Do We Need Car Air Purifiers?

Car air purifiers sound like a great idea. Soon after I started making DIYs, people emailed me asking if we could make a DIY for their car. My initial reaction was, “Great idea!” On the highways, we’re surrounded by all of that awful pollution. Best to get a car air purifier.

But as a methodical nerd, I wanted to do some tests first. The results of those tests led me to a conclusion I didn’t expect: you don’t need a car purifier. Here’s how I got to that conclusion.

Uber Test

I took to the streets of Shanghai with my Dylos DC1700 laser particle counter. (Are particle counters accurate?) Here’s what the air outside was like:

The number on the left is the number of particles 0.5 microns and above. For that channel, “safe” is somewhere around 2,500. So the air that day wasn’t pretty. The number on the right is larger particles—2.5 microns and above.


Next, I got into an Uber and kept taking readings. Here’s what the air was like when I just got into the car:

car air filter test wildfire

Way better! The 0.5 micron particles were only about 25% of outdoor air.

And a few minutes later:

car filter PM2.5 test China


Look, ma, no purifier! We were down to 11% of outdoor air.

So why’s the air so much cleaner?

It turns out car air systems have filters in them already, and apparently they do a pretty good job of filtering out particulate pollution. Here’s an example of a car filter on Amazon for only around $10.


Tests in a Regular Car

This simple test suggested to me that the car’s built-in air filtration system reduced particulate levels significantly—even the dreaded particles smaller than 2.5 microns. But was it something special about that car?

To be sure, I teamed up with Ohad, a Smart Air aficionado to do tests in his Mazda in Israel. He drives a regular 2010 Mazda 3–no special filters.

car filter test Mazda 3


Ohad took his Dylos particle counter with him when he drove his car. When he got in the car, he turned on the AC, used “recirculate air” mode, and kept the windows closed. That’s it. Ohad took measurements every minute for five minutes. That’s not very long, so this is a fairly conservative test of the car cabin.

But wait, does Israel really have bad air pollution? It’s no Beijing, but several parts of Israel have recently experienced dangerous levels of smog. In Ohad’s tests, baseline 0.5 micron counts ranged from 2,464 (near the WHO limit) to over 10,000 (four times the limit). So the air has plenty of particulate to do tests.


I averaged across all 8 tests, and here’s what I found:

Car air filter test air purifier HEPA

The Mazda’s built-in air filtration system drastically reduced the number of particles. When averaged over the course of 5 minutes with the fan on its maximum setting, the results showed a staggering 97% reduction of 0.5 micron particles and 99% drop in 2.5 micron particles!

But wait, savvy readers might ask: how do we know it’s the car’s air system and not just that there’s a person breathing in all those particles? Or maybe it’s because particles settle over time?

Ohad also ran an identical test with the car air system turned off. That way we can know if the reduction is truly because of the air system.

Car air filter test air purifier wildfire

With the air system off (the red line), particle levels drop a bit, probably because of settling.But with the air system on (blue line), particulate levels dropped much faster.

Across 5 tests with the AC off, particulate went down (right), but not nearly as much compared to the 8 tests with the AC on high (left):

Car air filter test air purifier PM2.5 pollution


After seeing the data, I’m convinced we don’t need special car purifiers to protect against particulate pollution. The air systems already in cars dramatically reduce particulate pollution.Here’s how to do it:

1. Keep the car windows closed.

2. Use the “recirculate air” mode for the highest particulate drop.

3. Turn the fan on high (at least at the beginning of the trip).

4. Check and replace your filters every few months.

5. Think twice about whether you really need a car purifier.

Exception: Gas pollution

There is one exception to these tests: Car filters capture particles, but they won’t capture gases, like NO2. Thus, it may still be worthwhile to consider using a carbon filter. (Does carbon really work?)

Are you saying no one should ever use a car purifier?

If I owned a car in China, I wouldn’t bother buying a car purifier, but I won’t say no one should buy one. For example, people who are extra sensitive to pollution or parents with small children might consider using a purifier to get extra protection.

As always, I’m making the raw data and more testing details publicly available for fellow nerds. Check that test out for a full comparison of AC high, AC low, and AC off tests.

Breathe safe!

