There’s a massive vacuum cleaner in the middle of a Rotterdam park and it’s sucking all the smog out of the air. A decent portion of it, anyway. And it isn’t a vacuum, exactly. It looks nothing like a Dyson or a Hoover. It’s probably more accurate to describe it as the world’s largest air purifier. The Smog Free Tower, as it’s called, is a collaboration between Dutch designer Daan Roosegaarde, Delft Technology University researcher Bob Ursem, and European Nano Solutions, a green tech company in the Netherlands. The metal tower, nearly 23 feet tall, can purify up to 1 million cubic feet of air every hour. To put that in perspective, the Smog Free Tower would need just 10 hours to purify enough air to fill Madison Square Garden. “When this baby is up and running for the day you can clean a small neighborhood,” says Roosegaarde. It does this by ionizing airborne smog particles. Particles smaller than 10 micrometers in diameter (about the width of a cotton fiber) are tiny enough to inhale and can be harmful to the heart and lungs. 

Ursem, who has been researching ionization since the early 2000s, says a radial ventilation system at the top of the tower (powered by wind energy) draws in dirty air, which enters a chamber where particles smaller than 15 micrometers are given a positive charge. Like iron shavings drawn to a magnet, the the positively charged particles attach themselves to a grounded counter electrode in the chamber. The clean air is then expelled through vents in the lower part of the tower, surrounding the structure in a bubble of clean air. Ursem notes that this process doesn’t produce ozone, like many other ionic air purifiers, because the particles are charged with positive voltage rather than a negative. “The proposed technology, while not new, would need to be well demonstrated on a large scale in a highly polluted urban area,” says Eileen McCauley, a manager in the California Air Resources Board’s research division. She adds that there are concerns around efficacy and logistics like how often something like this would need to be cleaned.

But Ursem himself has used the same technique in hospital purification systems, parking garages, and along roadsides. air purifier with heatStill the tower is by far the biggest and prettiest application of his technology.air purifier aus Indeed, it’s meant to be a design object as much as a technological innovation. best eco air purifierRoosegaarde is known for wacky, socially conscious design projects—he’s the same guy who did the glowing Smart Highway in the Netherlands. He says making the tower beautiful brings widespread attention to a problem typically hidden behind bureaucracy. “I’m tired of design being about chairs, tables, lamps, new cars, and new watches,” he says. “It’s boring, we have enough of this stuff. Let’s focus on the real issues in life.”

Roosegaarde has been working with Ursem and ENS, the company that fabricated the tower, for two years to bring it into existence, and now that it’s up and running, he says people are intrigued. He just returned from Mumbai where he spoke to city officials about installing a similar tower in a park, and officials in Mexico City, Paris, and Beijing (the smoggy city that inspired the project) also are interested. “We’ve gotten a lot of requests from property developers who want to place it in a few filthy rich neighborhoods of course, and I tend to say no to these right now,” he says. “I think that it should be in a public space.” Roosegaarde has plans to take the tower on a “smog-free tour” in the coming year so he can demonstrate the tower’s abilities in cities around the world. It’s a little bit of showmanship that he hopes will garner even more attention for the machine, which he calls a “shrine-like temple of clean air.” Roosegaarde admits that his tower isn’t a final solution for cleaning a city’s air.

“The real solution everybody knows,” he says, adding that it’s more systematic than clearing a hole of clean air in the sky. He views the Smog Free tower as an initial step in a bottom-up approach to cleaner air, with citizens acting as the driving force. “How can we create a city where in 10 years these towers aren’t necessary anymore?” he says. “This is the bridge towards the solution.”Update January 2016: My data below speaks for itself, but people should be aware of possible quality problems reported here. I think I’m done testing air purifiers. Yes, I’ve said this before, and I admit I often enjoy crunching the numbers, but I think I mean it now. The only possible way I could be persuaded to test another model would be an amazing breakthrough in technology. But it would still have to be reasonably affordable, even under 1,000 RMB per machine. Actually, it’s not, because I sold all my IQAirs and my Blueair and switched to six air purifiers which cost me a total of 5,200 RMB.

a total of 5,200 RMB, for six new machines. I hardly doubt I need to remind most of you that’s half the price of only one of the many popular machines on the market now. And as this article will show, I’ve proven that these new machines are keeping my family’s indoor PM2.5 under 10 ug/m3, which is the goal of the World Health Organization and thus should be everyone’s goal (read more about that here). So why would I even want to test anything else? As to which machine, many readers probably already know my answer because I started to test them last year: the Xiaomi 小米空气净化器 at 899 RMB, covering 48 square meters (apparently there’s a newer, smaller model as well for only 699). I initially blogged about Xiaomi last year discussing my test results of indoor air purifiers under 1,000 RMB. After last year’s testing, I had replaced two imported models in bedrooms with the Xiaomi purifier, and my bedroom air has been just fine since the switch. Just a few weeks ago I made the really big leap, returning two large air purifiers in my front rooms with three Xiaomi purifiers (I needed one extra as the coverage was comparatively less).

