Ever since I coughed through a Beijing airpocalypse, I’ve been saying clean air doesn’t need to cost an arm and a leg. I set up the social enterprise Smart Air to put out open-source data on purifiers and provide low-cost purifiers to people in places like China and India based on that data.

So when the Xiaomi 2 air purifier came out for under $150 (10,000INR / 1,000RMB), 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 Smart Air test bedroom 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 2 air purifier oddly calls “Favorite 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).

Results

The Xiaomi purifier scored as one of the worst purifiers I’ve ever reviewed. On average, it removed only about 60% of 0.5 micron particles over the last 4 hours of the test.

And oddly enough, the auto mode and the high mode results were nearly identical. That’s a mystery.

Here’s how the Xiaomi results compare to earlier results from other purifiers using the same method, same particle counter, in the same room.

The data shows the Xiaomi performed worse than the $30 DIY purifier I’ve been showing people how to make for years!

Below is a normal test day. I put a dashed red line representing the World Health Organization’s 24-hour PM2.5 limit (25 micrograms).

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 (watch a video of it doing so here). To be sure this is what the machine was doing, we put the Xiaomi on the highest setting and tracked noise levels overnight from the Smart Air Lab.

You can see the noise remaining high for three hours, then it returns to the ups and downs of auto mode:

Here is the same graph with particle counts (measured by the Dylos). The particle counts rise sharply every time the machine turns off.

The data shows that the Xiaomi has a loose 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. And it’s far higher than levels where we know PM2.5 has serious health effects from empirical research. Then it turns on again when PM2.5 gets up to 70 micrograms—almost three times the limit.

Really? It won’t stay on high?

The fact that the Xiaomi air purifier can’t be kept on continuously all night is so strange that Anna asked Xiaomi 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 purifier 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.

Maybe the Xiaomi Mi2’s auto mode is using the looser Chinese standard for clean air of 35 micrograms. Yet even with that high number, it was still above the limit 77% of the hours the Xiaomi Mi2 was running.

Update: we subsequently ran tests on the particle counter inside the Xiaomi Mi2, and what we found was shocking.

Wait, are you sure?

I wanted to be really careful about this review. 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:

We tested with a different particle counter in a different room.

I re-analyzed the data throwing out any days with large fluctuations in outdoor air.

I analyzed days when outdoor pollution was low to average (< 150 micrograms).

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 air purifier, 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.

Bottom Line: Performances of Xiaomi Mi2 From this data, my conclusion is that the fan and the filter are fine, but the Xiaomi air purifier 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 test 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.