Xiaomi Auto Mode Leaves Air Unsafe for 86% of hours

For the last three years, I’ve been saying clean air doesn’t need to cost an arm and a leg. So when Xiaomi came out with a purifier for under 1,000 RMB, I assumed it works just fine. But I had never gotten around to testing it—until now. And what I found shocked me.

The Test

Smart Air co-founder Anna ran a brand new Mi2 in the same 15m2 bedroom in Chaoyangmen where we’ve tested the DIYs, IQ Air, Blue Air, and Philips. Anna ran six tests on auto mode and six tests on the highest setting (which the Xiaomi oddly calls 最爱档 -“most love mode” or “Favourites mode”).


Anna turned the purifier on in the morning when she left home, and turned it off when she returned home, so there was no one in the home during the tests. The doors and windows were closed during the tests. (More details on the test method and all original data are here.)

A Dylos Pro tracked ≥ 0.5 micron and ≥ 2.5 micron particle counts during the test. Test data shows these 0.5 micron readings correlate highly with the US Embassy’s PM2.5 readings  (r = .90).

As in my earlier tests, I calculated effectiveness as (the number of particles before turning the purifier on) versus (the average number of particles over the last four hours).


The Xiaomi scored as one of the worst purifiers I’ve ever tested. On average, it removed only about 60% of 0.5 micron particles over the last 4 hours of the test.

Xiaomi effectiveness

Here’s how the Xiaomi results compare to earlier results from other purifiers using the same method, same particle counter, in the same room.

Average % of particulates removed

Below is a normal test day. I put a dashed red line representing the World Health Organization’s (WHO) 24-hour PM2.5 limit (25 micrograms).

Xiaomi room test

The Xiaomi Really Wants To Be in Auto Mode

What’s wrong? The key is that—even on the highest setting—the Xiaomi reverts back to auto mode after 3 hours. To be sure this is what the machine was doing, we put the Xiaomi on the highest setting and tracked noise levels overnight.


You can see the noise remaining high for three hours, then it returns to the ups and downs of auto mode:

Xiaomi noise

Here is the same graph with particle counts (measured by the Dylos). The particle counts rise every time the machine turns off.

Xiaomi noise vs PM 2.5

The data shows that the Xiaomi has a rather low standard for “safe.” The Xiaomi is turning off when PM2.5 reaches 40 micrograms, which is far higher than even the WHO’s 24 hour limit. Then it turns on again when PM2.5 gets up to 70 micrograms—almost three times the limit.

Really? It can’t be kept on high?

The fact that the Xiaomi can’t be kept on continuously all night is so strange that Anna asked Xiaomi’s customer service about it twice to make sure we’re not making a mistake. They confirmed that, no matter what, the machine will go back to auto mode after three hours (full transcript available in the supplemental materials).


How often is the air unsafe?

I calculated the percent of hours that the air was unsafe during the tests using this rule:

After the purifier was on for at least 1 hour, for any hour where outdoor air pollution was unsafe (> 25 micrograms – the WHO 24 hour limit), how many hours was indoor air also unsafe (>25 micrograms)?

The Xiaomi left air unsafe for a shocking 86% of the time. The other similarly sized machines in my earlier tests left air unsafe only 7-16% of the time.

% of unsafe hours

Now perhaps the Xiaomi is using the looser Chinese standard for clean air of 35 micrograms. Even with that high number, 77% of the hours the Xiaomi was running, pollution levels were still above the limit.

Wait, are you sure sure?

I wanted to be really careful about this. I’ve seen other tests showing that the Xiaomi 1 works just fine, including these tests from Dr. Saint Cyr (although I’ve seen people write about flaws too, such as this guy who kept the filters wrapped in plastic, turned it on turbo mode, and the app told him his air got miraculously cleaner). So I double and tripled checked the data:

  1. We tested with a different particle counter in a different room.
  1. I re-analyzed the data throwing out any days with large fluctuations in outdoor air.
  1. I analyzed days when outdoor pollution was low to average (< 150 micrograms).
  1. I compared it to other tests done just two weeks before in the exact same room with the exact same particle counter.

