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

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:


6 stage filters air purifier


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


7 stage filters air purifier


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.

IQAir purifier expensive


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


IQAir purifier expensive


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.)

DIY air purifier filters


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).


DIY purifier effectiveness


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.


HEPA filters


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.

HEPA filters definition


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


HEPA filters efficiency


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


specific particles' sizes


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

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)…


test on activated carbon's effectiveness

formaldehyde I ordered straight from the factory…




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


formaldehyde test


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.


activated carbon filters Smart Air


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:


Dyson states that their air purifiers are special


But What About…

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


but wait...



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.


washing HEPA filters


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


particles captured before and after washing filters

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.


CO2 scrubber


Diesel exhaust?

Diesel exhaust contains gas pollutants like NO2 and particles.


diesel exhaust


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).


HEPA filters ultrafine particles


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.


Is Molecule really better than other purifiers' filters


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.

Breathe safe!


The Sqair air purifier Kickstarter

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To make your air purifier effective in a room you have to shut windows and doors and plug leaks – but if you sleep in the room carbon dioxide builds up, which the activated carbon filter cannot reduce. So you have to allow some outside air. How much? And how does one measure and ensure it?

Good questions Vikram! You’re right you’ll need to trade-off between high CO2 levels and high PM2.5. For this situation, I’d suggest you open your window for short periods of time to let out CO2. Do this when PM2.5 levels are low, either on days when the AQI is lower, or during the afternoons as that is typically the least polluted time. After opening your windows for 15-20 minutes, you’ll want to close them to stop more PM2.5/10 coming in. If you have a very powerful air purifier, you might even be able to run your purifier with your windows open… Read more »


Thanks for your reply, Paddy. It’s a good practical approach to open your windows in the afternoon. I have a Smart Air Blast Mini as well as a couple of air monitors. But the question was motivated by the fact that when I’m sleeping for eight hours at night, the CO2 build-up is faster than the inleak of air. Luckily I have an old house, so the inleak rate is pretty high – but I’ve tried to plug the obvious leaks. So what I’m really asking is: is there an optimum level of inleak (litres/min) that I should have to… Read more »

Right, that’s a complex question and very difficult to give a ‘one answer fits all’ type answer. If you have an air monitor, then what I suggest you do is open your window slightly, then wait 1-2 hours and see if the Blast Mini is still keeping the pollution levels down. If it is, you can try opening the window a bit more, and keep doing this until you get a balance between open window but still low PM2.5 counts. Just remember: the longterm effects of PM2.5 are far worse than those of CO2 (when it’s at a level of… Read more »


I live in Delhi, where air purifiers are indispensable.

My concern is co2 buildup. Outside air quality is so poor that we do our very best to seal off our houses completely.

Do you think that in a typical home scenario, co2 buildup is a concern? If so, how would you address it?

CO2 buildup is definitely an issue in well-sealed apartments. The good news is CO2 doesn’t affect your health long term, it just makes you feel tired or drowsy in the short term. If you have a strong and powerful air purifier, one solution could be to open the window a fraction and keep your purifier running on high. This way you can allow some CO2 to escape, but also make sure PM2.5 are kept low. If you do choose this option, I recommend you have an air pollution monitor to make sure your purifier is actually able to reduce pollution… Read more »

Chiu Ying Wong

thanks for the analysis. It has almost everything I need to know to keep my indoor air clean. 2 questions remain: how do you know when activated carbon is no longer useful? “full”? is activated carbon “washable”?

This is a really great question Chiu, and one of the most difficult questions to answer! Unlike for HEPA filters, it’s much more difficult to put an exact number on the lifespan of a carbon filter. This is because indoor VOC levels vary greatly between different people’s houses. The best way to tell when a carbon filter is ‘full’ and can no longer be used is to open up your purifier and smell the filter. If the filter smells sort of sour, or acidic, then it is time to replace your filter. By taking your carbon filter out of your… Read more »


Thanks for sharing your knowledge.

I read a study saying there is some ionizer that emits no ozone.

Do you know how to make a DIY VOC air purifier from charcoal granules? I tried, but there were a lot of fine dusts from the granules.

Hi Emma, good question about VOCs – to capture VOCs, you’ll need a carbon filter. Here’s a write up on whether carbon actually works at filtering formaldehyde. DIYing a carbon filter might be pretty tough, but if you want to give it a go then you’ll need a net that can capture the fine granules and make sure they don’t dislodge from the filter!

Chris Mclester

Thanks so much for pursuing your passionate curiosity about the confusing mysteries of the air purifier industry. As an allergy sufferer and pet lover living in Florida, we are never without an abundance of daily assaults.
Air purifiers are nothing short of costing a significant portion of your budget. I’ve always wondered who the Wizard of Oz was behind the technology.
What a gift you and your team have brought to the world. Will anxiously await your new product. 🙂