And for the record, I am getting no promotional money or free anything from Xiaomi. In our nine years in China we’ve always used well known imported air purifiers to cover our front rooms, which are essentially one large open space covering 83 square meters. But the price gouging in China always galled me. So after last year’s testing of purifiers under 1,000 RMB, I was always thinking, why can’t I see if I can also replace these big machines in the front rooms with something of more value yet equally effective? The timing was perfect as we’ve had multiple December days way over 200, 300 — and the worst two days of pollution I’ve ever seen in Beijing in my nine years here, our 2015 Airpocalypse with readings over 600 and an eerily dark, yellow sky in the afternoon of December 1st. Let’s jump right in to the only thing that matters: hard data. And maybe the most important test was Airpocalypse 2015. So here’s the bottom line: data from our Laser Egg PM2.5 monitor showed an outdoor average PM2.5 concentration of 465 ug/m3 over those couple days, and inside the front rooms only 25 ug/m3.

That’s an astonishing 95% steady improvement over the worst pollution you’ll pretty much ever see here, and although it’s not at my ideal of 10 ug/m3, it’s still far below the 35 that most Chinese environmental teams shoot for anyway, and the 95% reduction is certainly way below the usually recommended 80% goal. And the machine’s weren’t even on the top, super-noisy speed but the middle speed, which is still fine for general conversation and TV watching. Here’s the graph from the Laser Egg app, with the red line showing outdoor PM2.5 and the blue line showing my living room PM2.5 (the left side axis is PM2.5 concentration in ug/m3 and the bottom axis is just the number of data entry points): On more “normal” days the data was much closer to, and usually under, my aggressive goal of PM2.5 concentration under 10 ug/m3. Here’s the data from earlier November: Even a quick glance at the graph above tells me everything I need to know: my living room air (the blue line) is generally exactly around 10, and always under 20 even as the outdoor air climbs up.

And as Beijing’s annual average is 70-90 ug/m3, my three Xiaomi machines easily get me to my goal of 10 during those typical days — and at a nice auto setting with very comfortable sound level. For calculated averages over this week using the Excel spreadsheet data from the Laser Egg, my front room average was 6.1 ug/m3 when outdoor air was a relatively clean 61; 9.8 during heavier days averaging 136 outside; and a still respectful 14.4 when outdoor air tanked for a couple days at 220 (that’s an impressive 93% reduction). You notice there are a few spikes on the blue line, but all of those are easily explainable as with any other air purifier I’ve had: either the windows were open while we were using our laundry dryer, or we were cooking, or the machines were accidentally off for a while and no one had noticed. Here’s one more graph from my bedroom, again demonstrating even more clearly than last year’s test that a Xiaomi on quiet setting is perfectly effective, and easily just as effective as any machine I’ve ever tested, including all the far more expensive models.

The calculated average PM2.5 concentration was 8.0 ug/m3, again nicely below my goal of 10. It was 8.6 ug/m3 when the outdoor air was a horrible 224 over two days; and 7.4 ug/m3 on two clearer days, when the outdoor air average was 69: So let me be perfectly blunt to those in China who say that an indoor PM2.5 of 10 ug/m3 is impossible and too expensive: I say that’s nonsense, especially now, certainly on the money side. A goal of 35 ug/m3 still gives everyone a 15% increased mortality risk, as the . I do realize this is a sensitive concept for some readers, as many people have spent a lot of money on air purifiers, or even work for those companies. And it’s fair to point out that these don’t last as long as others (three months, although they’re only 179 RMB), and (rated H11) is not as efficient as most other higher-end machines’ filters, and maybe the machine’s solidity isn’t so great, and perhaps they may break down sooner than other machines. We simply don’t know yet, as these are new machines only a year on the market — which in itself should give people some pause.

Also, the built-in sensor is quite inaccurate and pretty much useless to follow — but I’ve never found sensors ever reliable on any air purifier, and I always control all of them manually anyway (and none of the major brand machines ever had a sensor anyway). The sensor’s more concerning issue is that it thinks that a PM2.5 concentration of 75 ug/m3 and under is safe. That’s simply too high, and even the Chinese government rating of 35 would be more proper. I of course think 10 is best, but for a machine made in China I would think 35 is reasonable. So if people are using a Xiaomi and relying only on the sensor on auto speed, then indeed they still may be inhaling not great air. This again is why everyone, using any type of air purifier, should not be relying only on any machine’s auto setting ever (my personal opinion). You need to get a separate monitor and use that to test your air.) And yes, Xiaomi is mostly a cell phone company first. But let’s be very clear here: I’ve mentioned many, many times that anyone living anywhere in polluted areas, whether China or India or Los Angeles, has an ultimate goal to get your indoor air pollution under the World Health Organization’s goal of PM2.5 under 10 ug/m3.

The scientific data is very clear; any PM2.5 over 10 starts to have health effects. So people in polluted areas absolutely must have air purifiers indoors, where you spend 90% of your lives. At the very least, put one in your bedroom. And for heaven’s sake, make sure your children’s rooms are all protected. So you’re more than welcome to get any air purifier machine you want, as long as you’re reaching your goal of PM2.5 under 10 ug/m3, and routinely monitoring your air afterwards (perhaps with the new Laser Egg, only 499 RMB) to make sure all is working fine. In terms of which air purifier is best, again I’ve mentioned often that all that anyone needs for indoor protection is a good filter attached to a good fan. I’ve tested many models over many years, in real world circumstances in my homes, and no particular model at any price point ever was so obviously better than the rest. This is why I am perfectly comfortable with my current setup. And this is why I am done with testing.