How to Make a DIY Air Purifier

Based on my research into how different filters work, I concluded that a HEPA filter is all you really need to make an effective air purifier and fight particulate air pollution somewhere like China or India. Given the fact fancy air purifiers run for 8,000 RMB in Beijing, or 100,000 INR in Delhi and most people don’t have that kind of money, I thought I’d try making my own DIY air purifier.


I give data on how well this DIY air purifier works here (1, 2, 3, 4), but here I’ll explain how I made it.


#1 Buy a fan

It’s important that it has a flat front so that you can strap the HEPA filter to it.

Fans with flat fronts usually have a recessed motor unit, so look for that. Here’s the one I bought for 58 RMB (about US$8.50):

diy air purifier fan


#2 Buy a HEPA filter

For my very first homemade air filter, I first bought this one for 108 RMB:


HEPA filter for homemade DIY air purifier


Since then, I tested HEPAs from all the manufacturers I could find to see if I could make an even more effective DIY air filter, and found one that captures more particles and has better air flow. Because I buy it in bulk and ship it through the shop for my social enterprise I set up, it costs less (75 RMB) than that first HEPA I bought. I’ve tested this HEPA filter with my particle counter, so I know it works.


#3 Pull the grating off the front of the fan

This is where the DIY fun starts! It helps if you have pliers for this. Then turn the power setting to 3 and pull the knob off – we’ll be using this DIY air purifier on the highest setting.

The knob gets in the way of the HEPA. Without the knob, I turn my DIY air purifier on and off by plugging and unplugging it.


Step 3 in making a DIY air purifier


#4 Strap on the HEPA

Use string to strap the HEPA filter onto the front of the fan, and you’re done! Your very own DIY homemade air purifier.

The metal bar in the middle will stick through the filter a little bit. You could saw the metal bar off, but my tests show it doesn’t seem to make a difference. Or better yet, make yourself a fancier DIY air purifier by finding a fan without a metal bar in the front, like the DIY air purifier I ship through Smart Air.


Rear view of DIY fan air purifier

front view of HEPA attached to make DIY air purifier

#5 Admire your DIY air purifier

Total cost for this DIY air filter: 166 RMB (US$27). Compare that to 8,000 RMB ($1,300) for the fancy purifiers here in Beijing. I now ship a simple fan and a filter that’s more effective than my original design for 228RMB (US$37).


Your next question is probably, does this DIY air filter work? See a live test here, how well it works if we use a stronger fan like on the DIY Cannon, or comparison tests with the big brands.


And you might be wondering, “shouldn’t the HEPA go on the back of the fan?” I always thought the answer would be “it works just as well both ways,” but I tested it on my DIY air purifier, and the results surprised me. Click here to read more on that!

Do Pollution Masks Really Work?

When a billion people in China (and quite a few expats) woke up to the severe air pollution in almost every city in China, it forced a billion people to become experts in a complicated scientific question: Do masks work?

Since then, I’ve given talks with hundreds of people all around China about how to protect themselves from air pollution. In those talks, I’ve heard doubts from smart, skeptical people. Here I’ll answer those doubts because, fortunately, smart, skeptical scientists (plus one dedicated nerd—yours truly) have empirically tested these questions. 

Update: if you’re living in India, we’ve done some India-specific tests with some of the most common masks available in India – including kids’ masks!


1. “There’s no way they capture the really small particles.”


The skeptic case: 

The most dangerous particles are the smallest particles, but masks are so thin. How could they possibly get the smallest particles?


The scientific test:

Researchers from the University of Edinburgh tested different common masks by running a diesel generator (to mimic car exhaust) and piping the exhaust through different masks. They used a particle counter to see how many particles made it through the mask. Here’s my super scientific rendering of the setup:



One important detail: the particle counter they used measures down to .007 microns. We’re talking about truly tiny particles here!

First they tried a simple cotton handkerchief. Sometimes I see bikers in China wearing these.



Not great, 28% of particles blocked.

Next they tried a cheap surgical mask.



Surprisingly good! (Fit tests generally show lower results–see below–but still a lot higher than most people’s intuition.)

Next they tried several bike masks.



Most were around 80%.

Then they tried several cheap 3M masks.



They all scored over 95%. Pretty good!

Conclusion: Masks capture even very small particles.


2. “OK, they capture the small particles, but when you wear them, all the air just leaks in the side.” 


The skeptic case: 

Masks work in theory, but those tests aren’t on real faces! When you actually wear them, you can’t get a good enough fit, so they’re basically useless.

The scientific test:

This question is tougher to answer because you have to measure the mask while you’reactually wearing it. For that, you need a really expensive fit test machine. Fortunately, I begged and begged 3M until they let me use their lab in Beijing:



The blue tube is sampling air outside the mask, while the white tube is sampling air from inside the mask (more details on the methods here ). 