None of these analyses changed the result (see details here).  Note however, that we did only test one Xiaomi Mi2, so there is a possibility another machine would perform differently. I invite anyone with a Mi2 to replicate my tests using the same method, and I’m happy to publish the results.

What now for Xiaomi?

From this data, my conclusion is that the fan and the filter are fine, but the Xiaomi has a programming flaw. Even if I use it on the highest setting, I’ll be breathing air far above the safe limit for most of the night. That’s a problem.

Is it just a Xiaomi problem?

To be fair, I cannot recommend any auto mode I’ve tested. I’ve tested the Philips AC4072 on auto mode, and it averaged 59% reductions in 0.5 micron particles, which is pretty close to the Xiaomi results. So I think this is more of a problem with auto modes rather than the Xiaomi specifically.

Yet I can still recommend the Philips. Why? It can stay in medium or high as long as you want, and my data shows it works fine all night as long as it’s not on auto mode.

I’m confident that Xiaomi can fix this flaw by simply allowing people to run it on high without reverting back to auto mode. But until that happens, I cannot recommend using the Mi2.

Open Data

As always, I’m publishing more details on the test method and the original data for fellow nerds.

How Accurate is the 1 Micron Dylos?  Official PM2.5 Comparison Test

Everyone who buys an air quality monitor wants to know how it compares to the official numbers. When I bought a Dylos DC1100 Pro laser particle counter,  I did a comparison test with the US Embassy. I put my Dylos Pro outside the window of my apartment (at Nanluoguxiang) 70 times and compared the numbers to the US Embassy’s Twitter feed at the same time. I found that the two numbers were very strongly related: r = 0.89 (remember the highest possible correlation is 1).

I did that test with the Pro model, but there’s also a standard model that’s $70 cheaper. At Smart Air, we call it the Standard Dylos.

How does this cheaper machine compare to the government measurements? If it’s similarly accurate, great! That means people can save money and get roughly similar results.


Pro vs Standard

The Dylos Pro and Standard are almost the same judging from their front looks. So what’s the real difference?

The key is the size of particles they measure. Both of them have 2 size ranges. For the small particle channel, the Dylos Pro measures particles .5 microns and above, while the Standard Dylos measures 1 micron and above. On the large particle channel, the Pro measures 2.5 microns and above, while the Standard measures 10 microns and above. That means the Pro measures an extra range from .5 to 1 micron.


The machine governments use is called the Met One BAM. It measures particles 2.5 microns and below. If we put the measurement ranges of the BAM, the Dylos Pro, and the Standard Dylos together, here’s what it looks like:


The Pro overlaps a bit more the government machine than the Standard. For that reason, I’d expect the Pro correlates better with the government machine. But just how well do they compare empirically?

The Test

Luckily I’m not the only nerd interested in air pollution problem. An American living in Guangzhou bought the Standard and teamed up with me to test it against the US Consulate.

From November 12th to 17th, she collected data with her Dylos Standard 70 times outside her apartment,which is close to Sun Yat-Sen University in Guangzhou. She also recorded the numbers of PM2.5 concentration released by the US Consulate.



Remember how well the Pro correlated with the US Embassy? Here’s what I found before (r =.89).


Later I did another test by putting my Dylos Pro about 100 meters away from the US Embassy (much closer than my apartment). This time it turned out that the two numbers correlated at r = .9 – slightly higher than in the first test.


How does the Standard Dylos compare? Here’s what we found:


They correlate at r = .85! Although that’s a bit lower than the correlation between Dylos Pro and US Embassy, the two numbers are still strongly related. I think that result would surprise most people.

Converting the Dylos 1 Micron Readings to PM 2.5

Using this data, we can get a rough formula to convert between the Dylos 1 micron and the official numbers. For example, the 24-hour WHO limit is 25 µg/m³, which is approximately 1,250 on the Standard Dylos.


The simplest formula to convert the Dylos 1 micron to PM 2.5 micrograms is:

Dylos 1 micron count × 0.02 = PM 2.5 µg/m3

If we use that formula to convert the Dylos numbers to micrograms, here’s how well the numbers compare:


On average, the formula was off from the government numbers by 9.04 micrograms.


