This Data Explains Why You Should Never Use Your Purifier’s Auto Mode

Lots of things in our life are becoming smart. Our phones are smart. Our cars are getting smarter. Purifiers seem like a great candidate for “an education,” but test data shows smart purifiers routinely fail, leaving users with dangerous air.


The idea behind smart purifiers’ auto mode is simple. The purifier has a small air quality monitor inside. When the air is bad, the purifier turns on. After the air gets better, the purifier goes to sleep.


The benefits are obvious. The purifier will use less energy and make less noise, since it can spend more time on a low speed.


Real-World Tests of Auto Modes

Smart Air co-founder Anna tested two of the most popular auto modes in China—the Xiaomi 2 and the Philips AC4072.



These tests are simple and realistic. Over 13 days in an ordinary 15m2 Beijing apartment, Anna turned the Xiaomi or the Philips on auto mode before bed.



She used a Dylos Pro laser particle counter to track 0.5 micron and 2.5 micron particles every hour until she woke up in the morning. The Dylos has shown high accuracy (r = .89) in several comparison tests with official PM2.5 numbers (12). I calculated percentage reductions from before turning on the purifier versus the average over the last four hours before waking up.




The pictures below show the Xiaomi at the start and end of a test. Despite running in a closed room all night, the Xiaomi left indoor air in the unsafe range.



The Xiaomi averaged 62% reductions, and the Philips averaged 59% reductions.



For comparison, this is more than 20% worse than what I found by just strapping a filter to a small fan with a Velcro strap.



But wait, maybe the Philips and Xiaomi are just bad purifiers. Maybe they use low-grade filters, or maybe their fans aren’t strong enough.


Is there something with the purifiers?

Our other tests say no. In tests of the high setting, the Philips scored near the top of my earlier comparison tests.



And in tests of the Xiaomi’s high setting, it also performed relatively well but cannot be kept on high. The machine switches to auto mode whether users want it or not at the third hour.  Really? You really can’t set the Xiaomi to stay on high? 



But is it actually unsafe?

OK, OK, skeptical readers might say, “well a 60% reduction isn’t bad. Isn’t that good enough?”


It’s certainly better than nothing, but based on my reading of the research on the health effects of air pollution, this is meaningfully bad air. On average, indoor air was double the WHO 24-hour PM2.5 limit (and that’s the looser of their limits!).



Why Are Auto Modes So Bad?

The tests on the high setting above showed that it isn’t a problem of the fan or the filter. These machines work well on high. So what explains it?


Cause 1: The Built-In Air Quality Monitor is Wildly Inaccurate

The auto mode is governed by the built-in air quality monitor. The problem is, I systematically tested three different Xiaomi’s against three independent particle counters, and found the Xiaomi monitor was wildly inaccurate. At its worst, it was underestimating actual PM2.5 by 218 micrograms!



So sometimes these machines might think the air issafe when “toxic” would be a more accurate description.


Cause 2: The Xiaomi Has a Loose Definition of “Safe Air”

It could also be that they just have a loose definition of “safe air.” For example, China’s PM 2.5 limit is 35 micrograms, whereas the World Health Organization’s annual limit is 10 micrograms. Is 10 micrograms too low? I think not, because studies have found important health effects even below 20 micrograms.


What Users Should Know

Auto modes routinely leave indoor air far above safe limits. I do not recommend using auto modes (or the Mi2, which forces people to use auto mode after three hours).


Open Data

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

Thomas Talhelm

Thomas is an Associate Professor of Behavior Science at the University of Chicago Booth School of Business and the founder of Smart Air, a social enterprise to help people in China breathe clean air without shelling out thousands of dollars for expensive purifiers.

7 thoughts on “This Data Explains Why You Should Never Use Your Purifier’s Auto Mode

  1. I have commented on your other posts as well.

    1) dont rely on the auto mode. just set it to “favorite” mode at a reasonable level which isnt too loud.

    2) to be clear, favorite mode allows you to set the speed exactly. the max speed in favorite mode is 34-37m2. However, if you go into the settings menu, you can enable turbo mode within favorite which allows you to go all the way up to 42-45m2.

    3) for me, even the normal max speed (without turbo) is too loud. This is just a function of any purifier which is monstrously huge. the sweet spot is 17-20m2.

    4) smartAIR would do its readers a lot of service to analyze xiaomi’s 2’s performance at 17-20m2 setting looking at both pm2.5, below .5 and also the decibel level at this speed.

    5) i repeat (from the comment on your other post) in my experience, the xiaomi does NOT revert back to auto after 3 hours. it has maintained the speed set in favorite for as long as it receives power. dont know why smart air isn’t acknowledging this.

    6) lastly, the DIY and Cannon options that Smart Air sells are extremely ugly and not smart at all. i can’t even stress how amazing it is to control the xiaomi’s via iOS app. the Blast Air’s, which aren’t as hideos as the Cannon DONT SHIP OUTSIDE OF CHINA! So what option is SmartAir leaving us? Dont understand how SmartAir PHDs are being so biased, as their posts otherwise come accross as objective.

    1. Hi Amin, I’m happy to post any data you’ve got on whether the Xiaomi can stay on high past three hours. One way to do it is to use a decibel meter for your phone. The decibels will correspond to the fan setting, as long as the room is otherwise quiet.

      We’ve repeatedly tested the Xiaomi using every setting we can find to see if it can remain on high past three hours and continue to find that it reverts back to auto mode. We’ve also asked Xiaomi customer service on multiple occasions, and they say the same thing. We’ve saved the chat records and posted them here:

      Objectivity is very important to me. That’s why (1) I’ve published all the original data and chat records and (2) remain open to any new data documenting whether the Xiaomi can remain on high.

      And you’re absolutely right that the DIYs are not pretty! They’re simply filters strapped to fans. When I “designed” them, my goal was to get clean air, not design something that looks beautiful. If the looks of the machine are important to you, there are plenty of more finished looking machines out there.

      Sorry we can’t ship the Cannon or the Blasts to Pakistan! It’s not our choice. Because they’re heavy, the shipping is prohibitively expensive. Beyond that, China Post has regulations against shipping motors internationally. Go figure!

      Regardless, I hope you can get some clean air where you are! One option is that we can ship HEPAs to you, and you can source a fan locally. I recommend that to people who ask about shipping to the US.

  2. The new Xiaomi 2S (alongside the Pro and Max) use laser sensors instead of infrared to measure air quality. Would you mind testing the Auto Mode of one of those models?

    I expect their measurements to be almost identical to those of your Dylos Pro.
    So you could rule out Cause 1 and see of Cause 2 stands.

    Best Regards

    1. Hi Ben, thanks for the suggestion! I’ll have to run some more tests. However, my particle counter tests did include the 2 Pro. I’ve heard some people claim that Xiaomi says that has a different particle counter in it (although I can’t find it on their website). Regardless, it was just as inaccurate as the regular Mi2.

  3. Contrary to what Amin says, I’m told that even the Xiaomi 2s goes to auto mode after 3 hours. However, there is a fix! You have to use the smartphone app to turn it on to the high setting every 2.5 hours. For example, you set it to high at 9 a.m. and again at 11:30 a.m. and so on. Then it never goes to auto mode.

    1. Good question! The truth is that no room we live in is truly sealed. Dirty air from outside is constantly leaking back into your room through the door, windows etc. So as soon as you slow down the speed of your purifier, if it’s not strong enough to filter the dirty air leaking into your room, then the particle count will increase. Take a look at the data in this post which shows how quickly air pollution increases after switching off the Cannon in a Beijing flat:

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