Smart Air co-founder Anna Guo and Beijing-based Dr. Richard Saint Cyr also tested masks, so I combined all of our data. Here’s how well the masks worked on our faces:



How well do masks work for the broader population? 

It’s important to make clear: masks that fit my face well might not fit other people’s faces well. However, there is evidence from a broader population that masks fit most people well. A scientific study of 3M masks on 22 Chinese people found a median fit score of 99.5%–essentially the same as the top results from Dr. Saint Cyr and me.

Best yet, effective masks don’t cost a lot of money.



Exception: Gas Pollutants

Masks are great, but note that these are tests on particulate pollution (such as PM2.5). Most commercially available masks don’t target gas pollutants like NO2 and O3, so masks are not giving 100% protection.


3. Is there a documented health benefit of wearing a mask?

This is probably the hardest question to answer. However, there are two solid studies that have randomly assigned people in Beijing to wear masks or not and measured their heart rate and blood pressure (1 2). 



While wearing masks, people had lower blood pressure and better-regulated heart rates.



Conclusion: Masks capture even the smallest particles—even while people are wearing them—and they have documented health benefits. That should be enough to satisfy even the skeptics!

Protect yourself from cities with bad air pollution: A Beginner’s Guide

So you’ve arrived to a polluted city–Beijing or Bangalore, Delhi or Shanghai. What can you do to keep air pollution from trimming years off your life?

When I moved to China in 2007, I wasn’t a dummy. I was a college grad, and I knew about air pollution. But I knew almost nothing about protecting myself from air pollution.

Since then, I’ve spent ten years researching whether masks actually work, building my own purifiers, and testing them against the expensive imports. I’ve learned a lot about the effects of air pollution, and how to survive the smog. I’ll spare you the decade of research so that, in just minutes, you can equip yourself with the four most important tips I’ve learned about staying safe in the world’s most polluted cities. Then you can spend the rest of your time thinking about things other than air quality and air pollution.


1.     The Health Effects of Air Pollution


The health effects of air pollution


Bad air quality effects wages war on even young, healthy people’s bodies a lot faster (even without noticeable symptoms) and at a lot lower levels than I had ever imagined.

When I look at small particulate (PM2.5) air pollution readings, I sometimes see warnings like this one for Beijing:  


Beijing polluted city air quality levels


“Unhealthy for sensitive groups?” Well, I’m not a “sensitive group.” I’m young. I’m healthy. So I don’t need to worry. Plus, I’m usually only in those polluted cities in China or India for a few months. I’m probably fine.

Then I saw this air pollution study. Researchers gave real or fake air purifiers to 35 healthy college students in Shanghai — a top polluted city — and tested their blood.


Effects of poor indoor air quality on young healthy people


After just 48 hours, the students who were breathing Shanghai pollution had a war going on in their blood and the health effects of air pollution were obvious—their bodies were raging with inflammation, their blood pressure was up, their blood vessels were constricting, and their blood was preparing to form blood clots.

Yet the students breathing clean, purified air had significantly less damage in their body. For example, they had less inflammation.


air purifiers reduce blood inflammation


And they had less constriction in their blood vessels.



Another crazy thing about that study is that its biggest effects weren’t in people’s lungs. It was in their blood and inflammation throughout their body. It’s clear, using an air purifier reduced the impacts of air pollution on people’s health.

This hits on an important fact I discovered. Air pollution doesn’t kill like we think it does. When the World Health Organization tallied up air pollution deaths per year, they found that lung cancer only accounted for 14%. Other lung problems like bronchitis were another 14%. So what’s the other 72%—the large majority of air pollution deaths? Heart attacks and strokes. It comes from that war air pollution plays in our heart and blood vessels.



But wait, the media always reports on those scary air pollution days when the air outside looks like the end of the world – but those are pretty rare – and they’re just talking about one of the most polluted cities like Beijing, Delhi, Shanghai or Mumbai. The air quality isn’t normally that bad, right?



What’s more, Air quality in smoggy cities like Beijing has gotten better in recent years.
All true. But the more studies I read, the more I’m convinced that there’s no safe level for PM2.5 air quality levels. For example, scientists used statistics to best estimate of how many people die each year from air pollution, at different levels of PM2.5 air pollution.


Statistics on air pollution deaths per year


Take a look at the black line showing total worldwide deaths each year from air pollution. It goes up pretty fast. Even at 25 micrograms, it’s up pretty high. Now here’s where 2017—Beijing’s best year on record—falls on that graph. 


Statistics on air pollution deaths per year, polluted Beijing data


This is what all that air pollution facts mean to me: air pollution is waging war on our bodies, even if we don’t have obvious symptoms, even if we’re young and healthy, and even if it’s at levels far below the ones that make the news. If you’re in virtually any city in China, or any city in India, you’re breathing air that is killing people and you need to protect yourself from this harmful air pollution.