The Standard correlates highly with the government numbers—only slightly less well than the Pro model.


The conclusion is based on only one test in Guangzhou. We could draw stronger conclusions if we had more tests done in different locations and in different seasons.

Open Data

As always, I’m making the data and more testing details publicly available for fellow nerds.

P.S. Thanks to Yiwen Wang for help translating and creating figures.

Cost of air pollution

Death in the Air Infographic by World Bank

The World Bank released a new report titled “The Cost of Air Pollution: strengthening the economic case for action” and in it they detail how air pollution is now the 4th leading risk factor for deaths worldwide. That’s worse than the deaths attributed to alcohol and drug use, HIV/AIDS, and even malaria. Besides the other reasons for reducing air pollution (climate change, our health, etc.) the economic one is probably the one that will communicate the strongest to everyone as air pollution costs the global economy in terms of foregone labor income to the tune of $225 Billion each year globally.

Click here for full report.

Click here to view the infographic in higher resolution.

Air pollution has emerged as the fourth-leading risk factor for deaths worldwide. While pollution-related deaths mainly strike young children and the elderly, these deaths also result in lost labor income for working-age men and women. The loss of life is tragic. The cost to the economy is substantial. The infographic below is mainly based on findings from The Cost of Air Pollution: Strengthening the economic case for action, a joint study of the World Bank and the Institute for Health Metrics and Evaluation (IHME).
Air pollution has emerged as the fourth-leading risk factor for deaths worldwide. While pollution-related deaths mainly strike young children and the elderly, these deaths also result in lost labor income for working-age men and women. The loss of life is tragic. The cost to the economy is substantial. The infographic below is mainly based on findings from The Cost of Air Pollution: Strengthening the economic case for action, a joint study of the World Bank and the Institute for Health Metrics and Evaluation (IHME).
Source Pollution

10 Facts About Air Pollution

We get many questions about air pollution in our office, and understandably so. It’s a topic that isn’t well understood or well-reported about in certain parts of the countries in which we work. In some cases, it is difficult to distinguish research-backed findings from common beliefs. To contribute to collective learning, here is a quick list of top 10 facts about air pollution.

  1. Air pollution is made up of chemicals, particulates, and biological materials. Common  components include, but are not limited to: nitrogen, sulfur, carbon monoxide, carbon dioxide, dust, and ash.
  2. Air pollution is caused by both human and natural contributors. Industries, factories, vehicles, mining, agriculture, forest fires, volcanic eruptions, and wind erosion all cause air pollution.
  3. According to the Global Burden of Disease report (2013), air pollution contributes to more than 5.5 million premature deaths every year. Another report by the International Energy Agency estimates the number to be 6.5 million deaths per year.
  4. Research has linked air pollution to multiple diseases: acute lower respiratory infections, chronic obstructive pulmonary disease, lung cancer, tuberculosis, low birth weight, asthma, and cataract.
  5. According to the WHO, 98% of cities in low- and middle-income countries with more than 100,000 habitants have unsafe levels of air pollution.
  6. Of the top twenty most polluted cities in the world, 13 are in India and 3 are in China. Delhi ranks as 11th most polluted, whereas Beijing ranks as 57th most polluted.
  7. Over half of India’s population—660 million people—live in areas with unsafe levels of air pollution.
  8. On average, Indians living in polluted areas will lose 3.2 years of their lives due to air pollution.
  9. In 2014, India and China tied at 155 among 178 nations in rankings measuring how countries are tackling air pollution in the world, despite both countries having some of the worst air quality in the world.
  10. Pregnant women who live in  high traffic areas have a 22% higher risk of having children with impaired lung function than those living in less polluted areas.
Flickr Photo

Is Summer Air Better than Winter Air?


Summer is here, bringing with it clearer skies and certainly cleaner air. Right?

Summer always seems to drive out the dense clouds of pollution that suffocate many Indian cities. However, while summer air is in fact cleaner than air during other seasons, it’s still far from safe according to the standards set by the World Health Organization (WHO).

During the winter, cold air traps pollutants close to the ground, a process called an “inversion.” Summer heat prevents this inversion, which does improve the air quality. However, average air conditions in India are still clearly not ideal.