Does Closing the Windows Starve Us of Oxygen?

Air purifiers work best in a closed room environment and this worries some who are concerned that there wouldn’t be enough oxygen coming in when all doors and windows are closed. Yet recent tests in a real apartment — with as many as seven people in a small room — show less than 1% change in oxygen levels in a closed room.


Fears of Low Oxygen

Fears of low oxygen levels are easy to find. Quora user Arghya Roy, says a closed room running air condition leads to low levels of oxygen.



Playing on this belief, companies rushed in with products that fill this supposed void. For example, the popular Three Dad’s purifier claims to solve the oxygen problem by using “submarine technology” to create oxygen.



The logic is simple: The room is closed, humans breathe in oxygen, so oxygen levels should decrease over time. Or should it not?


The Experiment

Smart Air tested this question by tracking oxygen levels with an iBrid MX6. We used the Smart Air office as an extreme testing site because the humble Smart Air office has seven people packed into just 34.5m² room.



On top of that, the office windows and doors are sealed with insulation tape to prevent polluted air from getting in (the office has no central air system too). We tracked oxygen levels throughout normal workdays, keeping the door shut except for when people left and came back from lunch at midday.



Even in these extreme conditions, oxygen levels showed almost no change. By the end of the day, oxygen levels had dropped by 0.3%. Simply put, humans don’t take in as much oxygen as we think we do. Based on oxygen alone, estimates are that an average person could survive in a completely sealed room for 12 full days!



The Change Is in Carbon Dioxide

While oxygen levels are pretty much constant over the day, what does change is the amount of carbon dioxide in the air. Tracked by the Air Visual Node, carbon dioxide levels went from 0.08% to 0.35% — more than tripling over the course of a day!



How Can Carbon Dioxide Triple when Oxygen Remains Unchanged?

If humans breathe in oxygen and breathe out carbon dioxide, how can one remain almost unchanged while the other skyrockets? The key is that regular air has a lot more oxygen than carbon dioxide. Oxygen constitutes about 20%, while carbon dioxide is only 0.038% of the air. Thus, carbon dioxide is capable of tripling from such a small number, while oxygen is mostly unchanged.



Carbon Dioxide Can be Harmful

Although we don’t have to worry about running out of oxygen, too much carbon dioxide can be harmful. At levels in the Smart Air office during the test (1,000-3,500PPM), people felt drowsy and may even perform worse on cognitive tests.


The Simplest Ways to Lower CO2 Levels:

#1  Open your windows from time to time

Yes, periodically opening the windows will let in polluted outdoor air, but our tests have found that purifiers, take the Cannon and Blasts for example, can cut PM2.5 levels by 50% in just 10 minutes after closing the windows.



#2  Leave the windows opened, just a tiny gap

If you have a strong purifier, open the windows by just a bit and leave it there. Our tests have found that purifiers can still be effective even when the window is left opened.


#3  Get a place with a central air system

Choose an office or apartment with a central air system that brings in outdoor air (hopefully purified ones too). If your apartment or office, like many of the ones in China, doesn’t have a central air system, you can get an air exchange system installed. But it is expensive.


Bottom line:

In indoor spaces, oxygen levels are not of much concern even though CO2 can be. So the next time you come across a marketing hype about purifiers that claim to create oxygen, you know you don’t quite need that. Breathe safe (and smart) folks!

Are Purifiers Useless If Windows are Opened?

Tests show that keeping the doors and windows closed will keep particulate levels at 50% less than outdoor air on average. But some people really want to keep their windows open. For some people, it’s an old wive’s tale that windows must be kept open. For others, it’s a way to reduce chemicals coming from new furniture or recent remodeling.

Can we have it both ways? Or air purifiers useless with when our windows are open? CK from the Smart Air team put this question to the test by running the DIY Cannon air purifier in the 12m2 Smart Air testing room six times with the window open and closed.

The Test

The effect of having your window open will be more prominent when outdoor air is hazardous, so CK chose two days when outdoor air was bad (February 14th and 15th) for his tests. During these two days, Beijing’s AQI averaged 239 and 290 according to the US Embassy.


To keep the baseline level of pollution consistent across tests, CK opened the window to let in outdoor air until it reached the outdoor level.