2.      Solution #1: Protect yourself indoors


Solving poor indoor air quality is easy. Purifiers are just fans and filters. Skip the marketing noise. Build your own or buy a low-cost, data-backed purifier. When I started searching for air purifiers, I saw lots of people buying $500 Blue Air’s and $2,000 IQ Air’s. I wondered, does clean air need to be so expensive? Is there any truth to their claims about special technology or imported HEPA filters?


Air purifiers can be cheap


I found out the main ingredient in purifiers—HEPA filters—were invented back in the 1940s. 


HEPA filters invented in 1940s - cheap technology


HEPA filter captures air pollution like allergens, PM2.5 and soot


Fans aren’t expensive either. So I built my own and tested whether it worked. 


Low cost effective air purifier


The results were clear—air purifiers work.


DIY air purifiers are effective


My DIY air purifier improved air quality and reduced particulate in my bedroom by 86%. It captures even really small particles below 2.5 microns, and you can even make your own for a tiny fraction of what that fancy Swiss machines sell for.



In my tests and other independent tests that I describe here, there’s basically no correlation between price and effectiveness. Put another way, there are low-cost purifiers that remove just as much particulate as the $1,000 machines.



So, my startup Smart Air designed purifiers that are—you guessed it—just fans and filters. These DIY purifiers cost about $35 in China, India and the Philippines, and our super strong Blast Mini’s cost about $200 and can be delivered in China, Philippines and India. Clean air can be simple and cheap.

What you can do to protect yourself: Make your own DIY air purifier, order a low-cost DIY purifier or the super quiet Blast Mini air purifier, or choose any other empirically backed purifier.


3.       Solution #2: Protect Yourself Outside with a Simple Air Pollution Mask



I’ve encountered lots of people who are skeptical of pollution masks. People have told me that masks are so thin, there’s no way to capture small particles. Or they do capture small particles, but unless they’re professionally fitted, the pollution leaks in, so they’re basically useless. Even news media outlets repeat this idea.

Before I saw the research on masks, I would have guessed they kinda work all right. Turns out masks are wildly more effective than I thought, even against really tiny .007 micron particles, even when regular people are wearing them, and even when measured by physiological outcomes like lower blood pressure and better regulated heart rate. In short, masks are wildly effective.


Statistics on effectiveness of air pollution protection masks


And just like with purifiers, some of the most effective masks are also the cheapest. 


Cheap and effective air pollution masks review


I used to spend US$80 to buy the UK Respro mask in Beijing. Now I spend less than 10% of that to buy 3M N99 masks that are more effective and more comfortable (here’s the data). Avoid the marketing noise.

What you can do: Choose any of the low-cost, scientifically tested masks in the data above.

Which masks are good choices? I cover tests of masks and practical considerations like comfort in this answer: What is the best pollution mask for a vacation in China?

4.     Solution #3: Test the Air You’re Breathing

If you want to reach real air pollution nerd status, get an air quality monitor and know what you’re breathing. They’re also great for letting you know if your purifier is working.But are they any accurate? Smart Air tested three: the Laser Egg, the Air Visual Pro, and Dylos (left to right).


Review of air quality monitors and air pollution particle counters


We compared data from these particle counters to data from the U.S. Embassy in Beijing, which is close to our office:


China government air quality monitors comparison


We ran the machines for six days. Here are the results for the first (72-hour) test outside our office in Beijing:



All three particle counters tracked the official pollution readings really well! On average, they were about 5-9 micrograms off from the official readings.


accuracy of air quality monitors measure pm2.5


What you can do: Order a Laser Egg or the pricier Air Visual air quality monitor and discover what’s in the air you’re breathing. 

Air Pollution Fact Roundup:

Bottom Line: I take these simple steps to protect myself from air pollution and breathe clean air, then I spend my time enjoying myself when I visit polluted cities like Beijing and Delhi. Clean air is that simple. 

What is the difference between PM2.5 and PM10 with respect to the atmospheric pollutants?

The difference between PM10 and PM2.5 is size. “PM” refers to particulate matter—particles in the air. Those particles are things like organic dust, airborne bacteria, construction dust, and coal particles from power plants (for example, check out this study from researchers in Shanghai who analyzed what those particles are really made of).


Now on to size. The “10” and the “2.5” refer to microns (AKA micrometers). Microns are tiny. Here’s an idea of how small microns are compared to human hair:


Next, there’s a hidden (unlabelled) detail in the terms “PM10” and “PM2.5.” That is the “smaller than” piece. Each pollutant type is defined as that size and below. So PM 10 is particles 10 microns and below. PM2.5 is 2.5 microns and below. (That means PM 10 includes PM 2.5.)