Here’s a map of today’s pollution levels across India:


pollution levels
Source: https://aqicn.org/map/india/


On a day like today, when the AQI in Chennai, Hyderabad, Kolkata, Mumbai and New Delhi is in the ‘unhealthy’ or ‘very unhealthy’ range, we often wonder at Smart Air if the pollution in summer really is any better than the winter.

We got to the bottom of it by analyzing the US Embassy’s data in New Delhi and US consulates’ data in Mumbai, Chennai, Hyderabad, and Kolkata. So is summer air really better than winter air? We took the data from the past two years (June 2014 to June 2016) and broke it down into four seasons: winter (December to February), summer (March to June), monsoon (July to September), and post-monsoon (October to November). Next, we calculated the average particulate pollution (PM2.5) levels for each season.

Across the five cities we looked at, PM 2.5 levels were 26% better in the summer—118 micrograms in the winter compared to 49 micrograms in the summer. That means summer air is better.

Let’s take a look at the difference in PM2.5 between the five cities during different seasons:



US Embassy Air Quality Data
U.S. Department of State Data, June 2014 – June 2016. Air quality data may not be validated or verified


But how good is “better?” Here in India, “better” is nowhere near “safe.” Over the course of the two years we analyzed, average annual pollution levels in all five cities never fell below even the WHO’s more lenient (24-hour) exposure limit (25 micrograms per cubic meter). In fact, the average pollution levels across all the cities we tested was about 500% the WHO annual limit (10 micrograms) and 200% of the more lenient 24-hour limit (25)!


The lowest summer pollution level we found was Chennai (31 micrograms). But even that lowest summer level still surpassed the WHO limits.

Below are the 2-year graphs for each city. You can see that each city has two distinct swells in PM2.5 levels during the winter, each followed by 2 clear dips during the summer. Interestingly enough, comparing the summer and winter levels of each city from 2014-2015 to 2015-2016 shows some cities’ PM2.5 levels improving, while others’ increase between years. Most notably, Chennai’s winter pollution levels dropped significantly between years as did Hyderabad’s, while New Delhi and Kolkata experienced clear increases. However, we’re not sure whether or not this improvement and worsening of PM2.5 levels can be attributed to cities’ environmental efforts (or lack thereof).

The conclusion? The evidence is quite clear: summer air is in fact better than winter air. However, despite all the blue skies and warm days we’ve been having lately, there’s still a need to protect yourself inside and outside the house. Don’t mistake “better” for “safe.” Neither summer nor winter air meets WHO health standards and summer air is still of significant concern to public health.


Chennai US Department of State
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.


US Embassy Air Quality
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.


US Embassy Air Quality Data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.


US Embassy Air Quality Data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.


US Embassy Air quality data
U.S. State Department Data – June 2014 to June 2016. Data may not be fully verified or validated.


Do pollution masks 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.

Here are the two most frequent skepticisms I hear about 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:

图片 1

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.

  1. “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’re actually 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).

Beijing-based Dr. Richard Saint Cyr also tested masks, so I’ll combine my data with his. 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: fit test results on my face won’t always be the same for other people’s faces. 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. And you certainly don’t need to buy the most expensive masks on the market to breathe clean air.


A note on gases: Note that these tests are about particulate pollution. Most commercially available masks don’t target gas pollutants like NO2 and O3, so it’s not 100% protection.

  1. 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 you’re wearing them. And they have documented health benefits. That should be enough to satisfy even the skeptics!

Poor Man’s Fit Test

Which mask works best on your face? I was fortunate enough to visit a lab to do a super fancy fit test, but very few of us have access to this $10,000 machine. So what should normal folks do?

图片 1

While visiting the 3M lab, I learned about what I’m calling the poor man’s fit test. It’s not as accurate as a real fit test, but it will help you identify big leaks. It’s pretty simple:

  1. Put on the mask. Make sure the metal is bent tightly around your nose.
  1. If the mask has two straps, make sure one strap is below your ears and one above like this:maxresdefault
  2. Lightly hold the mask in place and inhale sharply. While inhaling, pay attention to see if you feel a sensation of air or coolness around the edge of the mask. Pay particular attention to the area around the nose.
  3. If you feel air leaking, adjust the mask and try again. If further adjustment does not solve the problem, try a different mask.