When the particle count reached the same level as outdoors, CK ran the Cannon on high for 20 minutes. He ran three tests with the windows open and three tests with the window closed.

Air purifier window open air purifier window closed


With the window closed, the Cannon reduced particulate by an average of 90% after 20 minutes. With the window open, the Cannon air purifier managed just a 60% reduction in particle count.

Air purifier window open vs. window closed

Window open versus closed purifier test

The data shows that it’s clearly better to close the windows with running your air purifier. No shocker there! But here’s the more interesting part: even with the windows open, it’s still possible to significantly reduce indoor particulate by turning on an air purifier.


Bottom Line

Closing your windows and turning on your purifier will give you the lowest PM2.5 levels in your home. That’s obvious.

However, if you have to balance the PM2.5 threat against high levels of VOCs or CO2 indoors, it is possible to get some ventilation AND significantly reduce PM2.5 levels indoors with your window open and purifier on.

One suggestion is you equip yourself with a super-charged purifier such as the Smart Air Blast or Blast Mini. Having your windows open whilst running a purifier like this can give you a good balance between low PM2.5 levels and low VOC or CO2 levels!

What’s CADR and how is it different from airflow?

When buying a purifier, you’ll sometimes see companies use the word “airflow” (风量) to describe an air purifier’s performance and other companies the word CADR (clean air delivery rate) as seen in the picture below.



They’re both a measure of purifier performance, but are they the same thing? Can I compare one purifier’s CADR value with another purifier’s airflow value?


Short Answer

No. CADR and airflow are not the same thing. If you see somebody claiming they have the same CADR and airflow (like the screenshots attached below), you know something is up!



Long Answer

The real answer gets into some nerdy (but actually profound) details about how people measure CADR and airflow.

Here’s what you need to do for a CADR test:



(For total nerds, the government guidelines describe the full methods [in Chinese])


How People Test CADR

Step 1: Pollute a sealed room to crazy high pollution levels (normally by lighting cigarettes).

Step 2: Run your purifier.

Step 3: Take PM2.5 concentration readings every 2 minutes for 20 minutes.

Step 4: Use these numbers to work out the rate of purification of the room (the CADR).

The hard part about doing this test is that you need a lot of complex equipment, a large testing room, the time and patience to do the test, and some data analysis skills.


How People Test Airflow

To measure airflow, the only thing you need is a wind speed measure (fancy name “anemometer”):

How to Test Wind Speed

Step 1: Place the anemometer against the filter and look at the number.

Step 2: Multiply this speed by the area of your filter.

That’s it! So much easier.

This method works all right, but it has a problem. Since this method only measures wind speed, it only tells us how much air is coming out.

What doesn’t it tell us?

  1. How clean is that air coming out?
  2. Is any of that air actually coming in from a leak or the middle of the air vortex and then being pushed back out?
  3. How efficiently is the machine mixing that air in the room?
  4. How much of the air that comes out of the purifier gets sucked back into the machine quickly, rather than being pushed farther out into the room?

Air flow tests assume perfect values for all of these variables. For example, it assumes that all the air coming out is 100% clean. Because all the assumptions are rosy, air flow values tend to be much more impressive than CADR.

Bottom line: Air flow is less rigorous than CADR and will almost always be much higher than CADR. Do not directly compare the two numbers.

Couldn’t We Convert Airflow into CADR?

The nerd in me thought that if we just multiply the airflow with the efficiency of the filter (for example, our filters capture 99.9% of particles above and below 0.3 microns), then we can measure the amount of clean air coming of the filter. But this still leaves out one vital bit of information: this doesn’t consider real life conditions on how air recirculates and mixes in a room.

When you measure airflow, you’re assuming all the air passes the filter just once. This doesn’t happen in real life, and predicts higher effectiveness than happens in real-world tests. The air in a real room is constantly being mixed (like stirring sugar into our morning coffee), which means that already purified air will recirculate and pass through the purifier for second or even third time. Re-purifying already purified air is not optimal, but it is inevitable. And that leads to a lower than expected cleaning rate.

This explains why CADR values are lower than airflow values, and shows that comparing one purifier’s CADR with another purifier’s airflow value just won’t cut it.

