We get many questions about air pollution in our office, and understandably so. It’s a topic that isn’t well understood or well-reported about in certain parts of the countries in which Smart Air works. In some cases, it is difficult to distinguish facts from belief.
据英国《卫报》, 乌兰巴托是世界大都市空气污染最严重第二。 2008-2011年，乌兰巴托PM 2.5平均值接近150微克；北京2014年平均PM 2.5是98微克；世界卫生组织24小时上限是25微克。
欢迎加入我们DIY沙龙，了解如何用简单的DIY空气净化器减少家里的有害颗粒污染。Smart Air创始人Thomas Talhelm将解释非常贵的空气净化器的工作原理及如何利用同样的道理保护自己和保护自己的钱包。参加者在现场制作自己的DIY空气净化器带回家。
数据显示很多人的直觉（包括我自己）是错的：PM 2.5在夜间的时候最高（上海除外）。在五个城市，PM 2.5是下午最低。所以如果要安排野餐或者室外活动，一般来说最好安排在下午或者凌晨。
Breathe safe, Guangzhou!
So far, I’ve been testing air purifiers by taking a baseline measurement of particulate pollution in a room, and then turning on the purifier and testing whether the counts drop. I’ve used that method to test the DIY and more expensive machines.
However, I recently bought a second particle counter, so my collaborator Gus suggested another method: run one particle counter in the bedroom that has the purifier, and run another particle counter in a different room that does NOT have a purifier. The benefit of this method is that the control room represents the counterfactual–what would have happened if we hadn’t turned on the air purifier.
Thus, if a northwest wind hits Beijing and makes the outdoor air a lot cleaner, we can separate the effect of the outdoor air fluctuations from the effect of the purifier. In that situation, my old method would artificially raise our estimates of effectiveness. Changes in outdoor air can also artificially lower our estimates of effectiveness if the outdoor air gets dirtier after we turn on the purifier.
In previous tests, I corrected for this by averaging over multiple tests. I also analyzed the data after removing days in which outdoor air pollution fluctuated a lot (for example, I do that sort of analysis in the extra nerd notes here).
But it’s always nice to use different types of tests to make sure an effect is real, so Gus did this experiment. He set up one particle counter in his room and one in his kitchen:
He let the particle counters run for several hours, and then a timer turned on the Original DIY in his room. (The kitchen had no air purifier.) Here’s what happened:
The difference between the bedroom and the kitchen air quality can approximate the effect of the air purifier. It looks like Gus would have been breathing 16,000 of these 0.5 micron particles in the air in his bedroom if he hadn’t turned on his DIY purifier.
And it’s pretty clear that the kitchen air quality (where we don’t have a purifier running) is following outdoor air quality:
(Be aware that I’m overlaying these two lines on the same graph, but the Y-axes are different. This is NOT saying that indoor air is as bad as outdoor air. Indoor air is usually cleaner than outdoor air.)
Conclusion: Similar to earlier tests, the double particle counter test shows that the DIY purifier is removing particulate pollution from the air.
As always, I’m including more details for fellow data nerds below.
What about the 2.5 micron particles? In the main text, I present the 0.5 micron results, but what about the larger particles? The results there are similar:
Remember that because these particles are larger, they are more affected by people moving around. That’s probably why there are large spikes from 7 to 9pm, and why counts level off after midnight. However, even after the numbers level off, the purifier is reducing the counts in bedroom even further.
Timing: Careful readers will notice a slight lag between the bedroom and kitchen numbers. That’s because after entering its hourly mode, the Dylos particle counter takes its first measurement after a random number of minutes. (After that, it’s every 60 minutes.)
Therefore, we cannot guarantee the kitchen and bedroom numbers are taken at the exact same time. However, after the fact, we can tell when the measurements were taken. Thus, we know that the bedroom numbers were taken at 27 minutes past every hour, and the kitchen numbers were taken at 36 minutes past the hour. Those numbers are reflected on the X-axis.
Percentage effectiveness: I also computed the percentage reductions in particulates using my regular method of averaging the last four hours before the DIY was turned off. Then I compared that as a percentage of particulate in the kitchen.
According to that calculation, the DIY removed 86% of 2.5 micron particles and 87% of 0.5 micron particles. Compare that to 92% of 2.5 micron particles and 84% of 0.5 micron particles in my earlier tests. However, I am cautious about drawing strong conclusions from one day’s data. It would be best to average the results of several testing days.
A while back, I posted data comparing how effective the DIY purifier was compared to a 6,000 RMB Blue Air and 11,000 RMB IQ Air. In that test, I calculated effectiveness as percent difference from outside air. At the time, I said I didn’t like that way of calculating effectiveness, but I did it that way so I could compare my results with the results of Dr. Saint Cyr.
