What combination of air filters do I need to protect against everything?

This will sound like a grand claim, but two filters are all you need. Here’s why (with data).


Very Reasonable Reasons To Be Skeptical

Why might you be skeptical? For one, big purifier companies are in an arms race to tell you they have more filters than the next guy. Simpleton Thomas here is telling you you just need two, but these guys have six (proprietary!) layers of filters:



But wait, those guys are stone age chumps, because these guys have seven stages! Also proprietary.



I’m not an expert in air pollution. I’m just a social psychologist who does a lot of research in China. Spending time in China put me on a path to care a lot about the polluted air I often breathe. In the midst of all that caring, I saw purifier companies telling me I needed to spend $2,000 for “the cleanest air guaranteed.” That made me suspicious.


Does it really cost $2,000 to make these things? Is there really something meaningfully different about “plasma arrays” and imported Swiss HEPA filters?



So I started digging. I found out the technology behind air purifiers is stupidly simple. Purifiers are just fans and filters.

They’re so simple that I “built” my own purifier. It’s just a HEPA filter strapped to a fan. (Here’s how you can build your own DIY purifier.)


But I wanted to know if it really worked. I’m not an expert in air pollution, but I am a scientist, so I understand data at least. I bought a laser particle counter to test for microscopic pollutants in my air. I was pretty geeked.



The data showed my $30 DIY purifier cut the amount of 0.5 micron particulate in my room by 84% on average. A souped up version with a stronger fan reduced particulate by 97% (I made all the data and methods open source).



I was so excited I wanted to tell the world. I started a group called Smart Air to ship these no-nonsense purifiers to other air-breathers-in-need. As I started ramping things up from DIY home project to social enterprise, I tested all the filters from all the manufacturers I could find. What I found was that I really just needed two types of filters because of a simple fact about natural kinds:


There are essentially two kinds of air pollution. That’s the principle behind why all we need is two filters.


1. Particles: HEPA filter

For every particle—and I mean every particle—a HEPA filter is all you need. They’re nothing fancy. They were invented back in the 1940s, and they’re just a mat of synthetic fibers.



These capture essentially ALL particles. At first I was misled by the common definition of HEPAs—that they capture 99% of particles 0.3 microns and above.


Reality is far weirder. It turns out HEPAs (and masks and furnace filters) are actually better at capturing particles smaller than 0.3 microns.



People have asked me about lots of specific particles—do HEPAs capture dust mites? Pet dander? Asbestos? Bacteria? Mold? The answer is yes.



Here’s an easy way to answer whether a HEPA filter will capture the particular particle you care about: Is it a particle? If yes, then yes.


2. Gases: activated carbon filter

The other category of air pollutants is gases. This includes chemicals that usually come from indoors—things like formaldehyde coming from furniture, chemicals coming from cleaning products, and even chemicals coming from wrinkle-free “treated” clothes.

Gas pollutants can also come from outdoors. Outdoor gas pollutants are usually gases like NO2, SO2, and ozone, which often come from car exhaust and industry.

How do we get rid of these gases? HEPA filters won’t help because HEPAs capture particles. Instead, activated carbon will capture these bad guys.

Activated carbon is surprisingly versatile. I ran real-world tests to find out whether carbon works, and the data showed carbon captured volatile organic compounds coming from cigarettes (probably including a fair amount of benzene)…


formaldehyde I ordered straight from the factory…



…and whatever unknown chemicals were coming out of the Smart Air office’s new flooring and wallpaper.



It’s a little bit hard to track down every single gas that carbon can capture, but scientific studies have found that carbon also captures ozone, NO2, and SO2. Thus, a simple activated carbon filter will help reduce the major gas pollutants I’m concerned about.



So that’s it! Two filters will protect you against basically everything—at least the most common air pollutants you are likely to face.


Carbon filters have two important limitations

Activated carbon filters are great, but they’re not as awesome as HEPAs. For one, HEPA filters capture 99% of particles in a single pass, but carbon filters aren’t as good. For example, this study found carbon filters captured different chemical gases anywhere between 5% to 60% in a single pass—sometimes lower.


Carbon also gets “full,” so the more carbon the better. Unscrupulous purifier companies put a tiny bit of carbon into a filter so they can put carbon on the spec sheet. But that carbon will get full in a hurry and then lose effectiveness.