How Much Lower Is CADR Than Airflow?

At Smart Air, we use airflow and CADR tests when testing purifiers like the Blast purifiers we just released. We use both because they both have advantages (airflow tests give a quick rough estimate of a purifier’s effectiveness; CADR is more precise). Because we do both types of tests, we can see how much smaller CADR values are on average.


Average across many different purifiers we’ve tested, we found that CADR tends to be around 60% of airflow results.


How to choose: When comparing purifiers, look for CADR over airflow (and pay attention to whether the company is reporting one or the other). And keep in mind that CADR tests are often done by third-party testing companies, whereas airflow is typically measured by the purifier manufacturer.

HEPA Longevity Test: 170 Days

When I started Smart Air, a lot of people asked me how long the HEPAs last. Several people criticized the DIY on Zhihu because they said you’d probably have to change the HEPA so often that it’d end up being more expensive than the expensive brands.

At the time, I really wanted to pull out a nice round number, but I couldn’t think of any way to answer the question without getting hard data first, so I started doing tests (well, actually Gus did). At 90 days, we found it worked as well as in the beginning. At day 130, we found a 4% decline. Now we’ve finished 170 days!


Gus used the Original DIY and the same HEPAs we ship from Smart Air every night in the 12.3 m2 bedroom in his Beijing apartment. Gus used the same method as my previous tests to calculate effectiveness—the percentage reduction of particles .5 microns and above from the room air overnight.

To smooth out the variability in any single datapoint, I averaged the effectiveness over each 10-day period. (More details on the methods here.) Here’s what a single test day looks like:



At day 100, the effectiveness dropped by about 4%. It stayed at around that level, until day 140, when it dropped 5-10%. After that point, the total effectiveness has bounced between 65-80%.



In this test with real-life Beijing air, the Smart Air HEPA lasted 100 days at about 8 hours a day at full effectiveness (729 hours to be precise). People who want every percent of effectiveness should change HEPAs after 100 days. People who don’t mind the 4% drop, I’d recommend replacing it by 140 days (1,028 hours).

For now, the test continues! I’ll post the final results when we finally run this HEPA into the ground.

As always, I’m posting the raw data as a download here, and there are more details on the methods in my earlier post.

Is air pollution affecting your productivity?

We know that air pollution is bad for our health, it causes heart attacks, strokes and lung cancer. But how about the more day to day impacts of air pollution? Feeling tired, drowsy or does going to the office make you sick? The data shows that poor indoor air quality is affecting you more than we once thought. The answer is somewhat daunting:

What we already know

The studies on the effects of outdoor air pollution are no surprise to most people. For example, a study found that air pollution reduced the productivity of workers picking apples outdoors in California. Another study found that people in Chinese marathons run slower when pollution is bad. What we might not know is how air pollution affects us in the office, here we’ll take a look at two studies that show its effect on productivity.



#1 The Ctrip Study

A team of researchers tracked the productivity of 4,910 employees at two Ctrip call centers in China. For these workers, measuring performance and productivity in the office was easy–the number of calls employees handle.


They found that when the air quality index (AQI) increased by 10 points, employees answered 0.35% fewer calls. On days where the air quality index was hazardous, the call centers handled about 20,000 fewer calls.



On days when the AQI is 150, Ctrip would need to hire an extra 145 people to cover those calls. Imagine if every company needed to have their own pollution warriors for bad pollution days.

Of course, the alternative is for Ctrip to put in an army of purifiers to keep their air pollution levels down. Maybe something like the Smart Air Blast army:



#2 The German Stock Market Study

Another team of researchers analyzed air pollution and the activity of more than 100,000 stock traders in Germany from 2003 to 2015. They found that a 12ug/m3 increase in PM10 decreased the probability that stock traders would log into their computers and trade stocks by 10%. This was true even controlling for factors such as weather and the day of the week (because markets are not open on weekends). 



If they had been breathing clean air, there’d have been more logins and more stock traded. Imagine if every stocktrader was breathing clean air. The stock exchange may well look like this:



How Is Air Pollution Affecting Productivity?