What PM is not

Finally, it’s helpful to think of what PM is not. Particulate pollution does not include gas pollutants like ozone and NO2.

For fellow nerds curious to read more about PM 2.5:

  1. I describe how governments measure PM 2.5 here (including the trick of how they get rid of any particles in the air larger than 2.5 microns)
  2. I describe what PM 2.5 does to our bodies in this answer.
  3. In this answer, I detect PM 2.5 in my home in Beijing and test whether a simple DIY air purifier I made can clean it up.

Hope this helps!

Plants Have No Detectable Air Purification Effect

Despite the popular conception that plants are purifiers, plants have a detectable effect on particle or gas pollutants, according to the latest test data from real-world homes.

The belief that plants are effective air purifiers is widespread. Arguably the most frequently cited evidence is the NASA study on plants. It feels scientific, authoritative. This page cites it, this pagethis page.



This entire Wikipedia entry is devoted to it.



Let me claim some nerd cred. I read the original study–all of it. I found that the NASA study was done in small, completely sealed plexiglass containers surrounded by growing lights. For an idea of size, here’s what their setup looks like next to a stick figure version of myself.



I’m not sure what your home is like, but that’s nothing like my home. In NASA’s defense, it’s a decent approximation of a spaceship.


The NASA setup also has a key difference with our homes. They pumped in the chemical at the beginning, and then that was it. But in our homes, most sources of chemicals like formaldehyde constantly emit the chemicals.


That’s why the real question is not IF plants have an effect, but HOW MUCH of an effect do they have? The NASA study convinced me that plants can remove chemicals like benzene and formaldehyde. But it didn’t demonstrate how much that matters:


  1. Is that effect large enough to actually matter in a real home?
  2. Is that effect enough to overcome a source that’s constantly emitting chemicals, which is the case when we have new furniture, new products, even some types of permapress clothes?


Fortunately, some people have tested the effect of plants in real homes.


Are Plants Good Purifiers?

#1 Gas Pollutants

I’ve seen some pretty fantastic claims about plants, like this TED speaker who said with a few plants, we could live in a sealed bottle.



Fortunately, these guys in China tested whether plants can remove volatile organic compounds in a more realistic setup.



Unlike our homes, their room is still sealed, but it makes three big improvements from the NASA test:

  1. It contains a constant source of VOCs (in this case, office desks).

  2. The experiment room is much larger than NASA’s box—more like a room in our homes.

  3. The room has no special growing lights (just like most people’s homes).


They tested multiple times with an army of plants that people have specifically said reduce VOCs (such as the snake plant, aloe, and spider plant).


And what happened? VOC levels actually increased from .17 ppm to .24 ppm during the tests. Even an army of plants wasn’t enough to deal with the gas pollutants coming from normal office furniture.



Conclusion: This test suggests that the effect of these plants on VOCs is so small it’s not clearly detectable under normal conditions.



  • Some plants actually emit VOCsas can soil. So it’s possible that VOCs went up because of the plants!
  • The Chinese team claimed to be testing formaldehyde, but these detectors aren’t that specific. They detect VOCs (“VOC” is a big category, including lots of things from perfume, to alcohol, to chemicals like formaldehyde). So we have no idea which VOCs they’re detecting. That means it’s possible that these plants were decreasing a harmful VOC like benzene and increasing some non-harmful VOC.
  • Did they use the wrong plants? There is some decent evidence that the effect on VOCs depends a lot on the type of plant. These researchers specifically chose plants that people claim are effective, but the results could be different with other plants.


#2 Particulate Pollution

OK, so plants don’t seem very good at VOC gases. But what about tiny particles in our air, particulate pollution like PM2.5?



I tested this by putting a similar army of plants into the Smart Air Beijing office. I tracked particulate levels over 4 tests with plants in the room and 4 tests without plants. Here’s what I found:



Here’s a comparison between plant effectiveness and a DIY purifier in that same room.


Conclusion: These plants had no detectable effect on particulate pollution indoors.


Is there NO evidence that plants help?


I should say that some people claim to have found effects of plants in real-world conditions. For example, these researchers claim that 9 potted plants in a classroom in Portugal reduced particulate by 30% and VOCs by 73%!


That stretches my imagination, at least. From the data I’ve seen, my conclusion is that there is not enough evidence to support the idea that plants meaningfully reduce VOCs or particulate in real-world conditions. At the same time, I think we need more real-world tests.


Given the evidence that plants can remove some harmful VOCs in sealed chambers, I’d err on the side of putting plants in my home. It’s unlikely they’ll hurt, and there’s some possibility they’ll help with gas pollution. Plus, they are nice!


P.S. Another way of asking whether plants are effective purifiers is to look at several months of particulate pollution data in a room full of plants. I did exactly that in an apartment in Beijing. Here’s what I found.