## 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?

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!

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])

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

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!

Methods:

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:

Results:

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%.

Conclusion:

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:

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:

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

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:

That ionizer shoots out negative ions:

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

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.

### 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:

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:

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:

…or to clean a HEPA filter vacuuming it

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

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.

### 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.

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.

## 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.

Methods

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.

Results

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.

Conclusion

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:

## The Xiaomi Particle Counter Is So Inaccurate It Should Not Control the Purifier

Tech companies love to talk about how automation, internet of things, and the connected house are going to make the machines we use every day more convenient. But does it work? Anyone who’s used Apple’s Siri or “talk to text” feature know that the promises of technology sometimes fall short.

Here’s the Xiaomi air purifier, the Mi2 to be precise.

And here’s the air quality readout coming from a small particle on the side of the machine.

### The Promise

And here’s what it promises to do—detect how bad air is in your home, turn the purifier on when air is bad and turn it off when air is good. If it can do that, it means we can breathe clean air without the fan on high all the time. That’s awesome because it means less noise and less wasted electricity.

### The Test

We previously tested how good the Mi2 purifier was at cleaning the air, and found shocking results: it left air unsafe 86% of the time. This time around, we wanted to do a more methodical test of the particle counters inside the Mi purifiers.

I tested the Mi1, Mi2, and the more expensive Mi2 Pro version against three particle counters. None of the Mi’s were new, but the Mi2 was relatively new, used to do just a few weeks of testing.

I set up the two Laser Eggs and the Air Visual Node on a chair next to the built-in particle counter on the Mi2.

I burned a cigarette in a closed 12m2 room and then turned on the purifier on high until the air got clean again (about 30 minutes from start to finish). That way we can test for accuracy from clean levels to truly toxic levels. I set my phone to take pictures of all the readings every 30 seconds.

### How Accurate SHOULD It Be?

But wait, before I get to the results, I want to set expectations. I don’t expect the Xiaomi particle counter to be really accurate. It’s a cheap particle counter inside a machine that costs less than some of the particle counters I’m about to compare it to. We need to have realistic expectations.

So what are realistic expectations? I don’t ask for it to be great. All I ask is that that it works well enough to do what it’s designed to do—run the auto mode.

### Results

Even against modest expectations, the Xiaomi was off by a lot. When the air was bad, the Xiaomi was off by an astounding 218 micrograms.

To give a sense of how large that discrepancy is, the WHO 24-hour limit is 25 micrograms. The Xiaomi’s error alone was over 8 times the WHO limit.

Here’s what that looked like live.

The Xiaomi seemed like it almost stopped counting past 50 micrograms. At that rate, the Xiaomi was saying the air inside was at orange or “unhealthy for sensitive groups” when it was really in the purple “very unhealthy” range.

OK, so the Xiaomi undercounts—severely at times. It turns out that’s not the only problem. If we zoom into the low range, the Xiaomi was overcounting.

I suppose a 9-microgram discrepancy might sound like not a big deal, but on the other hand, the Xiaomi was overestimating the real number by a factor of 10.

### The Xiaomi 1 Is Inaccurate Too

Maybe the Mi2 I got was just broken. Maybe the shipping guy dropped the machine on the way to my home and damaged the particle counter.

To test that possibility, I tested an older Mi1 against the Dylos Pro (which also scored wellagainst the official PM2.5 numbers). The results showed the same pattern as the Mi2.

I also tested the Mi2 Pro, and it showed the same pattern. Thus, this seems to be a consistent problem with Xiaomi purifiers.

### How Do We Know Those Other Numbers Are Correct?

Hang on, aren’t we assuming the Laser Egg and the Node are the right numbers? How do we know that those are the right ones, and the Xiaomi is the wrong?

Smart Air ran comparison tests of the Node and Laser Egg with official PM2.5 numbers for six days. The Node and the Egg correlated with the official PM2.5 at a very respectable r = .98, with an average error of 4.8 micrograms for the Node and 6.5 micrograms for the Egg. That makes me confident their numbers are a good approximation of the true concentration.

### Bottom Line

The Xiaomi particle counter is extremely inaccurate—so inaccurate that it should not be used to control your purifier. The problem is, Xiaomi doesn’t give users a choice (which I explain below).

### This Could Explain the Xiaomi Left Air at Dangerous Levels in Our Tests

Smart Air tested the Xiaomi Mi2 air purifier in a real Beijing apartment for 12 nights, and the results shocked me. I honestly thought it’d do a fine job. After all, purifiers are just fans and filters. But the Xiaomi left the air at unhealthy levels for 86% of the time.

The fact that the Xiaomi so severely underestimates pollution levels could explain why it so often leaves the air at those unsafe levels. I found similarly atrocious results when I tested the Philips auto mode, which convinces me that the technology behind air purifier auto modes just isn’t good enough yet. I would not use an auto mode in my home.

### Why This Problem Is More Than Just an Accuracy Problem

The Mi2 is fine purifier when it’s on high. Our open-source tests show that it does a great job on high (check out the first three hours in the test graph above). But the problem is the Mi2 forces users to use auto mode. No matter what you do to the machine, it will switch to auto mode after three hours. Sounds weird, right? We asked customer service three times just to be sure.

