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.


The Sqair air purifier Kickstarter

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I have my Xaoimi in my bedroom with all doors and windows closed. If I open the ensuite bathroom door for just a few minutes the reading on my Xaoimi will change. Doesn’t this mean that it is detecting the bad hair from the bathroom? Which would indicate that it’s working well and doing it’s job?

Hi Cecilia! You’re right that the Xiaomi may well be detecting that the air quality has got worse since opening your bathroom door. But what we would class as ‘working well’ would be ‘providing accurate results’. We’ve found that the Xiaomi sensor isn’t accurate, so whilst it may be able to detect the trends in air pollution (like the levels increasing when you open your bathroom door), it may not be giving the actual correct reading. That’s dangerous if the purifier under-estimates the indoor air pollution (and turns itself off as a result).

David Tise

You say that the Xiaomi works fine on high setting. How then is it possible that being used in a closed room on low or sleep setting will not give you the same clean air over time. Won’t it clean and re-clean the same air until no or few particles are left in the room. I just don’t understand why it would have to be on high setting to work effectively in a closed environment. I am in the market for an Air Purifier, and had pretty much decided on the newest version of the Xiaomi Pro, I am lost… Read more »

Great question David! The answer lies in the fact that no room is ever truly ‘sealed’. Ever room will have small gaps between windows or doors, which will let in outdoor air pollution. If the clean air the purifier is pushing out isn’t enough to overcome the dirty air coming in (as is the most often case when purifiers are in auto mode) then the purifiers will never get the air in the room clean.

David Tise

Thanks for your response. As it happens, a few Xiaomi Pros became available yesterday and I went for it….it’s size, shape, and method of delivering “clean” air works best for my situation. I have also ordered an external metering device and will double check it’s effectiveness. It will be interesting to see if the internal and external meters agree. I purposely chose a different brand for the external. My fingers are crossed. Hopefully I’ll have both working within the week. The air in Bangkok has been great the last few days, I may have to wait a bit for any… Read more »

Clara Williams

HI David, how did the Mi perform? Would be very interested to know!

Fe Ev

Hi David, I’m in your same situation in Bangkok. Would love to hear what results you are getting with fhs xiaomis. And do they also turn auto after 3 hours?


Won’t the technique of putting a fan with a strapped on filter cause the fan the break? I’m pretty sure normal household fan aren’t optimised for that much pressure as the filter requires a lot of pressure to push through. Wouldn’t it just break the fan after a period of time?

You’re spot on that most household fans aren’t designed to also push air through a HEPA filter. However we’ve been shocked to find out that after shipping over 30,000 of these simple DIY filters, they work surprisingly well! I’ve heard of reports of people using their simple DIYs for over 4 years with no issues, so the answer is likely that the fan and motor are still powerful enough to push the air through, or that the air resistance of the HEPA filter isn’t so high that it damages the fan.


Hi! So this tests applies to the Xiaomi Mi Air Purifier PRO version? This one have a Laser sensor, and I’ve been reading is really accurate… Haven’t tried max setting much… I always use it on Auto Mode and always shows less than 20 or even 10 on the screen, I live in Coslada, Madrid, Spain.

Hi Luciano! This test applies mainly to the Xiaomi 2 air purifier, however we did test the Pro Air Purifier sensor. Check out the data in our previous report explaining the accuracy the Xiaomi’s air quality sensor.

Paul The Rock

I think that there is a problem with China limits… If you look to auto mode for Xiaomi it thinks that bad air is when PM2.5 daily averge are 75 (which is China limit), but in Europe f.e. the limit is 35… so Xiaomi leaves room with usafe air for europena standard but not for china standard. I think that it switches to lower power too soon for us, but not fot China limits (which is the country where it is sold officialy…). The right solution for Xiaomi should be a adjustable level for PM, but it needs a chenge… Read more »