Now I’m posting data from new tests that improve on the old tests in three ways:
- The new tests calculate effectiveness as the percentage reduction from indoor air. This gives smaller percentages because outside air is dirtier than inside air. In my opinion, this is a more rigorous way to test air purifier effectiveness.
- The new tests were conducted by my Smart Air collaborator Anna Guo in her apartment. This helps test whether the results are generalizable. In other words, this way we can know that my original results were not a fluke just based on the layout of my room or how well my windows are sealed. Anna’s room (15 m2) is also slightly larger than mine (13.5 m2).
- Finally, the new tests are with the Meidi fan I’m now shipping to people from my Smart Air site. Thus, these results are more relevant for people using our DIY filter kits. (However, the Meidi fans are very similar to the blue Kadeer fans I was using before. I haven’t seen a noticeable difference in the results with the two fans.)
Methods: Anna used my particle counter to take a baseline 2.5 micron reading before turning on the filter. Then she turned on the filter, put the particle counter on the other side of the room from the DIY purifier, set it to take hourly measurements, and went to bed.
To calculate effectiveness, we took the average 2.5 micron reading of the four hours prior to waking up and calculated the percent reduction from the baseline reading before turning the filter on.
Filter: We used the same DIY kit with the Meidi fan that we’re shipping from Smart Air:
Results: Here’s what the data looks like. This data is from September 9th:
In these real-world tests, we need to be careful to check that outdoor particulate didn’t go down during the test. If it did, we would think the decrease is due to the purifier when really it’s due to outdoor air changes. To test that, we can overlay the outdoor PM 2.5 readings from the US Embassy:
Outdoor air actually got slightly worse during the test, so the reduction inside is not because outdoor air got cleaner.
And here’s the average reduction in 2.5 micron particles over the three test nights:
Conclusion: The DIY filter removed an average of 92%. That’s similar to the earlier result of 94%. I suspect the new result is 2% lower than before because this new calculation uses reduction from inside air rather than outside air.
Again, the effectiveness ratings are similar to Dr. Saint Cyr’s tests with the Blue Air and IQ Air, although remember that his room is larger (20 m2), and the post does not specify the length of testing time or the particle counter used. Ideally we’d have tests with the same particle counter in the same room.
As always, I’m posting more details on the methods for fellow nerds here:
If Particle Counting has just one take-home message, it is that you can protect your yourself from particulate pollution in China, and it costs a lot less than filter companies want you to think. However, I don’t want to mislead people into thinking that turning on a filter will definitely solve any possible problem. I talked with Louie Cheng, who founded Pure Living China, a company that tests for pollutants in homes and offices, and we came to a few conclusions about the limits of filters:
- Particulates are not everything.
Particulate pollution is a big deal in China, and it’s bad for your health. There are lots of studies out there showing this, but the one that pops into my head the most is the study showing babies exposed to more air pollution are born with smaller heads. If that’s not scary, I don’t know what is.
However, particulates are not everything. Gas pollutants can be a problem too. Although I suspect that almost every home in China has particulate pollution while only some have gas pollution, there are dangerous gases out there like radon, carbon monoxide, and the broad category of “volatile organic compounds.” Unfortunately, even the fancy filters have trouble consistently capturing a wide range of gases.
If you’re considered about gases, consider getting a home test from Pure Living China. They’re not cheap, but I’d consider it if I had unexplained health problems or a child at home.
I try to write “particulate pollution” rather than “pollution” because not all pollutants are particulates. However, it’s easy to just covert “particulate pollution” to “pollution” in our minds.
- Some particles are too small for filters.
A quick look at the size of different particles and gases makes it clear that particulate filters can’t get everything:
HEPAs are rated to .3 microns (micrometers), and I’ve seen reports that they can get particles of .1 microns, but there are still leftovers. For example viruses and gases will pass through air filters (but not bacteria). (This, by the way, explains what I call “the smoker paradox.” More on that later.)
- Opening your window is good (sometimes).
The air outside in China is scary, but sometimes it is actually good to open your windows. That is because some pollutants can come from inside. Some common sources are formaldehyde used in some furniture, cooking without a good vent, and construction work.
In those cases, it can actually be a good thing to open your window, particularly on blue sky days.
Conclusion: Air purifiers will get a lot of pollutants out of your air, but there are limits to what you can filter out. Consider opening your windows on clean days–and particularly if you have renovation going on indoors or are cooking without an adequate vent.
Thank you to Louie Cheng for sharing his expertise.