Of course air pollution makes people sick and take more sick days. Employees will also have to take care of sick family members. However, the CTrip study didn’t count the effect of sick days, so the effect of air pollution wasn’t from sick days.

Instead, recent research suggests that, even if we’re not getting physically sick, air pollution stull affects our brains. For example, air pollution raises blood pressure, and elevated blood pressure causes decreased cognitive performance. Air pollution also affects mood, which could decrease productivity.


The Total Cost of Pollution

Researchers found that air pollution was affecting 4,910 workers at Ctrip call centers and 100,000 stocktraders throughout Germany. How big a cost is that when we multiply it by the billions of workers around the world affected by air pollution? The OECD estimates that all of the sick days and rising health costs from will cost 1% of global GDP in 2060. That’s more than the cost of 3 million BMWs!



What Can Organizations Do?

One reason many organizations haven’t done anything to fight air pollution in their buildings is probably the sheer cost. Building filters into central air systems often cost tens of thousands of RMB and require approval from building management. Meanwhile, the stand alone purifier market is full of expensive machines.

Machines like the new Blast that are powerful enough to clean large offices yet inexpensive are still rare, but can help change the picture. For example, the Shanghai World Financial Center commisioned an analysis for its office space and found that using Blasts to clean its air would cost less than half of a central air system.



Bottom Line

A growing body of research suggests that air pollution affects our productivity, even at our indoor desk jobs. This productivity loss is taking a toll on the global economy, but new purifiers are making it much more affordable to clean the air.

Smog doesn’t kill like you think it does

The World Health Organization tallied up all the deaths due to outdoor air pollution based on different causes, and the major killer isn’t what most people think. 



Asthma, lung cancer, and emphysema come to mind when we think about air pollution. But lung problems are only a small portion of the deaths caused by air pollution. Lung problems don’t even count for half.


How exactly do people die from air pollution?

This graph shows the three major categories of deaths from air pollution. What would you think goes into those boxes?



My first thought would be lung cancer. Survey says?



Just 14%! Not nearly as large as I imagined.


Well, OK, lung cancer is just one type of lung problem. How about emphysema, bronchitis, and other lung diseases?



Hmm, we’re still only at 28% of deaths due to air pollution. So what’s hiding in that big blue piece of the pie?



Heart attacks, strokes, and other diseases in our blood vessels. Really? It makes so much sense to look at our lungs because that’s where the air goes. But it turns out that we’re looking in the wrong direction. We should’ve been looking here:


Does air pollution really affect our heart and blood vessels? Here are three points of evidence.


1. More people have heart attacks on days with bad air pollution

A study in Boston found that on days with higher particulate air pollution (blue line), more people had heart attacks (red line).



Now, we’re not talking about a wild epidemic of heart attacks. The base odds of heart attacks on any given day are low, probably one in millions. So a 50% increase is important, but it moves the odds from 1 in a million to 1.5 in a million.


2. Wearing a mask in polluted Beijing reduces blood pressure

But hey, heart attacks are for unhealthy old people, right? I’m just 32 years old. Well, two studies randomly assigned healthy adults in Beijing to wear a mask outside or not. No one had a heart attack, of course, but people without a mask had higher blood pressure and lower heart ratevariability.



3. Breathing purified air improves blood inflammation and constriction of veins

OK, OK. But maybe you’re a healthy young person, just 20 years old. Maybe your lungs are even “used to” the pollution. My friends in China joke that Chinese people have the strongest lungs in the world.


Researchers in Shanghai randomly assigned healthy college students to use a real purifier or a fake purifier for 48 hours in their dorm rooms. Here’s what the PM2.5 levels were like in their dorm with a fake purifier (blue) and a real purifier (yellow).



They tracked all sorts of effects on biomarkers. When they looked at effects on blood inflammation, the effects were pretty big!



Same for blood coagulation:



And constriction of veins:



But when they looked at effects on lung function, the effects were tiny and not statistically significant:



That blew my mind. All this time we were looking in the wrong place!


If someone in Beijing or Delhi fell over and died of a heart attack, would most people think, “Ah, air pollution…”? Before I saw this research, I wouldn’t have.


Bottom line: Even if we aren’t coughing, even if we don’t have asthma, air pollution is still taking a toll on our heart and blood vessels.

