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.

Xiaomi Auto Mode Leaves Air Unsafe for 86% of Hours – Supplemental Data

Data and Test Details


The app asks what size the room is. How Xiaomi uses that data is a little opaque, but it would be logical that settings for larger rooms will run the purifier harder, so we used the highest allowable setting, 34-37m2.

It should be noted that the Xiaomi also has a turbo mode, which I’ve heard people describe as “sounding like a jet taking off.” This really isn’t meant to be used while people are in the room. Instead, it’s meant to be used temporarily before returning home.

Tests in a Different Room

We’ve used this same bedroom in Chaoyangmen, Beijing for many of our tests—of the Original DIY, Cannon, Philips, Blue Air, and IQ Air. So I doubt there’s something weird about this room that somehow hurts the Xiaomi but not other machines.

But you never know! So to be sure, we conducted an overnight test in the Smart Air office near Sanlitun. The results are shown in the noise test above. They show the same pattern as the other room tests. Thus, the results don’t seem to be something weird happening with that particular test room.

Tests with a Different Particle Counter

We’ve used the Dylos Pro for most of our tests, so this is constant across tests. But particle counters can break or lose accuracy over time. Thus, we carried out the noise/particle test above with a different Dylos Pro. The fact that the pattern of the results is the same suggests the results are not because of any problems with the particle counter. (That conclusion is also hinted at by the fact that the Xiaomi performed well for the first three hours while on high.)

How Bad Was Outdoor Air?

It’s important to analyze how bad outdoor air was during the tests because my analyses have shown it’s harder to achieve a high percentage reduction when outdoor air pollution is bad. (Or put another way, it’s easier to achieve a high percentage reduction on relatively clean days.) For example, here is the relationship between effectiveness and outdoor air pollution for the Blue Air 203:

Blue Air

However, theoretically, the Xiaomi auto mode shouldn’t be affected by outdoor air pollution. If it can accurately detect air pollution levels and turn on the fan in response, the results shouldn’t be affected by outdoor air pollution levels (until we get to levels that are too high for even constant high mode to clean). But to be conservative, I analyzed hourly outdoor air pollution data from the US Embassy, about 7 kilometers from Anna’s home.

During the auto mode tests, outdoor air averaged 128 micrograms. During the high-mode tests, outdoor air averaged 246 micrograms. Both of these are higher than the average in Beijing (90-100 micrograms). So it’s worth seeing how the Xiaomi did on days with lower pollution.

I analyzed the four days with lowest outdoor air pollution (average 89 micrograms; 11/29, 12/2, 12/5, 12/7). On these days, the Xiaomi averaged a 56% reduction in 0.5 micron particles and 87% in 2.5 micron particles. Thus, the poor results in the main tests do not seem to be because of outdoor air pollution levels.

Outdoor Air Fluctuations

Besides the baseline level of outdoor air pollution, it’s also important in these real-world tests to look for large fluctuations in outdoor air pollution. If outdoor air gets a lot worse during the test, it can look like the purifier is not cleaning the air very well. And on the flip side, if outdoor air pollution goes down a lot during the test, it can look like the purifier did a great job.

First, I analyzed the data after removing any test day where outdoor air fluctuated more than 100 micrograms from beginning to end (4 days total). That left 8 test days. The result was very similar: on these stable days, the Xiaomi average a 67% reduction in 0.5 micron particles and 86% in 2.5 micron particles.

Next, I analyzed all days where outdoor PM2.5 micrograms changed no more than 50% from baseline at any point during the test. This “at any point” criterion is more stringent, and it left three test days (12/11, 12/12, 12/18). Again, the results were nearly identical: a 69% reduction in 0.5 micron particles and 83% in 2.5 microns. In sum, the poor results did not seem to be caused by fluctuations in outdoor air pollution.