If your mask does not have an exhalation valve, you can also do the test while exhaling sharply.

Breathe safe!

Do ionizers actually clean air?

The other day, someone on Quora asked whether ionizer fans 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 products like this:

Ionizers 2

 Amazing! A “miraculous 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” purifiers, I tell them to steer clear.

Careful not to overgeneralize

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, 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:


Is Air Pollution a Problem in Mongolia?

图片 1

According to The Guardian, Ulaan Bator is the world’s second most polluted mega city. From 2008-2011, the average PM 2.5 level was nearly 150 micrograms. In 2014, Beijing averaged 98 micrograms; the WHO 24-hour limit is 25 micrograms.



Is it a bustling manufacturing sector? Manufacturing is still a tiny percentage of the Mongolian economy. Cars filling the streets? Not that either.


It turns out, most of Ulaan Baatar’s pollution is from people burning coal to keep warm. A little heat is necessary where winter temperatures dip to -40 C.

Yet all that air pollution causes 1 in every 10 deaths in the capital, according to an academic study. That makes clean air an urgent need in this city of 1.2 million people.

Smart Air will host its first-ever Mongolia workshops in Ulaan Baatar! Join us in the Mongolian capital September 11th and 12th.

Join us to learn how DIY purifiers can help remove dangerous particulate pollution from inside the home. Smart Air founder Thomas Talhelm will explain how even the priciest purifiers use a fan to push air through a HEPA filter. Workshop participants will make their own in just 10 minutes, and they’ll take it home to start breathing easier right away.

Types of Filters

Premise: This blog is for people concerned about air pollution. There are many valid reasons people want air purifiers: pet allergies, pollen, and asthma. However, these are not what I’m concerned about while living in China. So I assess purifiers solely based on whether they can help protect me from particulate air pollution.

With that in mind, here are the three most common types of filters in portable air purifiers:

  1. UV light filters are designed to kill bacteria. In China, I’m concerned about air pollution, not bacteria, so UV filters are unnecessary. But beyond that, Consumer Reports says that UV filters in most air purifiers don’t even kill bacteria:

The Environmental Protection Agency cautions that air cleaners outfitted with ultraviolet light are unlikely to kill bacteria and mold because they won’t be in contact with UV light long enough to have any effect.


Conclusion: UV filters aren’t what I need.

  1. Activated carbon filters use charcoal screens to catch certain types of chemicals and organic matter that happen to interact with carbon. In other words, these filters catch some things, but not particulate in general. In sum, these will do a little bit, but they aren’t the whole solution.


Activated charcoal is good at trapping other carbon-based impurities (“organic” chemicals), as well as things like chlorine. Many other chemicals are not attracted to carbon at all – sodium, nitrates, etc. – so they pass right through. Howstuffworks.

Conclusion: Carbon filters may help (particularly for certain types of gases that carbon can get), but they don’t target all particulate matter. It would be a mistake to use ONLY a carbon filter to get rid of particulate pollution.

  1. HEPA filters are the solution. “HEPA” sounds fancy, but it’s just a standard that means the filter catches 99% of particles .3 micrometers and above (they also get particles smaller than .3 micrometers–the .3 designation is based on the overlapping point of different types of filtering). That covers a lot of particulate pollution–the most-often cited figures are for 2.5 micrometers.


HEPA filters aren’t rocket science. They work pretty simply: particles get stuck in the fiberglass fibers (using a few types of physical filtering processes). In fact, if you have a vacuum cleaner, it probably has a HEPA filter in it. You can get one for about $10.

Contrast that with the major purifier companies like IQ Air and Blue, which have proprietary HEPA filters with names like “HyperHEPA” that cost $200. These may have benefits, such as a longer lifetime or smaller pore size, but it’s not clear to me that’s necessary or worth 20 times the price.

Bottom line: If your concern is particulate air pollution, a HEPA filter is really all you need. And for that, you don’t need spend 8,000 RMB. You can make your own for 166 RMB.