What are ionizers and do they work?

The other day, someone on Quora asked whether ionizers actually purify the air. This is an important question because ionizer purifiers are all over the place. For example, I was at a friend’s apartment in the US, and I saw his tower fan had an ionizer button on it:


Ionizers 1


It’s also important because several friends in China have sent me links to ionizer products like this:


Ionizers 2


Amazing! A “miraculous air purifier” that removes PM 2.5 and formaldehyde in just 30 seconds. And all that for far cheaper than regular purifiers and even cheaper than building your own purifier.

If this is true, my life in Beijing is now so much easier. But is it true?


So how do ionizers work? 

Here’s my bedroom, with an ionizer and bad particles in the air:


Ionizers 3


That ionizer shoots out negative ions:


Ionizers 4


Those ions cause the particles to stick to surfaces, like my bed, the wall, and the floor:


Ionizers 5


That’s the principle behind ion generators. It’s hard to see it happening with these tiny particles, but you’ve seen it on a visible scale if you’ve seen someone rub a balloon on their hair and then stick it to a wall.


Ionizers 6

But wait #1

A summary of scientific tests of air purifiers found that most ionizers have no noticeable effect on particulate levels (p. 8). Their conclusion is that most ionizers are too weak to have an effect. Studies do show an effect if they use very strong ionizers–much stronger than most ionizers on the market (p. 19).


But wait #2

OK, so regular ionizers don’t work, but we can use a big one! The problem is, when you put that many ions into the air, it produces ozone. Ozone is harmful, so that’s not good!


But wait #3

Even if we use a really strong ionizer and even if we can accept the ozone, you might have noticed that the ionizer didn’t actually filter out the particles. It just made them stick to my bed, wall, and floor.


First, that’s gross. Since the particles floating around here in Beijing include things like arsenic cadmium, and lead, I’d rather not have them stick to my pillow.


Second, they’re still a danger. The particles are just sticking to my bed. So let’s say Thomas comes home:


Ionizers 7


When I sit down on my bed, I’ll dislodge those particles, and they’ll float back into the air. Here’s my super scientific rendering of that process:


Ionizers 8


Those problems are what led Consumer Reports to publish tests and warn people not to buy the Sharper Image Ionic Breeze. Sharper Image sued Consumer Reports; Consumer Reports won.

So when people send me links asking about these “miraculous” ionizer purifiers, I tell them to steer clear.


Careful not to over-generalize

But let’s not 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, be it PM2.5 or PM10, and they are backed by empirical tests (1, 2, 3, 4, 5, 6). Here’s a little test I did with HEPA filters in Beijing:


What’s more, HEPA filters don’t give off other harmful pollutants as they work, unlike the ionizers that give off harmful ozone.

Can I Clean or Wash HEPA filters?

HEPA filters are a nearly perfect solution for air pollution. They are backed by tons of scientific tests; they’re pretty cheap; they use about as much energy as a lightbulb; they even have documented health benefits.

Just about the only remaining problem is that you have to throw out the HEPA filters every few months. So wouldn’t it be great if we could clean them and re-use them? Here we’ll try and answer how to clean HEPA filters.


People have lots of opinions! On the question-answer site Quora, I’ve seen people say you can clean HEPA filters by banging the HEPA to get the dust out:


clean a HEPA filter by banging out the dust


…or to clean a HEPA filter vacuuming it


clean a HEPA filter by vacuuming


…or even wash it. Then I’ve seen other people say you can NEVER wash a HEPA.


you can never clean a HEPA filter


But one thing I haven’t seen is anyone actually test the question. Well now I’ve got data. That’s because CK from Smart Air decided to test out the best way to clean a HEPA filter. Here’s CK’s story.



Vacuuming the HEPA

First I tried vacuuming a HEPA. I took a dirty HEPA filter, ran tests on its performance, vacuumed it, and then ran more performance tests. In all, I tested 8 dirty HEPAs given to us by avid Smart Air users! They all used in China for between 2-4 months.


Testing Effectiveness

Particle Capture: I measured the percent of 0.3 micron particles captured at the outlet of the HEPA before and after washing. 0.3 micron particles are a great benchmark because they are the hardest particle size for HEPAs to capture.