Carbon bottom line: If you’re worried about gas pollutants, look for a carbon filter with lots of carbon and change the filter frequently.


If it’s really so simple, why don’t more air-breathers know this?

Just two, inexpensive filters. That’s pretty awesome, right? Why don’t more people know about this?


One big problem is that air purifier companies don’t make money from simplicity. They make money from claiming they have proprietary technology (that they’re often unwilling to have tested by third parties), fancy imported filters (that they also don’t want tested), or some other advantage that conveniently involves you paying a few hundred more dollars.


One of the most common tricks out there is to package the facts above as if it’s something special about their purifier. Here’s Dyson doing just that:



But what about…

But wait, you really need just two filters? Surely it’s more complicated than that. What about…




If there’s one other filter to add to your purifier, a pre-filter is it.


Pre-filters don’t improve performance. What they do is capture large particles, which your HEPA filter would capture anyway. The benefit is that they spare your HEPA filter, allowing it to last longer.



Many smells are volatile organic compounds, which means carbon filters help capture smells.


Washable HEPA filters?

A big problem with HEPA filters is that they eventually get full and have to be thrown away. Some companies claim to have washable HEPA filters to solve that problem. Some purifier “experts” tell you to wash your HEPA.



The data nerds at Smart Air tested both vacuuming and washing HEPA filters, and the results showed that washing HEPA filters actually worsened performance.


I have yet to see an empirically proven washable HEPA filter.



First, what the heck is smoke exactly? Smoke contains particles and gases.



For particles, we know the HEPA covers us. For harmful gases like VOCs released from burning, the activated carbon filter has us covered. (But see carbon monoxide below.)



Carbon dioxide will build up if people are in an enclosed, unventilated room. At moderate levels, CO2 makes people tired. At very high levels you’re unlikely to find in your home, CO2 can make people sick or even die.


Activated carbon filters on the market don’t meaningfully reduce CO2. Short of buying a submarine grade CO2 scrubber, the best way to reduce CO2 is to bring in outdoor air.



Diesel exhaust?

Diesel exhaust contains gas pollutants like NO2 and particles.



HEPA filters capture the particles, and carbon filters will capture the NO2. But what about…


Carbon monoxide?

Experimental studies have found that activated carbon captures carbon monoxide, but I don’t know how meaningful the effect is. Carbon monoxide can be really dangerous. If I were in a room with a stove that might be putting out carbon monoxide, I would NOT rely on a carbon filter. I would move somewhere else!



Ultrafine particles?

Ultrafine particles are where the real action is, right? It’s all about the nanoparticles. I once got an email from a guy who sleeps in a plastic bubble because he’s afraid of nanoparticles in the air.

The crazy thing about HEPA filters (and Brownian motion) is that they’re actually better at capturing ultrafine particulate (< .1 microns) than particles around .1 microns (I cover some empirical demonstrations of that here and here).



But I’m splitting hairs. The truth is they’re incredibly good at capturing particles across the range. We’re talking about a difference between 99.X% and 99.Y%.


Ultrafine particulate bottom line: you don’t need to sleep in a bubble. HEPA filters will capture ultrafine particles too.



Ionizers are awesome! No filter needed, very low noise. (How ionizers work.)



The only problem is that tests find that ionizers are terrible purifiers (1, 2). Oh, and they create ozone, which is harmful. Skip the ionizer.


Ozone generators?

From what I’ve read, ozone generators are pretty good purifiers. The only problem is ozone harms and even kills people. Skip the ozone generator.


UV light?

Some purifier companies put ultraviolet lights in purifiers because UV kills bacteria. It does, but not in the short amount of time air whips through an air purifier (according to the EPA). Skip the UV light.



Plasma-iodide-plutonium-nano-ultra technology?

If look long enough, you’ll find some claim of some technology I haven’t talked about here.



It’ll sound better. It’ll cost more. And people will want to know if it works better.



Before forking over your hard-earned money, remember that simple technologies (like, you know, fibers and carbon) already clean your air and are pretty cheap. If the new technology is so great, I’d wait until I see 3rd-party tests empirically demonstrating that the new technology (1) is better, (2) lasts over time, and (3) doesn’t produce some new harm we don’t know about. That sounds like a pretty high bar, but air purifiers are pretty darn good to begin with.

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