Smart Air Filters featured on CNN Money

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Charles Riley, a journalist with CNN Money, recently published a long-form article on Delhi’s air quality and its impact on individuals—particularly children and those with asthma. Smart Air Filters was thrilled to be featured as one of the groups making Delhi’s air cleaner through our low-cost, DIY air filters. The article also featured Saurabh Bhasin, a lawyer taking up legislative battles with the Supreme Court and Dr. Naresh Trehan, who has called for government reforms to promote cleaner air.

Check out the full article on CNN Money’s website.




1. UV紫外线





2. 活性炭





3. 静电


有的净化器里面用静电,但也有很多知名品牌(比如Blue Air,IQ Air)不用静电。我猜是因为静电还会产生臭氧而臭氧对人体有害,会刺激呼吸道和减少抵抗力。同时,静电滤网还需要花更多的时间维修和清洗,所以美国的《消费者报告》(Consumer Reports)建议不要买带静电的净化器

4. HEPA滤网过滤器




像市面上常见的IQ Air和Blue Air这些空气净化器,里面都有HEPA滤网。不过他们给自己的HEPA取了一些高大上的名字(比如“Hyper HEPA”),然后要价上千块。相比DIY,那些净化器或许寿命更长一点、可能能过滤更细小的颗粒,但我觉得,不至于在它们身上“一掷千金”。




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The new Cannon kicks butt (scientific definition of kicking butt), but it’s noisier than the Original DIY. How noisy is it? As is my habit, I wanted to answer this question scientifically.

So I bought a decibel meter:


And I tested the Cannon, Original DIY, Blue Air 203/270E, and IQ Health Pro Plus on their highest settings from 1.95 meters away. Here are the results:


The cannon is noisier than I’d like, but it’s similar to the Blue Air on the high setting. To give you an idea of how loud that is, this decibel chart says that’s between “conversation at home” and “conversation in restaurant.”

It’s still louder than I’d like, but fortunately I’ve found that the Cannon is still very effective on the lower settings:


So I recommend running the cannon on a lower setting if you find it noisy.


  1. Cannon-owners can use the lower settings without sacrificing much performance.
  2. For people who are particularly sensitive to noise, the Original may be a better choice.
  3. For people who are VERY sensitive to noise, the Philips AC4072is expensive (2,700 RMB), but it’s quite quiet on the low setting.

As always, I’m posting the data and methods below for fellow nerds.



More Comparisons

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Dr. Saint Cyr recently pointed me to great tests of air purifiers from the Shanghai Consumer Protection Bureau:


The reason this type of research is so badly needed is that Western research (like this report from Consumer Reports) focuses on allergens, not general industrial air pollution. Allergens are probably more relevant for most Americans, but for those of us living in China, particulate air pollution is the real problem. And a lot of smog is smaller than pollen:


Thus, I was excited to see research that focused on PM 2.5 here in China (results here, in Chinese). In removing PM 2.5, 17 out of 22 models removed more than 90% of PM 2.5 in just 20 minutes in a 30-square-meter room. Pretty good!

Yet even the cheapest of the “non-famous” brands cost about 1,500 RMB. For that price, you could make 9 of my DIY filters, and that’s before eating the cost of proprietary replacement filters (US$200 a piece at IQ Air). Even with these cheaper brands, I still think consumers lack a truly affordable way to get clean air. Using the consumer bureau’s results, I calculated how much you’re paying for each percent reduction in PM 2.5.


Now, their tests weren’t perfect. The biggest downside I see is that they used cigarette smoke as the PM 2.5 source, rather than outside air. It’s not clear to me if cigarette smoke behaves the same as smog in filters. Also, many filters were less effective at removing the formaldehyde that they released in the room. That said, I don’t know how much formaldehyde is in the air normally.

But these results from Dr. Saint Cyr do use ambient air pollution in China (as do mine 1, 2). In tests with doors closed and the filters at their highest settings, Dr. Saint Cyr’s two Alen Air filters, Blueair, and IQ Air all got rid of at least 95% of PM .3. But the Alen Air A375 will set you back US$500 or RMB 5,000 imported in China. Here’s how to get the same results for 166 RMB.


DIY Compared to Expensive Filters

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I now have directly comparable data with the DIYs, Blue Air, and Philips: Direct comparison tests.

In earlier posts (1, 2), I showed that you can make an effective DIY air purifier to clean Chinese air pollution out of your home air. But just because the DIY filter is effective doesn’t mean it’s as effective as the Ferrari filters. How effective is the DIY filter compared to the expensive filters?

I’ll attempt to answer this question from several angles in different posts. Here’s one method: comparisons with the published data from the blog of the Beijing doctor Dr. Saint Cyr (which was an original inspiration for me that air purifiers could actually reduce air pollution). He ran tests of an 11,000 RMB IQ Air and a 6,000 RMB Blue Air. I used the same calculation of effectiveness and compared his results for the pricey machines with tests of my DIY purifier. Here are the results:


To make the comparisons more precise, I compared my filter results only to the tests he reports from the IQ Air and Blue Air on max power in his smaller room.

Dr. Saint Cyr calculated reductions based on outside air quality, which I do not think is the best method. I think it makes more sense to calculate reductions based on the room air before and after using the filter because my room air is significantly cleaner than outside air, even before using the filter (more on that later). But to make the data comparable, I calculated effectiveness using Dr. Saint Cyr’s method.

Now, the comparisons aren’t perfect. Dr. Saint Cyr’s smaller room is still 6.5 meters larger than mine. He also doesn’t say how long he ran his filters and whether the doors were closed. My tests were with the doors closed and overnight, so the filter had several hours to run.

(Update: via email Dr. Saint Cyr said his tests were with doors closed and testing times of 1 hour+.)

But even if the difference in room size bumps mine down 5% and the others up 5%, that would mean 11,000 RMB and 166 RMB get you similar results. To illustrate that, I calculated a graph of how much you’re paying for each percentage reduction in air pollution:


Of course, the ideal test would use all three filters in the same room with the same particle counter. To that end, I will hopefully get the chance to borrow a friend’s Panasonic filter and test it in my home. I’ll post those results when I get them.

Conclusion: Although the comparisons are imperfect, they show that a DIY filter is at least roughly comparable to expensive filters at a far lower cost. This illustrates my larger point: all you really need to combat particulate air pollution is a HEPA filter. In fact, Dr. Saint Cyr found the same thing in his tests of a much simpler Hunter brand HEPA filter. In a room with the doors closed, his HEPA got 91% of the pollution–similar to what I found.

For data nerds like me, I’m posting more details on my data and methods here:

Test Details: