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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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!

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.

Air Quality Around the World

In the last couple of posts, we have mentioned that 13 of the world’s 20 most polluted cities are in India. The rest are in Pakistan, Qatar, Turkey, Iran, and Bangladesh. But the top 20 rankings made us wonder, how does India compare to the rest of the world? To find out, we selected a few cities from around the globe and mapped the World Health Organization’s 2014 air quality data. Due to variations in the frequency of data collection, WHO’s database contains data collected anywhere from 2010-2013, varying by city and country. The map below shows the outdoor air quality as measured by PM 2.5, or particulate matters that are two and a half microns or less in width. PM 2.5 are approximately 30 times smaller than human hair. Due to their size, they can lodge deeply into lungs and lead to health problems.

Air quality is drastically different across continents, but even cities with seemingly clean air see high-pollution days. Recently, the city of Stuttgart, Germany issued an air pollution warning and asked residents to leave cars at home in lieu of public transport to bring down pollution levels. Though Stuttgart averages healthy levels of air pollution on an annual basis, the city saw 64 days of pollution above EU’s recommended 40 µg/m³ for PM 10 in 2014. Stuttgart government issued the air pollution warning after pollution levels rose to 89 µg/m³ (PM 10) on January 19, 2016.

Similar to Stuttgart, other cities in Europe have taken measures to bring down air pollution. Even though these pollution levels are significantly better off than India’s, they are still above WHO and EU’s recommended levels. In 2014, Paris—like New Delhi—implemented an odd-even scheme to bring down pollution levels. Milan took more drastic measures in 2015 by banning all cars, motorcycles, and scooters between 10 am and 4 pm for a three-day period to bring down pollution levels. Contrastingly, London has introduced a congestion charge for those seeking to drive in the city during the day on weekdays. Beijing has also taken various measures, including putting limits on cars, factories, and construction sites on days with high levels of smog.

As Delhi government continues its debate on how to move forward with the odd-even policy, scholars from the Energy Policy Institute at the University of Chicago the Evidence for Policy Design group at Harvard University examined preliminary data to understand the pilot period’s impact. They found that the odd-even pilot reduced hourly particulate air pollution concentration by 10-13%, but doubted the scheme could work long-term. In Mexico City, a similar scheme led to worse pollution outcomes when households purchased second cars  or old, polluting cars to overcome the odd-even rule. The scholars called for a pilot on congestion charges akin to London’s to understand how this could lead to long-term reductions in air pollution.

As Delhi’s debates on how best to curb air pollution continue, however, many of the other cities that see dangerously high levels of pollution in India necessitate a louder public debate. Here is a chart showing the top 20 polluted cities around the world as compiled by the World Economic Forum. If you live in one of these cities, start the conversation!


Filters and Fetuses

I don’t meant to be sensational, but I just got a picture from a friend of a friend who is pregnant and just started using the DIY filter three days ago. The filter is turning black already.

After all, scientific research suggests children are the most affected by air pollution. If the outrageous prices of filters are keeping pregnant women from cleaning their air, then the DIY filter has already started to change that.