% Hours of Unsafe Air

To calculate the percentage of unsafe air for the Blue Air 203/270E (3,600 RMB), and Philips AC4072 (3,000 RMB), I used the data in my previously published tests. For the Cannon, I used three series of tests: my original tests, tests where tested whether adding a pre-filter affects performance (below), and tests comparing performance after adding a carbon layer.

That gives a lot of data! I had 93 hours of data for the Xiaomi where outdoor air was above 25 micrograms, 98 for the Cannon, 80 for the IQ Air, 41 for the Blue Air, and 45 for the Philips.

% Unsafe Hours: How bad was outdoor air?

It’s important to compare just how bad outdoor air was during the tests. If one machine has lots of hours where air was just above 25 micrograms, it would be a lot easier for the machine to clean the air. So, I calculated the average outdoor micrograms for all of these unsafe hours. For reference, Beijing’s air has averaged about 90-100 micrograms for the last 7 years according to the US Embassy.

For these calculations, outdoor PM2.5 averaged 78 micrograms for the Philips, 113 for the Blue Air, 209 for the Xiaomi, 111 for the Cannon, and 146 for the IQ Air. The Xiaomi really stands out, so I re-analyzed data only looking at days with lower outdoor air pollution, with an average of 138 micrograms—lower than the IQ Air. The result was similar to the original analysis: 83% of hours were unsafe.

In calculating the percentage of time the Xiaomi didn’t give clean air, we used all tests except for one day since this was a short test day. If we include this dataset in our calculations, the Xiaomi gave unsafe air (PM2.5 concentration above 25µg) 85% of the time, giving the Xiaomi a 1% better percentage point, but it’s still not great.

To be even more conservative, I analyzed the two days with the lowest outdoor concentration, averaging 108 micrograms. That is lower than all of the other machines except the Philips. On these days, 68% of hours were unsafe. Thus, the Xiaomi was leaving far too much unsafe air, even on days with lower outdoor AQI.

The Xiaomi Test Compared to Other Recent Tests in the Same Room

I’ve published dozens and dozens of days of test data using a Dylos particle counter in this exact same room (1, 2, 3, 4, 5, 6). That makes me pretty confident these poor results are not some basic flaw of the test design. However, there is always the possibility that the particle counter will break or lose its accuracy slowly over time. Or maybe some neighbor is setting up a secret chuan’r stand nearby. Fortunately, I have data from just two weeks earlier in the same room with the same particle counter.

If something is wrong with the particle counter, we should get weird results for these tests. But over several tests with a new-and-improved Original DIY 1.1, the results are pretty much what I’d expect—a modest percentage higher than results for the DIY 1.0. Here’s a sample test day for the DIY 1.1 two weeks before the Xiaomi test:

DIY 1.1 room test

That test data looks normal to me. And that suggests that there’s nothing strange going on with the room or the particle counter recently.

Converting Particle Counts to PM2.5 Micrograms

The tests used a Dylos Pro particle counter. To convert to PM2.5 mcirograms, I took the 0.5 micron particles divided by 100. This formula comes from Dylos, and our prior tests show it’s pretty accurate compared to the US Embassy—a correlation of r = .90.

Original Data

Below is the original data for the Xiaomi tests, including the outdoor PM2.5 levels as recorded by the US Embassy. My comparison data for the Blue Air, Philips, IQ Air, and DIY Cannon are available through in my earlier post. To request a copy of the original data in Excel format, please email us at [email protected].

Xiaomi Noise + Particle Count

Xiaomi Raw Data 1

Xiaomi Raw Data 2

Xiaomi Raw Data 3

Xiaomi Raw Data 4

Xiaomi Raw Data 5

Xiaomi Raw Data 6

Xiaomi Raw Data 7

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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.

Shanghai Technical Inspection Bureau Mask Tests

It’s tough to get good mask data on a wide range of consumer masks. I’ve posted some of my own test data and test data from Dr. Saint Cyr. But another great source of even more masks comes from the Shanghai Municipal Bureau of Quality and Technical Supervision. I’ve linked to their tests several times, but now they’re down for some reason. Thanks to the Internet Wayback Machine, I was able to locate an archive of their results, and I’m posting them here so that everyone has access to this important data.