That means unless you wake up every three hours during the night and switch the machine back onto high, you have to use auto mode and the particle counter that controls it. I hope Xiaomi fixes this simple design flaw, but until they do, I would not use a Xiaomi in my home.

Read more for extra data and methods. I also test the possibility that the particle counter is inaccurate because it’s on the inside of the machine and so sampling air that is different from air outside the machine.

### Extra Data and Methods

#### Mi1 Test Method

I tested the Mi2 in the Smart Air office and the Mi1 at my home, so the room and methods were slightly different. In the office, I burned a cigarette to make the particle counts go up. At home, I don’t have any cigarettes, so I burned a piece of paper.

The size of the office room was 12m2. My room at home was larger, probably closer to 15m2.

#### Is the Xiaomi inaccurate because the particle counter is inside the machine?

I wondered if the particle counter is inaccurate because it’s on the inside of the machine and therefore not getting a good sample of air. One way to test this is to take the particle counter out of the machine, which isn’t very hard. Even when I did that, the numbers still consistently undercounted when pollution was high and overcounted when pollution was low. Thus, I don’t think the problem is the placement of the particle counter.

#### What are the Xiaomi numbers exactly?

One frustrating part of the Xiaomi is that it doesn’t label the air quality numbers. Are they micrograms, China AQI, US AQI, or something else? I can’t understand why they wouldn’t label the numbers.

This isn’t just a nerd concern. It could really affect the results because the relationship between micrograms and AQI isn’t linear.

If you dig around deeply enough through the Xiaomi, they do say that the numbers are micrograms. Thus, I compare micrograms to micrograms in the analysis.

#### Can the Xiaomi get below 10 micrograms?

The lowest number the Xiaomi registered was 9 micrograms, while the Node was registering 0.2 micrograms and the Eggs 1 microgram. That made me wonder, is it even possible for the Xiaomi to display numbers below 9? Is it programmed not to go below that number?

To get to the bottom of it, I turned on the DIY 1.1 and pointed it directly onto the Xiaomi particle counter. When I do the same test with the Dylos particle counter, the numbers go down to zero. But with the Xiaomi, the numbers stayed around 10 micrograms. Therefore, I think the Xiaomi is either registering phantom particles or programmed not to go below 9.

### Original Data

I’m making the original data available as an Excel file download here.

### Node Numbers

#### Why Were the Node Numbers Low?

In the main graph in the article, you’ll see that the Node numbers were lower than both of the Laser Eggs. The Node I used in the test was about a year old (although the Laser Eggs weren’t new either). One problem with older particle counters is that dust accumulates inside the machine and restricts the air flow. The guy behind AQIcn.org tested an old Dylos and found it was undercounting when concentrations were bad. Then he cleaned out the dust inside with compressed air and found it got up to higher numbers.

I suspect the same thing was happening with the Node in our tests. That can be a particular problem when the particle counter is subjected to really high levels of particulate, like in our cigarette tests.

## How accurate are common particle counters?

When I started Smart Air in 2013, I wanted to buy an air quality monitor. I had basically two options. I could buy a US\$260 Dylos or spend thousands of dollars on the crazy expensive particle counters. Since then, the market has exploded with new air quality monitors as cheap as 99 RMB. But are they any good?

### Putting Pollution Monitors to the Test

To get to the bottom of it, Smart Air tested three popular pollution monitors on the market: the Dylos DC1700, the Kaiterra Laser Egg, and the Node.

### Dylos

The Dylos is the trusty particle counter Smart Air has been using since 2013. We’ve used it to perform purifier comparison tests, and air quality tests in places like China and India.

### Laser Egg

Origins Laser Egg

The Laser Egg from Kaiterra (formerly Origins) is a popular, more technologically savvy device.

### AirVisual Pro

AirVisual Pro

The AirVisual Pro (formerly the AirVisual Node) is a fancier version, including a large screen, pollution forecasts, and even a CO2 monitor.

### The Government Comparison

We placed the machines outside the Smart Air office in Beijing, on Dongzhimen Waidajie. That’s about 1.3km away from the government PM2.5 monitor at the Agricultural Exhibition Center.

We ran the machines for six days. The Laser Egg and the AirVisual Pro give output in PM2.5 micrograms, and the Dylos gives number of 0.5 micron particles. As a result we converted it to PM2.5 micrograms using the semi-official formula (0.5 microns – 2.5 microns)/100.

Here are the results for the first (72-hour) test outside our office in Beijing:

### Low-Concentration Test

Next we tested days with extraordinarily low PM2.5, over a period of 48 hours. That’s helpful because concentrations in homes—where most people use pollution monitors—are also typically low. We know this from tests we’ve done on air quality in Beijing and Shanghai. So with this data we should be able to test how good the devices are at measuring low concentration levels.

Low-Concentration Tests

Eyeballing both graphs, all three machines did a pretty good job of tracking the official numbers. Combining both tests, we found that both the AirVisual Pro and the Laser Egg correlated r = 0.98 with the official PM2.5 numbers.

