filters are the solution. “HEPA” sounds fancy, but it’s just a standard that means the filter catches 99% of particles .3 micrometers and above (they also get particles smaller than .3 micrometers–the .3 designation is based on the overlapping point of different types of filtering). That covers a lot of particulate pollution–the most-often cited figures are for 2.5 micrometers.


HEPA filters aren’t rocket science. They work pretty simply: particles get stuck in the synthetic fibers (using a few types of physical filtering processes). In fact, if you have a vacuum cleaner, it probably has a HEPA filter in it. You can get one for about $10.

Contrast that with the major purifier companies like IQ Air and Blue, which have proprietary HEPA filters with names like “HyperHEPA” that cost $200. These may have benefits like a longer lifetime, but direct comparison tests show the super expensive machines capturing no more particulate than the non-proprietary HEPA (1), even measuring down to .01 microns (2).

Bottom line: If your concern is particulate air pollution, a HEPA filter is really all you need. And for that, you don’t need spend 8,000 RMB. You can make your own for 166 RMB.


The Sqair air purifier Kickstarter

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Great reading, whole blog, most appreciated thank you! If we go deeper, what do you think about quality of those impregnated carbon filters what are used as pre-filters for actual HEPA? Some ordinary e.g. Electrolux air purifier used in Europe market area use following pre-filter https://www.amazon.co.uk/Electrolux-EF109-Accessories-Purifier-Activated-Carbon-x/dp/B008PORWJ0 How is activated carbon attached to plastic (I think that it is not microglass fiber) filtration material? Can the activated carbon itself shed and cause pollution? Hard to find any kind of quality assurance and/or test document. Can the carbon itself contain some heavy metals and to what extent and so forth. What I… Read more »

I’m glad you brought up the ‘foam’ carbon filters Miika! We haven’t done any rigorous tests on these yet, but it’s on our list of things to test! We did do some preliminary tests when we were looking at what carbon material we should use for our purifiers. When testing it then, we found that it wasn’t as effective as our regular granular carbon (the effectiveness was roughly 65% lower!). Based on that one test, we can make an assumption it’s not as effective, but more testing would need to be done to be sure. Most carbon material that’s sold… Read more »

[…] draw too broad of a conclusion here. This doesn’t mean ALL air purifiers are junk. Instead, I use HEPA filters. HEPAs actually capture particles, and they are backed by empirical tests (1, 2, 3, 4, 5, 6). […]