I have separated the list into ‘Brands whose efficiencies are up to standard’ and ‘Brands whose efficiencies are not up to standard’ and it is noteworthy that many brands have efficiencies that are way below the standards set by the officials. Therefore, it is important to choose your masks carefully!

The Tables are shown below followed by the bar charts which can clearly differentiate the build quality of the masks. The masks are also labeled from 1-25 and a to m respectively for brands whose efficiencies are not up to standards and brands whose efficiencies are up to standards for easy reference.

1-910-1415-1920-25a-j    the rest

Capture 2 Capture

Keep in mind that these are tests of filtration effectiveness. They are not mask fit tests (which you can read more about here).

Winter is Coming

Another year, another winter. As winter arrives in Beijing and soon in Shanghai, I got to wondering: how much worse is winter air?

To get to the bottom of it, I analyzed the last eight years of US Embassy PM 2.5 data for Beijing and Shanghai. I found that the capital’s air has averaged 111 micrograms in the winter versus 92 micrograms for the rest of the year. Shanghai was a little better at 65 micrograms in winter versus 40 micrograms in summer.


Beijing summer-winter Shanghai summer-winter

Just how bad is that? The WHO 24-hour PM 2.5 limit is 25 micrograms (the year PM 2.5 limit is just 10 micrograms!). That means Beijing’s summers average three times the 24-hour limit, and winters average over four times the limit.

China whole year trend

Don’t live in Beijing or Shanghai? Then don’t get complacent! This trend is the same across China:

Breathe safe this winter!

How accurate are common particle counters? Comparison test

When I started Smart Air in 2013, I wanted to buy a particle counter, and 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 particle counters as cheap as 99 RMB. But are they any good?

Putting Particle Counters to the Test

To get to the bottom of it, we tested three popular particle counters on the market. We tested the Dylos DC1700, the Origins Laser Egg, and a new particle counter called the AirVisual Node.



Dylos DC1100
Dylos DC1100


The Dylos is the trusty particle counter Smart Air has been using since 2013.


Laser Egg

Origins Laser Egg
Origins Laser Egg


The Laser Egg is the popular, more technologically savvy device.



Air Visual Node
Air Visual Node

And the Node is a fancier version, including a large screen, richer information, pollution forecasts, better user experience and even a CO2 monitor.



The Government Comparison

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


Map of Smart Air Office and Agricultural Exhibition Centre



We ran the machines for six days. The Laser Egg and the Node give output in PM2.5 micrograms. The Dylos gives number of 0.5 micron particles, so 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 Tests


Next we tested on days with extraordinarily low PM2.5. That’s helpful because concentrations in homes—where most people use particle counters—are also typically low. So this data is good for testing how good the devices are at low concentration levels. We ran tests for 48 hours whilst the skies were clear.

Low concentration tests
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 Node 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 and suggest that these particles counters are 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 Node 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.


Low concentration PM2.5 deviation from Agricultural Center


Perhaps one worry to note is that the Laser Egg was consistently under-estimating PM2.5 while air pollution was in the lower range. This means there could be a risk that the Laser Egg underestimates the real pollution levels in the home, giving a false sense of security. However, even these deviations were not large.


picture2 picture1 picture3


The Airpocalypse Test

To test accuracy at extremely high concentrations, we burned a cigarette in a closed 15m3 room. Our goal here 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!

Airpocalypse test setup

For this test we also has 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%. Thus, we used the LD-6S as our baseline.


Airpocalypse test results
Airpocalypse test results


Looking at the data, it’s immediately clear that the Laser Egg and the Dylos had a hard time keeping up with these really high levels of concentration. In contrast, the Node and the LD-6S matched very closely, and were able to measure values over 1,000µg/m3. The chances of you needing to measure these values outside of experiments are very slim, but it shows that the Node is more accurate at these high levels.