For non-nerds, 0.98 is incredibly close to identical! The Dylos had the lowest correlation at r = 0.90, but still incredibly high (and similar to our previous test).

These correlations are all extremely high, suggesting that they’re all tracking government data well.

### Average Deviation

Another way to measure accuracy is to look at on average how far the numbers were from the government data. The AirVisual was the closest: it was off from the official numbers by an average of 4.8µg/m3. The Laser Egg was consistently further than the government machine, with an average deviation of 6.5µg/m3. The Dylos was off by an average of 9.1µg/m3.

Next we looked at deviation in the low range. The Laser Egg had higher deviation in the lower range. However, even these deviations were not large.

### The Airpocalypse Test

To test accuracy at extremely high concentrations, we burned a cigarette in a closed 15m3 room. Our goal was to see how well the particle counters were at reading concentration levels over a whole range of values, including toxic levels. With the help of cigarettes and a partner NGO in Beijing, we managed to get the concentration above 1,000µg/m3!

For this test we also had another machine (Sibata LD-6S) on hand as a reference. This is an industrial PM2.5 dust indicator, with an accuracy of ±10% and repeatability error of ±2%. The LD-6S was used as our baseline monitor.

It’s clear from the data the Laser Egg and the Dylos had a hard time measuring high levels of concentration. In contrast, the AirVisual and the LD-6S matched very closely. Both were able to measure concentrations over 1,000µg/m3. The chances you’ll need to measure concentrations this high outside of experiments are slim, but the AirVisual did surprisingly well.

### Take-Home Message

Overall, the three particle counters were reasonably accurate compared to the government machines. They’re all suitable for giving an approximate indoor air pollution (AQI) reading  in your home. Of all three, the AirVisual scored the highest. It had the lowest deviation from the government machines, and the highest accuracy in the “crazy bad” test.

### Usability

Since all three machines are reasonably accurate, the question then really comes down to: How easy it is to use the device? And what features do they have?

The Dylos (1800 RMB)

The Dylos easily loses this fight. It has no phone connectivity, and downloading the data is a terrible pain—and that’s if you have one of the old school pin connecter cables. With new, much cheaper particle counters now on the market, the price is also far too high for what you get.

The Laser Egg (499RMB)

The Laser Egg is an entry-point pollution monitor. It gives reasonably accurate results with a simple interface. It’s not feature rich, but it does what it says on the box. The Laser Egg is a great low cost way of testing the air in your home–ideal for making sure your purifier is doing the trick.

The AirVisual Pro (Node) (1,480RMB)

To our eyes, the airVisual offers the best features. For starters, it can measure CO2, temperature, and humidity. That makes it more of an ‘environment monitor’ than just a particle monitor. If you have indoor sources of air pollution (VOCs) like new furniture or remodeling, high CO2 levels can mean that those indoor pollutants are building up. However, at 1,480RMB (over \$200) it’s not cheap.

### Conclusion: which air quality monitor is top?

After passing our tests, we will start shipping the Laser Egg air quality monitor and the AirVisual Pro through our stores in China, India, and Mongolia. They’re both great options for anybody wanting a solid device for both home use and research (if you’re a nerd like us). Go take a look!

Over the next few months, we hope to get a larger pool of air quality monitors together and run more extensive tests. This is only the beginning! Once we’ve independently verified more devices, we may well be adding them to our shop as well.

## Is It True That Masks Last Just One Day?

We posted info and data on our findings on whether masks really can stop pollution. My judgment after reading up on mask tests and testing them myself is that the science is pretty well settled. Masks do work.

But there are still mysteries that need answers. Perhaps the biggest mystery out there is how long do these masks last?

### Really Just One Day?

I frequently run into people who suspect that masks only last one day, such as this person on Zhihu (and the 115 people who upvoted him):

Or this person who sent me an email:

### What Does 3M Say?

Last year I had the chance to visit the 3M lab:

Why? I think they’re worried about giving a specific timeframe because many users are in industrial environments with super high or very toxic pollutants. So giving a general “it lasts X days” answer could mislead people who work in completely different environments. But for folks like us in China who see a fairly “normal” range of pollution, we should be able to find a rough guideline.

### Test

Fortunately, when I went to do fit tests, I had a 9332 mask that I had been using for 11 days. And when I say “used,” I mean “used intensely.” It was summer in Beijing, and I wore the mask while biking around town and sweating heavily. I estimate I used the mask at least four times every day for those 11 days.

It’d be a lot better if we had lots of test data with more masks over more durations. But this data can at least test the popular conception that masks are single use or one-day use only.

So what did the data say? Here are the results for a completely new 9332 mask (left) and my 11-day-old mask (right).

98% after 11 days is pretty darn good! This contradicts the idea that you can only use these for a day or two.

### So, How Long Do Masks Last?

The truth is, we need more data. With this limited data, my guess is masks last at least two weeks. I use mine for 2-4 weeks, at which point the rubber bands break or they get so gross I want to throw them out anyway!

Keep breathing clean air, both inside and out!