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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The manual, full mode is way too loud for sleep. I just leave it on A. Has to better than no purifier.

My room was 80 when I turned it on, it’s now 19… That’s not so bad, right?

Amit Carbyne

I use “Automation” creating a schedule for manual mode. The purifier turns to manual mode for 10 min than runs at auto mode for 10 min resulting in 10 min of full power for every 20 min. So in 1 hour the air purifier runs 30 min manual and 30 min auto. Although it took few hours to create this automation.

Sherrin Thomas

I have a Philips AC 2887/20 and now a Dyson TP03 (tower). For both machines I have noticed that when pollution levels are high (100+), if I run them at high or turbo speed, the sensor shows the pollution level going up sharply – sometimes by as much as 100 µg/m3. One way to look at this would be that at turbo mode, the machine is processing more air, and so shows higher pollution levels, but it doesn’t seem like a very satisfactory answer.

Simon So

Thanks for doing this test, too bad I stumbled upon your article after purchasing the Mi Pro.


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).