Take-Home Message


Overall, the three particle counters were reasonably accurate compared to the government machines. In the estimation of the Smart Air team, all of them are suitable for giving an approximate AQI value in your home. Of all three, the Node scored the highest, with the lowest deviation from the government machines in both outdoor tests and the highest accuracy in the “crazy bad” test.




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.


The Laser Egg (499RMB)

The Laser Egg is an entry-point particle counter. It gives reasonably accurate results with a simple interface. It’s not feature rich, but it does what it says on the box.


The AirVisual Node (988RMB)

 To our eyes, the Node 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. CO2 is important if you have lots of people in a small space as it can give an indication of how confined the space is. If you have indoor sources of air pollution (VOCs) like new furniture or remodelling, high CO2 levels can mean that those indoor pollutants are building up. Its user design shows AQI and CO2 for the past 24 hours both indoors and outdoors, a forecast for the coming days, and suggestions to help you decide when to open your windows and wear a mask. We’ve found these features helpful in our office.


After passing our tests, we will start shipping the AirVisual Node through our Taobao shop  and website. It’s a great option 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 particle counters 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.


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).

US Embassy Beijing reads AQI of 0! Are the summer skies always clear?

Map of today’s pollution levels across China – 9th September 2016.


What a glorious day in Beijing! Right now, the US Embassy in Beijing is giving a PM2.5 value of 0. Is summer normally this good? And what’s the pollution like in other parts of China right now? (Short answer: not good! Long answer: read on!)

US Embassy Beijing’s Twitter account


A few months back we posted our analysis on the summer/winter variation in air pollution in Beijing. Using the US Embassy’s data for four more cities we’re able to paint a wider picture of the difference in summer and winter pollution levels across major cities in China.

This time around we’ve analyzed the US Embassy’s data for Shanghai, Chengdu, Guangzhou, and Shenyang. Using data from the past 7 years we have calculated each city’s pollution on a monthly and seasonal basis.

The result? Our analysis across these four cities confirmed the popular theory that summer air is better than winter air; PM 2.5 levels were on average 29% better in the summer across all cities.

It’s likely that during the winter months, air pollutants which would often disperse away from city centers remain locally confined due to inversion. Inversion is an atmospheric condition in which cold air is trapped beneath a layer of warm air close to the earth’s surface. Summer heat prevents this inversion.

Although summer pollution is “better” than winter, it’s doesn’t mean these levels are satisfactory or safe by WHO standards. The summer average across the Chinese cities we tested (60µg/m3) still exceeded the WHO yearly limit (10µg/m3) by 600%.


Of all the cities, the lowest summer pollution levels were seen in Shanghai and Guangzhou (49µg/m3, five times the WHO limit). The worst summer pollution levels (excluding Beijing) were seen in Chengdu. In fact, Chengdu’s winter average pollution levels are even worse than Beijing’s!



Pollution levels on a monthly basis:

We also plotted the average monthly pollution levels for all the cities with US Embassy data, these graphs can give a good idea of which cities have the worst pollution levels, and which months are the worst overall.

The above graphs show a clear annual trend in PM2.5 across each of the cities: pollution levels rise in “winter” months (October-March) and dip in“summer” months (April-September). July and August look to be the best months across most cities, although Beijing has a peculiar peak in air pollution levels in July – most likely due to the lack of wind to blow the pollution away. In fact, Beijing’s yearly variation in pollution is the smallest of all cities – it remains at a consistent average concentration above 80µg/m3.


December and January are consistently the worst months for pollution, which is most likely due to the burning more fossil fuels during winter for heating.


You can find our 200RMB & 470RMB air purifiers on Taobao and on our PayPal store. You can learn more about our purifiers and what masks we recommend by coming to one of our workshops.

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


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.