Air Flow: I measured air speed coming out of the HEPA before and after washing.

CADR: I used particle capture and air flow to estimate CADR.



Vacuum Results

On average, vacuuming increased relative CADR by about 10%. But there was also a lot of variance, ranging from a 14% increase to a 3% decrease. Based on that data, I’d say cleaning a HEPA by vacuuming it might help a little, but there’s also a decent chance it’ll do nothing at all.


What’s The Vacuum Doing?

HEPAs are made of thin fibers. The vacuum is probably pulling out some of the larger dust particles from the HEPA filter, but at the same time some of these fibers as well. You can see what looks like fibers that have been pulled loose in this picture.




In short: There may be a slight benefit of vacuuming HEPAs, but the benefit is small, and it can also damage the HEPA. Therefore, we do not recommend vacuuming.

Washing the HEPA

So, what if we wash them instead? I used the same 3 filters that were vacuumed before and washed them under a shower head. Then I gave them a day to dry.



Washing Results

I stopped testing after 3 trials because the results were so conclusive: cleaning the HEPA through decreased effectiveness by an average of 32%.



Why Was Washing So Bad for the HEPAs?

To get to the bottom of why washing was so bad for the HEPAs, I looked at wind speed and particle capture before and after washing the HEPA.

First, washing actually increased the windspeed.

Great, right!? But then particle capture actually went down a lot after washing.


effectiveness of HEPA fitler after washing

Washing the HEPAs probably broke some of the fibers or pushed the gaps between the fibers larger. That would explain why more air passed through after washing, but also more particles!

In short: Washing HEPAs with water breaks down the HEPA material and so is not recommended.

Bottom Line

Vacuuming HEPAs may improve performance, but the effect is small. Washing HEPAs strongly decreases performance. We do not recommend washing HEPAs.

Based on this data, it seems it really is not possible to clean your HEPA filter. We still recommend replacing your HEPA when its effectiveness drops. When is this? We tested this with the Smart Air DIY air purifier found replacing after 140 days is best. For the larger HEPAs, like those in the Blast and Blast Mini purifiers the huge surface area means they can last over 1 year.

Own another purifier? Learn why the standard numbers might not always be accurate.

Open Data

You can learn more about the testing methods, and view the original data in the Supplemental Data article. Click here to learn more.

How Big of a Room Can the Cannon Clean?

After I tested every fan in China I could find with a flat front…

…the results of one fan stood out above the rest. Because of its appearance and how much butt it kicked, I called it “the Cannon.”

In Anna’s 15m2 Beijing bedroom, it crushed particulates. It removed 97% of the small .5 microns particles.

But how well can it do in larger rooms? Most companies estimate this using flow rate, but those calculations depend on how well houses are sealed and how dirty the air outside is. Since air outside is dirtier in China, I think it’s necessary to do real-world tests.


Fortunately, I moved into a large 4-bedroom apartment, with a 30.5m2 living room. I tested the Cannon six times on high and tracked particulates with a Dylos particle counter. Each test lasted at least three hours.

I calculated the percentage reduction in particles from the first hour to the last hour.

This test also serves as an important extension of the earlier room tests because:

  1. These tests were run in the daytime. Several people have the intuition that pollution goes down at night because people are less active and fewer cars are on the road (but the data shows that intuition is false–PM 2.5 pollution is worst in the middle of the night in Beijing).
  2. These tests were run while people were moving around in the room and opening the door to the outside. This is more conservative than the nighttime tests because the Cannon has to fight influxes of outdoor air.


Over six tests, the Cannon removed 92% of .5 micron particles and 89% of 2.5 micron particles even with people moving around and opening doors.

In previous tests, 2.5 micron reductions were usually slightly larger than .5 micron reductions, so it’s a little surprising that the 2.5 micron reduction was 3% lower than the .5 micron reduction. My guess is that this is because people were moving in the room, and human movement affects the 2.5 micron readings much more than the .5 micron readings.


The Cannon can clean rooms at least 30.5 m2, which puts it above the 22.3 m2 Blue Air recommends for their 3,6000 RMB 203 model.

As always, I’m posting the raw data and more details on the methods for fellow nerds below.

Raw data: